How this is calculated
Every family except temperature works the same way: each unit carries a fixed factor to one base unit, so the value is multiplied by the factor it came from and divided by the factor it is going to.
Temperature is the exception, because Fahrenheit and Celsius do not share a zero. It uses the defining relation printed in NIST Special Publication 811: degrees Celsius equal degrees Fahrenheit minus 32, divided by 1.8.
Battery capacity is the other special case. An amp-hour is a quantity of charge rather than of energy, so it becomes watt-hours only when multiplied by the voltage the battery works at.
Worked example, at the defaults on screen. A 100 Ah battery at 12 V holds 100 × 12 = 1,200 Wh, which is 1.2 kWh, or 4.32 MJ, or 4,095 BTU. At an 80 percent depth of discharge you can spend 960 Wh of it; at the 50 percent floor a flooded lead-acid bank wants, 600 Wh.
The same 100 Ah on a 48 V bank is 4,800 Wh. Nothing changed but the label, which is why two banks cannot be compared in amp-hours alone.
The one conversion worth memorizing
Watts times hours gives watt-hours; watt-hours divided by volts gives amp-hours. Those two lines, either direction, cover most off-grid arithmetic.
The factors this page uses
These are definitions rather than measurements, so they do not drift. The values are those published by NIST in Special Publication 811, Appendix B.
| Unit | Exactly equals | Where it comes from |
|---|---|---|
| 1 kWh | 3.6 MJ · 3,412.14 BTU | Definition; BTU factor from NIST SP 811 |
| 1 BTU (international table) | 1,055.05585262 J · 0.293071 Wh | NIST SP 811, Appendix B.9 |
| 1 MJ | 0.277778 kWh | Definition of the joule |
| 1 therm (US) | 105,480,400 J · 29.3001 kWh | NIST SP 811, Appendix B.9 |
| 1 stere | 1 stacked m³ · 0.2759 cord | Definition; inverse of the cord above |
| 1 horsepower (mechanical) | 745.6999 W | NIST SP 811, Appendix B.9 |
| 1 ton of refrigeration | 3,516.853 W · 12,000 BTU/h | NIST SP 811, Appendix B.9 |
| 1 psi | 6,894.757 Pa · 0.0689476 bar | NIST SP 811, Appendix B.9 |
| 1 bar | 100,000 Pa · 14.5038 psi | Definition of the bar |
| 1 meter of water head | 9,806.65 Pa · 1.42233 psi | Water at 1,000 kg/m³, gravity 9.80665 m/s² |
| 1 US liquid gallon | 3.785412 L | NIST SP 811, Appendix B.9 |
| 1 imperial gallon | 4.54609 L | Weights and Measures Act definition |
| 1 cord | 128 cu ft · 3.624556 m³ | NIST SP 811, Appendix B.9 |
| 1 foot | 0.3048 m | International yard and pound agreement |
| Degrees Celsius | (°F − 32) ÷ 1.8 | NIST SP 811, Appendix B.9 |
Head figures assume fresh water at standard gravity. The US therm is defined as 100,000 BTU measured at 59°F, which is 99,976 of the international table BTU used everywhere else on this page, so the converter shows that figure rather than a round 100,000.
How the electrical units hang together
Most sizing mistakes are one of these arrows read backwards. Power is a rate, energy is that rate held for a period, and charge is energy divided by the voltage it was stored at.
American service adds a wrinkle: it is split-phase 120/240 V, and ANSI C84.1 sets the Range A service voltage at 114 to 126 V, so a nominal 120 V outlet is legitimately anywhere in that band. Wells, dryers and ranges want the full 240 V, which an off-grid inverter must be built or stacked to deliver.
European service adds a wrinkle: IEC 60038 harmonized it at 230/400 V, replacing the older 220/380 V and 240/415 V, so a socket once called 220 V and an appliance once labeled 240 V now sit on the same nominal supply. Rural properties often arrive three-phase, and reproducing that off-grid means three inverters or a decision to live single-phase.
Either way, a nameplate in amps means watts only once you know its voltage. The whole-house version of this arithmetic runs in the can I run my house off-grid calculator.
Where the conversions bite
Three places, over and over. First, a battery quoted in amp-hours at a voltage nobody mentioned, which makes a small bank look large.
Second, a generator or inverter rated in volt-amps. Apply a power factor of about 0.8 and a 5 kVA machine is a 4 kW machine.
Third, heat. A stove sold in kW and one sold in BTU per hour are the same machine either side of a 3,412 factor, and a fuel quoted as a higher heating value sits above the same fuel quoted as a lower heating value, by about ten percent for natural gas.
A worked case: a 30,000 BTU/h propane heater is an 8.8 kW appliance. No ordinary battery bank makes that as electric heat, which is the argument for keeping combustion or a heat pump in the plan.
A worked case: an 8 kW wood stove is a 27,300 BTU/h appliance. No ordinary battery bank makes that as electric heat, which is the argument for keeping combustion or a heat pump in the plan.
Solar has its own translation. A panel is rated at 1,000 watts per square meter and a 25 degree Celsius cell, so daily output is the rating times peak sun hours, then reduced.
The PVWatts Version 5 Manual (NREL, 2014) sets a default total system loss of 14 percent, built up in its Table 6 from soiling, shading, mismatch and wiring, and models the inverter separately at 96 percent. A battery in the path deserves a deeper cut than either.
Quick answers
How many watt-hours is a 100 Ah battery?
Multiply amp-hours by nominal voltage: 1,200 Wh at 12 V, 2,400 Wh at 24 V, 4,800 Wh at 48 V. Amp-hours cannot be compared between batteries until the voltage is stated.
How do I convert kW to kWh?
Multiply kilowatts by hours. A 1.5 kW load left on for four hours uses 6 kWh; the same 6 kWh spread over four hours is a 1.5 kW average draw.
How many BTU are in a kilowatt-hour?
3,412.14. The international table BTU is defined as 1,055.05585262 joules and a kilowatt-hour is 3.6 million joules, so the ratio is exact.
What is 1 bar in psi and in meters of head?
14.50 psi, and a column of fresh water 10.20 meters or 33.5 feet tall. That equivalence is why a pump can be specified either way.
How many cubic meters is a cord of wood?
3.6246, so a cord is 3.62 steres. A 16-inch face cord is a third of a cord, or 1.21 steres, and the depth is what the name hides.
A 120-term off-grid glossary
Each term below gets a plain definition, then the reason it changes a decision once you run your own supply.
- Electricity, wiring and protection — 36
- Batteries and charging — 24
- Solar — 22
- Inverters, generators and loads — 16
- Water and pressure — 12
- Heat, fuel and buildings — 10
Glossary: electricity, wiring and protection
Thirty-six terms that decide what copper you buy, what the fuse has to hold and why a number on a nameplate is not the number the wire sees.
Amp (ampere)
The rate at which charge flows through a circuit, the electrical equivalent of liters per minute. Wire, fuses and breakers are sized in amps, so amps decide the copper you buy while watts only decide the bill.
Amp-hour (Ah)
One amp flowing for one hour; the charge a battery can deliver, quoted at its own voltage. Ah becomes energy only when multiplied by voltage, which is why 100 Ah at 12 V and 100 Ah at 48 V are four times apart.
Watt (W)
The rate of energy use, equal to volts times amps in a direct-current circuit. Watts size the inverter and the wire; they say nothing about how long anything runs.
Watt-hour (Wh)
One watt drawn for one hour, the smallest useful unit of stored energy. Every off-grid design is a Wh budget, because the battery is measured in Wh and the day only has so many of them.
Kilowatt (kW)
One thousand watts, the unit generators, inverters and solar arrays are rated in. A 5 kW inverter and a 5 kWh battery sound alike and mean entirely different things, which is the single most common sizing error.
Kilowatt-hour (kWh)
One thousand watt-hours, the unit a utility bill counts and a home battery is sold in. It is the common currency: a bill, a battery, a generator run and a cord of wood can all be compared once each is written in kWh.
Volt (V)
The electrical pressure pushing current through a circuit. Choosing 12, 24 or 48 V for the battery bank sets every wire size, fuse and controller in the system, and it is nearly impossible to change later.
Ohm
The unit of resistance, the opposition a conductor offers to current. Resistance in undersized cable turns your stored energy into warm wire, which is loss you paid for twice.
Voltage drop
The volts lost along a run of cable, equal to current times the cable's resistance. Low-voltage systems are unforgiving: three percent drop on a 12 V circuit is a third of a volt, enough to make a controller misread a full battery as empty.
Alternating current (AC)
Current that reverses direction many times a second, the form household outlets deliver. Everything on an off-grid site that is not a battery, panel or pump motor runs on AC, and making it costs inverter losses.
Direct current (DC)
Current that flows in one direction, the form batteries and solar panels produce. Running lights, pumps and fridges natively on DC skips the inverter entirely, which is why boat and van systems are so efficient.
Hertz (Hz)
Cycles per second in an alternating-current supply. Motors, clocks and compressors care: a 60 Hz motor on a 50 Hz supply turns slower and runs hotter, so imported appliances need checking.
Power factor
The ratio of real power in watts to apparent power in volt-amps, from zero to one. Motors and older electronics pull more current than their watts suggest, so an inverter rated in VA delivers fewer usable watts than the label implies.
Volt-ampere (VA)
Apparent power, volts times amps, before power factor is applied. Generators and uninterruptible supplies are often rated in VA; multiply by a power factor of about 0.8 to get watts you can actually use.
Inrush current
The brief current spike when a motor, transformer or power supply first energizes. It is the number that disqualifies most small inverters, because the surge can be several times the running draw for a fraction of a second.
Locked rotor amps (LRA)
The current a motor draws at the instant of starting, when the rotor has not yet turned. Compressor and pump nameplates print it, and an off-grid inverter must clear that figure with the rest of the house still running.
Rated load amps (RLA)
The current a compressor draws at its rated operating condition, printed on air-conditioning nameplates. It is the running number to budget energy against, while LRA is the number to size the inverter against.
Duty cycle
The share of an hour a load actually spends running. A fridge rated at 150 W but running a third of the time costs 1.2 kWh a day, so duty cycle is what separates nameplate watts from real energy.
Phantom load
Power drawn by equipment that appears to be switched off. On grid power it is a rounding error; off-grid, a dozen standby devices at 3 W each drink 864 Wh a day.
American wire gauge (AWG)
The North American numbering for conductor size, where a smaller number means thicker wire. Battery cable is the one place where going two sizes up is almost always worth the money, because loss there is loss on everything.
Conductor cross-section (mm²)
The European way of naming wire size, as the actual area of copper. It converts cleanly to gauge: 6 mm² sits between 10 and 8 AWG, and cable bought on either side of the Atlantic has to be compared this way.
Ampacity
The current a conductor may carry continuously without exceeding its insulation's temperature rating. It falls with heat, bundling and conduit, so a cable rated in open air can be undersized once it is buried in a hot battery box.
Busbar
A solid copper or brass bar that gathers several cable connections at one point. It replaces a stack of ring terminals on a battery post, and a loose post connection is the most common cause of a melted battery terminal.
Shunt
A precision low-value resistor placed in the negative line so a monitor can measure current. It is the only way to know true state of charge, because voltage alone lies badly under load and while charging.
Class T fuse
A fast, high-interrupt-rating fuse designed for the enormous short-circuit current a battery bank can deliver. A lithium bank can push thousands of amps into a dead short, and an automotive fuse will weld rather than clear it.
Overcurrent protective device
The general name for the fuse or breaker that protects a conductor from carrying more than it can handle. It protects the wire, not the appliance, which is why its rating follows the cable size rather than the load.
Grounding electrode
The rod, plate or ring that ties a system to the earth itself. Off-grid sites usually have no utility ground, so the electrode is the whole reference for lightning and fault paths.
Bonding
Connecting metal enclosures and frames together so they sit at the same potential. It is what makes a breaker trip on a fault instead of leaving a panel frame quietly live.
Neutral-ground bond
The single deliberate connection between the neutral conductor and ground in a system. Off-grid setups often end up with two of them or none, and either fault stops residual-current protection from working.
GFCI and RCD
Devices that trip when current returns by any path other than the neutral, sold as GFCI in North America and RCD in Europe. They need a correct neutral-ground bond upstream to sense anything, so an inverter's bonding mode is a safety decision.
Split-phase
The North American 120/240 V arrangement, two 120 V lines in opposition sharing a neutral. Wells, dryers and ranges want the full 240 V, so an off-grid inverter must produce split-phase or be stacked to make it.
Three-phase
Three alternating supplies offset by a third of a cycle, standard for European 400 V service and larger motors. Rural European properties often arrive with three-phase service, and matching it off-grid means three inverters or an accepted downgrade.
Transfer switch
A switch that connects a panel to either the grid or a generator, never both. It is the legal and physical wall that stops a generator backfeeding a line someone else is working on.
Interlock kit
A mechanical plate that prevents the main breaker and a generator breaker being on at once. It does the job of a transfer switch on an existing panel for a fraction of the price, where the panel maker sells one.
Arc fault
A fault where current jumps a gap and sustains a hot arc rather than a clean short. Direct-current arcs do not self-extinguish the way alternating-current ones do, which is why solar strings need arc-fault detection.
Continuous load
A load expected to run for three hours or more, which by convention is derated to 80 percent of a breaker's rating. It is why a 20 A circuit is planned around 16 A, and the same margin is worth keeping on an inverter output.
Glossary: batteries and charging
What a battery actually holds, what the charger is doing in each stage, and where the capacity you paid for quietly goes. Twenty-four terms.
State of charge (SOC)
How full a battery is right now, expressed as a percentage of its rated capacity. Voltage is a poor proxy for it on lithium, whose curve is nearly flat between 20 and 90 percent, so a shunt-based monitor earns its cost.
Depth of discharge (DoD)
How far a battery has been emptied, the mirror image of state of charge. Lead-acid wants 50 percent as a working floor while lithium iron phosphate is comfortable at 80 to 90, which halves the bank you have to buy.
State of health (SOH)
Present capacity as a share of the capacity the battery had when new. A bank at 80 percent health still works but no longer carries the autonomy you sized for, and winter is where you notice.
Usable capacity
Rated capacity multiplied by the depth of discharge you are willing to use. A 200 Ah lead-acid bank offers about 100 Ah of usable charge; a 200 Ah lithium bank offers 160 to 180, which is the only fair comparison.
Nominal voltage
The label voltage of a cell or bank, used for naming rather than measuring. Lithium iron phosphate packs are nominally 12.8 V and rest at 13.4 to 13.6 V when full, so a controller set for a 12 V lead bank will undercharge them.
C-rate
Current expressed as a multiple of capacity, so 0.5C on a 100 Ah battery means 50 A. Datasheets quote capacity at a specific C-rate, and pulling harder than that gets you less energy than the label promises.
Peukert exponent
A number describing how much capacity a lead-acid battery loses as the discharge current rises. At an exponent of 1.25 a hard discharge can cost a quarter of the rated capacity, which is why lead banks are oversized on purpose.
Cycle life
How many charge and discharge cycles a battery delivers before falling to a stated share of original capacity. It is always quoted at a depth of discharge, so compare 3,000 cycles at 80 percent against 500 at 50 percent before comparing prices.
Battery management system (BMS)
The electronics inside a lithium pack that watch cell voltage, current and temperature and disconnect on a limit. It will cut the pack off rather than let it be damaged, so a system must be designed to survive that disconnect happening under load.
Cell balancing
Equalizing the charge of individual cells in a series pack so one does not hit its limit early. An unbalanced pack behaves as though it has the capacity of its weakest cell, which can be most of the bank's apparent aging.
Lithium iron phosphate (LiFePO4)
A lithium chemistry with a nominal 3.2 V cell, charged to about 3.65 V, valued for cycle life and thermal stability. It has become the default off-grid chemistry because it takes deep daily cycling and does not need to be returned to full each day.
Nickel manganese cobalt (NMC)
A denser lithium chemistry with a nominal 3.6 to 3.7 V cell, common in vehicles and compact power stations. It packs more energy into less weight but is less tolerant of heat and abuse, which matters more in a fixed bank than in a backpack.
Absorbed glass mat (AGM)
A sealed lead-acid battery in which the electrolyte is held in a fiberglass mat. It needs no watering and tolerates being mounted anywhere, at the price of a tighter charge-voltage window than a flooded battery.
Flooded lead-acid
The classic vented battery with liquid electrolyte and removable caps. It is the cheapest real storage per kWh and the only chemistry that can be equalized, in exchange for watering, ventilation and a 50 percent floor.
Bulk stage
The first phase of charging, where the charger delivers all the current it has and voltage climbs. It does most of the work; a bank that never leaves bulk before sunset is telling you the array or the controller is undersized.
Absorb
The phase where the charger holds a fixed higher voltage while current tapers, filling the last portion of capacity. Lead-acid needs its full absorb time or it sulfates, so cutting the day short is what actually kills most off-grid lead banks.
Float
A reduced holding voltage that keeps a full battery topped up without overcharging it. Lithium does not need float at all, and leaving a lithium bank parked at a lead-acid float voltage shortens its life for nothing.
Equalize
A deliberate controlled overcharge of a flooded lead-acid bank to stir the electrolyte and reverse stratification. It is a maintenance tool for flooded cells only, and applying it to sealed or lithium batteries damages them.
Temperature compensation
Adjusting charge voltage with battery temperature, upward when cold and downward when hot. A lead bank in an unheated shed is undercharged all winter without it, which looks exactly like a bank that has lost capacity.
Self-discharge
The charge a battery loses while simply sitting. It is a few percent a month for lithium and rather more for lead, so a cabin left over winter needs either a trickle source or a disconnect.
Round-trip efficiency
The share of energy put into a battery that comes back out again. Lithium iron phosphate returns roughly 95 percent and lead-acid nearer 80, and that gap is array you have to buy to make up.
Days of autonomy
How many days the bank can carry the loads with no charging input at all. Two days is the common target and three is a winter target, and it is the setting that moves the battery budget most.
Resting voltage
Terminal voltage measured after the battery has sat with no load or charge for several hours. It is the only voltage reading worth trusting for state of charge, because load sag and charge surface charge both distort a live reading.
Low-voltage disconnect
A relay or controller setting that drops the loads before the battery is damaged. It is the last line of defense for a lead bank: set at the right voltage it stops one deep discharge from costing the whole bank, and set too low it never trips in time.
Glossary: solar
Twenty-two terms for the panel, the string and the controller, including the two nameplate voltages that decide whether an array is safe to wire the way you drew it.
Peak sun hours (PSH)
A day's solar energy expressed as the number of hours it would take at a reference 1,000 watts per square meter. It is the one number that turns a panel's watt rating into a daily kWh figure, and it changes by season far more than most people plan for.
Irradiance
Instantaneous solar power falling on a surface, in watts per square meter. Panels are rated at 1,000 W/m², a bright clear noon, so any other moment produces proportionally less.
Insolation
Solar energy accumulated over a period, in kWh per square meter. Daily insolation and peak sun hours are the same number in different clothes, which is why the terms are used interchangeably.
Standard test conditions (STC)
The laboratory conditions a panel is rated at: 1,000 W/m², a 25°C cell and a defined air mass. No roof ever meets them, so the nameplate watt figure is a ceiling rather than an expectation.
Nominal module operating temperature (NMOT)
A more realistic rating condition, at 800 W/m² and a cell temperature reached in still, warm air. Comparing panels at this figure rather than at standard test conditions tells you more about a hot afternoon.
Open-circuit voltage (Voc)
The voltage a panel produces with nothing connected, the highest voltage it will ever show. It rises as temperature falls, so a string sized on a summer figure can exceed a controller's limit on the coldest clear morning of the year.
Maximum power voltage (Vmp)
The voltage at which a panel delivers its peak power. A controller has to be able to work at this voltage, and the gap between it and open-circuit voltage is where a PWM controller wastes energy.
Short-circuit current (Isc)
The current a panel delivers with its terminals shorted together. Fuses, combiners and controller ratings are all sized from it, with a safety multiplier on top.
Maximum power current (Imp)
The current at the panel's peak power point. It is the figure to size wire against under normal operation, while short-circuit current is the figure for protection.
Temperature coefficient
How much a panel's voltage or power changes per degree of cell temperature. Power typically falls by about 0.3 to 0.4 percent per degree above 25°C, so a hot roof in July can quietly cost a tenth of the output.
Maximum power point tracking (MPPT)
A controller that continuously converts the array's actual voltage down to the battery's, extracting the most available power. It recovers the energy a simpler controller throws away, and it lets you wire panels in series so the array cable can be thin.
Pulse width modulation (PWM)
A simpler controller that connects the array directly to the battery in pulses. It is cheap and reliable but pulls the panel down to battery voltage, losing the difference, so it only suits small matched systems.
String
A group of panels wired in series so their voltages add. Series wiring raises voltage and lowers current, which is how you get a long array run on modest cable, within the controller's voltage limit.
Array
The complete set of panels feeding one controller or inverter. Its total watt rating is the headline number, but the layout of strings is what decides whether shade on one panel costs you one panel or twelve.
Derate
Reducing a theoretical output figure to account for real-world losses. PVWatts Version 5 uses a default total system loss of 14 percent on top of a separately modeled 96 percent inverter, and off-grid systems usually deserve more.
Tilt
The angle of a panel from horizontal. Steeper tilts favor winter sun and shed snow, and a winter-optimized tilt is usually the right choice off-grid because winter is the limiting season.
Azimuth
The compass direction a panel faces. A fifteen-degree error either side of south costs very little, so a roof that is nearly right is usually not worth fighting.
Soiling
Loss from dust, pollen, salt and bird droppings on the glass. It is 2 percent in the PVWatts Version 5 default loss table and far more in a dry, dusty or agricultural setting where months pass without rain.
Clipping
The energy lost when array output exceeds what the inverter or controller can pass. A little clipping at noon is a deliberate and cheap trade, because it buys you more output in the poor light at either end of the day.
DC-to-AC ratio
The array's rated watts divided by the inverter's rated watts. Ratios between 1.1 and 1.3 are standard practice; off-grid arrays are often pushed higher still because winter mornings matter more than summer noon.
Bifacial
A panel that also generates from light reaching its back surface. The gain depends entirely on what is under it, so it rewards a light roof, gravel or snow and does almost nothing over dark ground.
Rapid shutdown
A requirement that conductors on a building be brought to a safe voltage quickly when the system is switched off. It is a firefighter provision written into the North American code, and it shapes what equipment is legal on a permitted roof.
Glossary: inverters, generators and loads
The machines between the battery and the outlet, and the decisions about which circuits get to stay alive. Sixteen terms.
Pure sine wave
An inverter output whose waveform matches the smooth wave a utility supplies. Motors, transformers, medical equipment and modern electronics all want it, and it is no longer expensive enough to be worth avoiding.
Modified sine wave
A stepped approximation of a sine wave produced by cheaper inverters. It makes motors run hot, upsets some chargers and buzzes audibly, so it belongs on tools and lights rather than on a house.
Continuous rating
The power an inverter or generator can deliver indefinitely at a stated temperature. Ratings are often quoted at 25°C, so a unit in a hot closet in August delivers less than the box says.
Surge rating
The much higher power a device can deliver for a few seconds. It is what gets a compressor or pump started, and both the size of the surge and how many seconds it lasts have to be checked.
Idle draw
The power an inverter consumes simply being switched on with nothing connected. At 20 W it is half a kWh a day, which on a small system can be more than the loads it was bought to serve.
Inverter-charger
A single unit that inverts battery to household power and also charges the battery from a generator or grid. It is the heart of most off-grid systems because it can blend battery, generator and solar without a person deciding each time.
Pass-through
The ability to feed incoming generator or grid power straight to the loads while charging the battery. Its amp rating caps what the whole house can draw when the generator is running, independent of the inverter's own rating.
Soft starter
A device that ramps a motor up instead of connecting it across the line. It roughly halves the starting surge, which is usually far cheaper than buying the inverter capacity that surge would otherwise demand.
Variable frequency drive (VFD)
A controller that varies a motor's supply frequency to vary its speed. On a pump or compressor it removes the surge and lets the machine match demand, which suits a battery system very well.
AC coupling
Connecting a grid-tie solar inverter to the alternating-current output of a battery inverter. It reuses existing rooftop equipment, but the battery inverter must be able to throttle the solar inverter or the battery will be overcharged.
DC coupling
Feeding solar into the battery through a charge controller on the direct-current side. It is the more efficient path for a system that spends most of its life charging a battery, which is every off-grid system.
Altitude derate
The output a naturally aspirated engine loses as air thins with height. Generator manuals typically deduct around three percent per 300 meters of elevation, plus a further deduction for high ambient temperature.
Prime and standby ratings
Prime is the power a generator can produce continuously in variable service; standby is a higher figure reserved for occasional emergency use. An off-grid generator lives at the prime rating, so a standby number on the box overstates what it will do all winter.
Specific fuel consumption
Fuel burned per unit of energy produced, usually liters or gallons per kWh. Generators are far thirstier at light load, so running one at a quarter load to keep a fridge alive is the most expensive electricity on the property.
Load shedding
Deliberately switching off lower-priority circuits when supply is short. It is the difference between a controlled winter week and a flat battery, and it can be automated with simple relays.
Critical loads panel
A small subpanel holding only the circuits the backup system will feed. It is the cheapest way to make a house backup-ready, because it lets a modest inverter serve exactly what matters.
Glossary: water and pressure
Twelve terms shared by pumps, tanks and rainwater, where pressure and height are two ways of saying the same thing.
Pounds per square inch (psi)
The North American unit of pressure, one pound of force spread over a square inch. Household plumbing runs at 40 to 60 psi, and every pump and filter on the property carries a rating in it.
Bar
The metric working unit of pressure, defined as exactly 100,000 pascals and very close to one atmosphere. European pump and filter labels use it, and the conversion to psi is near enough to multiply by fourteen and a half in a hardware aisle.
Head
Pressure expressed as the height of a water column that would produce it. It is the natural unit for pumps, because a pump that can lift water 30 meters can also make about 3 bar, and the same arithmetic covers both.
Static head
The vertical distance from the water surface to the point of delivery, with nothing flowing. It sets the minimum a pump must overcome before delivering a single drop, and it does not change with pipe size.
Friction loss
Additional head lost to pipe walls, bends and fittings while water is moving. It rises steeply with flow and falls steeply with pipe diameter, so one size larger in pipe often saves more than a larger pump would.
Gallons per minute and liters per minute
The rate at which a pump or fixture delivers water. Multiplied by the daily minutes of running it gives the tank size, and divided into a household's daily use it gives the pump's runtime.
Drawdown
The usable water a pressure tank delivers between the pump switching off and switching on again. It is far less than the tank's total volume, and a larger drawdown means fewer pump starts, which is what actually wears a pump out.
Cut-in and cut-out pressure
The pressures at which a pressure switch starts and stops the pump, commonly 30 and 50 psi. Widening the gap increases drawdown and reduces starts; the tank's air charge has to be set to match the cut-in figure.
Submersible pump
A pump and motor sealed as one unit and lowered into the well below the water level. It pushes rather than sucks, which is why it handles deep wells, and it is also why a start surge lands directly on the inverter.
Pump curve
The manufacturer's chart of flow against head for a given pump. Reading the duty point off the curve, rather than trusting the headline flow figure, is what stops a pump being bought a size too small.
Net positive suction head
The margin of pressure available at a pump inlet above the point where water would boil into vapor. Get it wrong on a surface pump and the pump cavitates, which sounds like gravel and destroys the impeller.
Runoff coefficient
The share of rain landing on a roof that actually reaches the tank. Around 0.8 to 0.9 for a metal roof and less for rough or absorbent surfaces, and it is the factor most catchment estimates forget.
Glossary: heat, fuel and buildings
Heating brings its own units, and an American and a European label can describe the same fuel with different arithmetic. Ten terms for that ground.
British thermal unit (BTU)
The heat needed to raise one pound of water by one degree Fahrenheit, defined internationally as 1,055.06 joules. It is the unit stoves, air conditioners and fuels are labeled in across North America, and 3,412 of them make one kWh.
Megajoule (MJ)
One million joules, the metric unit European fuel and appliance data uses. One MJ is 0.2778 kWh, so a gas figure in MJ divides by 3.6 to become the kWh everything else on the site is counted in.
Therm
One hundred thousand BTU, the unit natural gas is billed in across the United States. One therm is about 29.3 kWh, which is the conversion that lets a gas bill be compared with a heat pump's electricity.
Cord
A stack of firewood measuring 128 cubic feet, or 3.6246 cubic meters. It is the only firewood measure with a fixed definition, which is why a price quoted per truckload or per rick cannot be compared with anything.
Stere
One stacked cubic meter of firewood, the European trade measure. A cord is 3.62 steres, and a loose cubic meter of chips holds far less wood again, so ask which measure a price refers to.
Higher and lower heating value
Two ways of counting a fuel's energy: the higher value includes the heat recovered by condensing the water vapor, the lower value does not. American tables usually quote the higher value and European tables the lower, which makes the same fuel look about ten percent different.
Coefficient of performance (COP)
Heat delivered divided by electricity consumed, for a heat pump. A COP of 3 means a kWh of electricity moves 3 kWh of heat, which is what makes a heat pump plausible on a battery at all.
Seasonal heating efficiency (HSPF and SCOP)
Season-long averages of heat pump performance, HSPF in BTU per watt-hour in North America and SCOP as a plain ratio in Europe. Seasonal figures include the cold hours when performance collapses, so they predict a winter far better than a headline COP.
Degree day
A measure of how cold a period was, summing the daily gap between outdoor temperature and a reference. It is the fair way to compare one winter to another, but the reference differs by region, so an American and a European figure cannot be swapped.
R-value and U-value
Two inverses of the same property: R-value measures resistance to heat flow, U-value measures how readily heat passes. Insulation is the cheapest kilowatt-hour on any off-grid site, because a watt-hour never needed is one you never have to store.
Sources
- Every exact conversion factor. NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.8 and B.9; the exact 1,055.055 852 62 J international table BTU is footnote 9 of the same guide.
- Solar loss assumptions. NREL, PVWatts Version 5 Manual (Dobos, September 2014, NREL/TP-6A20-62641): Table 6 default total system loss 14 percent, default nominal inverter efficiency 96 percent.
- North American voltage. ANSI C84.1, Voltage Ratings (60 Hz): Range A service voltage 114 to 126 V on a nominal 120 V system.
- European voltage. IEC 60038 standard voltages: 230/400 V at 50 Hz, the harmonized replacement for 220/380 V and 240/415 V.
- Battery charging terms. Trojan Battery maintenance documentation for the bulk, absorption, float and equalize stages and its “50 percent or less” discharge guidance; published LiFePO4 cell datasheets for the 3.2 V nominal and 3.65 V cut-off.
- Peak sun hours. Defined against the 1,000 W/m² reference irradiance used by the National Solar Radiation Database.
Factors re-checked against NIST SP 811 in September 2026. They are definitions, so they will not move; the loss and voltage standards are the parts worth revisiting.
Go deeper
Units are the vocabulary; sizing is the sentence. Put your loads through the can I run my house off-grid calculator, read the voltages off the LiFePO4 voltage chart, size the copper with the DC wire size calculator, and follow the sequence in how to size an off-grid solar system and inverter and charger sizing. Heat units get their workout in the firewood BTU chart. More in the tools index.