How this is calculated
Start with stored energy. Amp-hours times nominal voltage gives watt-hours: 100 Ah at 12V is 1,200 Wh on paper, the same 100 Ah at 48V is 4,800 Wh. Amp-hours alone are not a quantity of energy.
Then take away what you are not allowed to spend. Lithium iron phosphate is worked to 80–90 percent of nameplate. Lead-acid stops at 50 percent: Morningstar’s design guidance puts a true deep-cycle battery at a 50 percent daily discharge, with 80 percent survivable but life-shortening if you make a habit of it.
Then look at what the load costs. Victron’s Phoenix range peaks between 87 and 92 percent efficient and does worse at light load, so at 88 percent a 55W appliance asks for about 62W of direct current, and the inverter burns several watts more simply being switched on. Usable watt-hours divided by that true draw gives hours.
Worked example, the default preset: 100 Ah at 12V is 1,200 Wh, of which 1,080 Wh are usable at 90 percent. A 55W refrigerator behind an 88-percent inverter idling at 8W pulls 55 ÷ 0.88 + 8 = 70.5W, or 5.9 amps. So 1,080 ÷ 70.5 is about 15 hours, and only 836 Wh of the original 1,200 ever reaches the cold box.
Quick answers
How long will a 100Ah battery run a fridge?
Roughly 12–18 hours in LiFePO4, against a full-size refrigerator averaging 45–70W around the clock. The same 100 Ah in lead-acid gives 6–10 hours, because half the nameplate stays off limits.
How do you convert amp-hours to watt-hours?
Multiply amp-hours by nominal voltage: 100 Ah at 12V is 1,200 Wh, at 48V it is 4,800 Wh. Comparing amp-hours across voltages is meaningless.
Why does the battery empty faster than the math says?
Four costs stack: the discharge limit, inverter conversion loss, idle draw, and the rate penalty on lead-acid. Together they cut a naive estimate by a third.
What is Peukert's law?
A lead-acid battery yields less total capacity the harder you pull. Victron's SmartShunt manual gives the case: a battery rated 100 Ah at the 20-hour rate may deliver only 56 Ah when it is emptied in two hours.
How many amp-hours do I need for a set number of hours?
Multiply the battery-side watts by the hours you want, then divide by nominal voltage and by usable depth of discharge. The reverse field above does it.
Runtime table: common batteries against common loads
Hours of runtime for 12V LiFePO4 at 90 percent depth of discharge through an 88-percent inverter idling at 8W, with Victron’s lithium Peukert exponent of 1.05 applied. Read down to your battery, across to your load.
| Battery | 10W | 25W | 50W | 100W | 200W | 400W | 800W |
|---|---|---|---|---|---|---|---|
| 50 Ah (600 Wh) | 27.9 h | 14.7 h | 8.0 h | 4.1 h | 2.1 h | 1.0 h | 0.5 h |
| 100 Ah (1.2 kWh) | 55.8 h | 29.7 h | 16.6 h | 8.6 h | 4.3 h | 2.1 h | 1.0 h |
| 200 Ah (2.4 kWh) | 111.5 h | 59.3 h | 33.3 h | 17.7 h | 8.9 h | 4.4 h | 2.1 h |
| 300 Ah (3.6 kWh) | 167.3 h | 89.0 h | 50.0 h | 26.6 h | 13.6 h | 6.7 h | 3.3 h |
| 400 Ah (4.8 kWh) | 223.1 h | 118.7 h | 66.6 h | 35.5 h | 18.4 h | 9.0 h | 4.4 h |
Above about 1,000W the 12V wiring and the inverter become the limit before the battery does — the point where a 24V or 48V bank stops being optional.
Chemistry changes the same row completely. All three lines below are the same 100 Ah at 12V:
| Chemistry | Depth | Usable | 25W | 100W | 400W | 800W |
|---|---|---|---|---|---|---|
| LiFePO4 | 90% | 1,080 Wh | 29.7 h | 8.6 h | 2.1 h | 1.0 h |
| Lithium NMC | 85% | 1,020 Wh | 28.0 h | 8.1 h | 2.0 h | 1.0 h |
| Lead-acid (AGM, gel or flooded) | 50% | 600 Wh | 16.5 h | 4.1 h | 0.8 h | 0.3 h |
Lead-acid falls away fastest at the right-hand end because the rate penalty compounds on top of the shallower discharge limit. At 25W it keeps 55 percent of the LiFePO4 runtime; at 800W, 30 percent.
What common loads actually draw
The arithmetic is only as good as the wattage you feed it. Take yours off the appliance label or read it on a plug-in meter for a day.
| Load | Draw to use | Where the figure comes from |
|---|---|---|
| Full-size refrigerator | 45–70W averaged over 24 h | Annual kWh on the appliance label divided by 8,760 hours. ENERGY STAR Most Efficient 2025 recognition (released September 2024) caps a standard-size refrigerator-freezer at 637 kWh a year, or 72.7W averaged |
| Chest freezer | ~25W averaged over 24 h | ENERGY STAR product page: a certified chest freezer uses about 215 kWh a year, or 24.5W averaged |
| Upright freezer | ~45W averaged over 24 h | ENERGY STAR product page: about 395 kWh a year, or 45.1W averaged |
| CPAP, humidifier off | 11W (0.93 A at 12V) | ResMed Battery Guide 198103/7 (2018), AirSense 10 with SlimLine tubing at 10 cmH2O |
| CPAP, heated humidifier and heated tube | up to 59W (4.92 A at 12V) | Same guide, ClimateLineAir tubing at 30°C with the humidifier at setting 8, 10 cmH2O |
| 55-inch television | ~92W while on | Department of Energy FEMP purchasing guidance: a 54.5-inch ENERGY STAR set is rated 168 kWh a year over 1,825 active-mode hours |
| Router and modem | 5–15W continuous | ENERGY STAR Small Network Equipment V1.0 (Rev. Nov-2013) sets base idle allowances of 3.1W for a router and 4.0–5.7W for a modem, before functional adders |
| 12V on-demand water pump | up to 90W while running, minutes a day | SHURflo Revolution 4008, 3.0 GPM at 55 psi, 7.5 A maximum at 12V |
| LED lighting | 5–10W per fixture | The lamp's own rating |
Peukert's law and why lead-acid shrinks under pressure
A lead-acid battery is rated at a gentle drain, normally over 20 hours. Pull harder and the acid cannot reach into the plates fast enough, so the energy you get out falls. Victron’s SmartShunt manual sets the default Peukert exponent at 1.25 and gives the blunt case: a battery rated 100 Ah at the 20-hour rate, C20, may deliver only 56 Ah when it is emptied in two.
The same manual tells you to set the exponent to 1.05 for a lithium battery, and those two numbers are what this calculator uses: 1.25 for any lead-acid, 1.05 for LiFePO4 and NMC. Your own battery’s datasheet may publish a different exponent, and it wins over the default. The rate-derating line under the result shows how much the correction is costing you at the draw you entered.
Inverter idle draw and conversion loss
An inverter is a load in its own right. Victron’s Phoenix VE.Direct datasheet, covering 250VA to 1600VA, lists zero-load consumption of 4.2W on the 12V 250VA unit rising to 10–12W across the 1200VA models, with maximum efficiency between 87 and 92 percent. Eco mode drops the same idle figure to 0.8–3W by shutting the output down until something asks for power.
Idle draw is trivial next to a kettle and ruinous next to a router. Ten watts of standby against a 10W load doubles consumption, and over a day that is 240 Wh gone to nothing. Small direct-current loads are worth running straight off the battery where the device allows it.
The overnight test. Leave the inverter on with nothing plugged in and read the battery monitor after eight hours. That is the tax you pay before a single appliance runs.
Temperature, age and the rest of the real world
Batteries are rated warm. Morningstar’s design guidance for lead-acid banks puts capacity down 20–25 percent at 32°F and at roughly 50 percent of rated capacity at 0°F. A winter cabin running on lead-acid is working with half the battery it paid for.
Lithium has the opposite problem, and it is about charging rather than running. Victron specifies its LiFePO4 Smart batteries for discharge from −20°C but for charging only between 5°C and 50°C, because charging colder than that plates lithium onto the anode and takes capacity away permanently. In an unheated space that limit bites before any runtime figure does.
Age takes its own cut. A cycling bank is conventionally called end-of-life at 80 percent of original capacity, so plan a well-used 100 Ah battery as an 80 Ah one and let the calculator work from the smaller number.
US edition
Loads run at 120V, so a 1,500W inverter draws about 12.5 A on the output side and about 142 A from a 12V battery at 88 percent efficiency. Appliance energy comes off the yellow EnergyGuide label in kWh a year; divide by 8,760 for average watts.
Europe edition
Loads run at 230V, so the same 1,500W inverter draws about 6.5 A on the output side while still pulling about 142 A from a 12V battery. Appliance energy comes off the European energy label in kWh a year; divide by 8,760 for average watts.
What each number in the result is for
The amps figure sizes your cable and fuse, and it is why 12V systems stop scaling: an 800W load behind an 88-percent inverter pulls about 76 amps continuous from a 12V bank, which means proper battery cable and a fuse rated to match.
The reverse figure is the buying decision. On the default preset a 24-hour target returns 157 Ah, so one 100 Ah battery was never going to do it. Carry that number into the off-grid battery bank calculator, which turns a single load into a whole system with autonomy days and recharge sized around it.
- Use average watts for cycling loads such as refrigerators; running watts for anything always on.
- Count the inverter idle draw once for the system, never per appliance.
- Derate an aging lead-acid bank to 80 percent of nameplate first.
Sources: Victron Energy SmartShunt manual (battery capacity and Peukert exponent), Phoenix Inverter VE.Direct 250VA–1600VA datasheet and Lithium Battery Smart manual; ResMed Battery Guide 198103/7, 2018; ENERGY STAR freezer product page and ENERGY STAR Most Efficient 2025 Consumer Refrigeration Products criteria, released September 2024; ENERGY STAR Small Network Equipment V1.0, Rev. Nov-2013; US Department of Energy FEMP television purchasing guidance; Morningstar design factors for lead-acid batteries; SHURflo Revolution 4008 pump specification. Figures checked September 2026.
Go deeper. Size a whole bank with the off-grid battery bank calculator, weigh the chemistries in LiFePO4 against lead-acid and LiFePO4 or NMC in a power station, and pin down wattages with the load audit worksheet. Then the two loads people ask about most: a CPAP off-grid and a mini fridge off-grid.