Off-Grid Power System Design in 7 Steps (and Where Every Number Comes From)

Most off-grid systems fail on a spreadsheet, before a single panel goes on the roof.

The usual sequence: get excited, buy solar panels, then discover the battery big enough to store all that power costs more than the panels did. The inverter faults every time the well pump kicks on. The county wants a permit for a system that, by definition, was never supposed to touch anyone’s grid.

None of that is a hardware problem. It’s a sequencing problem. The seven steps below are the order off-grid designers actually work in — each step’s output becomes the next step’s input — with the formula behind every number and a calculator wired to the ones worth automating.

You can spot a system that skipped the sequence from across the yard:

• The battery bank costs more than the array it’s supposed to charge
• The inverter faults every time the well pump or a table saw starts
• A generator nobody budgeted for is doing half the work by month two
• The permit office wants paperwork for a system that “isn’t connected to anything”

Run the seven steps in order and none of that happens to you.

The 7 steps, at a glance

Each step produces exactly one number the next step needs — work them in order and the sizing above never surprises the budget below.

Step	Solving for	Core formula	Tool
1. Load audit	Daily energy demand	Σ(watts × hrs/day) × 1.25	Load calculator
2. Array	Panels to recharge that load	kWh ÷ peak sun hours ÷ 0.77	Solar sizing calculator
3. Battery	Storage for your worst stretch	kWh × autonomy days ÷ DoD	Battery bank calculator
4. Inverter	What can run at once, and start	Continuous = simultaneous watts; surge = 3–6× largest motor	Bundled into the solar sizing result
5. Generator	Backup past the battery’s autonomy	Battery kWh ÷ recharge hrs ÷ 0.85	Backup power calculator
6. Water	Storage matching your power autonomy	Gal/person/day × people × autonomy days	Water calculator
7. Permitting	Whether you’re allowed to build it	Check before you buy, not after	Get matched with an installerSee the recommended gear

Step 1 — The load audit

Everything below is downstream of one number: how many kilowatt-hours you actually burn in a day. Not the spec sheet, not “average household” — your loads, in your building, added up by hand.

List every load with its running watts and real hours of use per day:

• Fridge/freezer (compressor duty cycle): ~60 W average × 24 h = 1,440 Wh
• LED lighting: ~100 W × 5 h = 500 Wh
• Well pump: ~750 W × 0.5 h = 375 Wh
• Laptop, phone charging, router: ~60 W × 10 h = 600 Wh
• Kitchen and misc plug loads: ~400 W × 1.5 h = 600 Wh

That’s 3,515 Wh — call it 3.5 kWh of real daily demand. Now add the number every DIY spreadsheet forgets: system losses. Inverter conversion, battery round-trip efficiency, and wiring resistance eat roughly 20–25% between the battery and the outlet, so multiply by 1.25. 3.5 kWh becomes 4.4 kWh — the real design target for every step that follows, not the number on the appliance tags.

Do this by hand once, then let the load calculator run it for your actual house — or go deeper with the full load-audit method for the loads a worksheet misses entirely.

Step 2 — Sizing the array

Here’s the part almost everyone gets backward: the array isn’t sized to your load. It’s sized to recharge the battery within the sun hours you actually get — a bigger number.

Formula: array kW (DC) = daily kWh ÷ peak sun hours ÷ 0.77. Peak sun hours (PSH) isn’t daylight — it’s equivalent hours of full 1,000 W/m² sun, and it swings hard by season and latitude: roughly 3–4 across much of northern Europe or the US Northeast in winter, 5–6 across the US Sunbelt or southern Spain in summer. Design to your worst month — the array has to carry you through January, not July.

At a conservative 4 PSH: 4.4 ÷ 4 ÷ 0.77 ≈ 1.43 kW DC — four to five 400 W panels with a little headroom. A whole-home load of 30–40 kWh/day runs the identical formula into an 8–13 kW array; only the inputs change.

The solar sizing calculator runs this against your location’s sun-hour data in one pass — or walk the full method in how to size an off-grid solar system.

Step 3 — The battery bank

This is where off-grid sizing stops resembling grid-tied solar. A grid-tied system can size to its annual average, because the utility is the battery — it absorbs summer surplus and returns it in winter. Off-grid has no utility to lean on. It has to survive its worst stretch alone.

Formula: usable kWh = daily kWh × autonomy days, then nameplate kWh = usable kWh ÷ DoD. For the 4.4 kWh/day cabin at 3 days: 13.2 kWh usable. In LiFePO4, commonly cycled to ~90% DoD, that’s about 14.7 kWh nameplate — roughly 306 Ah at 48V. Size the same 13.2 kWh usable in lead-acid (50% DoD, the conservative convention for cycle life) and the nameplate bank doubles to 26.4 kWh. Same autonomy, twice the boxes — most of why LiFePO4 won the chemistry argument despite costing more per kWh up front.

Run your own loads and autonomy target through the battery bank calculator — it compares chemistries side by side on your numbers.

Step 4 — Inverter and charge controller

Two different numbers hide inside “inverter sizing,” and conflating them is the single most common off-grid wiring mistake.

Undersize the surge rating and the inverter faults every time the pump starts — which reads as a broken inverter, and isn’t one. It’s correctly sized for the wrong number.

In the US, whole-home systems commonly stack two inverter/chargers for 120/240V split-phase output; in Europe, single-phase 230V is standard, and larger systems more often add one bigger unit or true three-phase instead. Either way, buy to the surge figure — the full inverter sizing guide walks all-in-one vs. stacked in both formats.

Step 5 — Generator or backup charge source

The battery in Step 3 is sized to your worst realistic stretch — three days, in the example. The generator’s job is everything past that: the fourth day, the week someone left a load running, the February worse than the design year.

Size it to bulk-recharge the battery within a runtime you’ll actually sit through, plus whatever runs while it’s going: generator kW ≈ (battery nameplate kWh ÷ recharge hours ÷ 0.85) + concurrent loads. Recharging the 14.7 kWh bank from Step 3 in four hours: 14.7 ÷ 4 ÷ 0.85 ≈ 4.3 kW — a 5–6 kW dual-fuel or propane unit covers it with room for the fridge and lights to keep running. Whole-home systems scale to 10–20 kW standby units, sized the same way against a bigger battery.

This is also where the two backup philosophies split: a generator you start on demand, or a second battery bank that starts itself. Both are legitimate — the backup power calculator sizes either path from your Step 1 numbers, and this winter-top-up guide covers propane/dual-fuel sizing.

Step 6 — Water storage

Going fully self-sufficient means water needs the same worst-stretch logic as the battery — easy to miss, because the well pump already showed up once, in Step 1, as an electrical load. Here it has a second job: sizing how much you store, not just how much you draw.

Formula: storage target = daily use per person × people × autonomy days. A conserving off-grid household typically runs 40–60 US gallons (150–230 liters) per person per day — well under the 80–100 gallons (300–380 liters) a mains-fed US household uses, because every off-grid gallon was pumped by a battery someone is watching. For three people at 50 gal/day and the same 3-day autonomy as the battery: 450 gallons, roughly 1,700 liters, so conservation mode — running the pump less to save charge — never means running dry.

Run household size, fixtures, and roof catchment area through the water calculator for an actual tank size instead of a rule of thumb.

Step 7 — Permitting and interconnection

This step belongs first on the calendar, even though it’s last in the sequence — check it before you spend, not after.

“Off-grid” describes the electrical topology, not the paperwork. In the US, the work typically falls under NEC Article 690 (PV systems) and Article 710 (stand-alone systems specifically), with sign-off from your local AHJ regardless of whether anything ties back to the utility. In Europe, the equivalent sits in your national wiring code — DIN VDE 0100 in Germany, NF C 15-100 in France, and their counterparts elsewhere, all harmonized to the shared IEC 60364 root standard. Add a well permit and septic setback if you’re building on bare land.

The gotcha that actually derails projects is rarely electrical — it’s zoning that quietly bars an “unoccupied” or non-conventional dwelling from existing off-grid at all, or a county requiring a minimum square footage or a grid-connection stub regardless. None of that shows up while you’re shopping for panels — it shows up at the counter, after you’ve already bought them.

Check it first — the full permitting & interconnection checklist walks the US and European paperwork side by side, then get matched with a permit-savvy installer before you order hardware.

Why the order matters more than any single number

Grid-tied solar is an averaging problem — oversize a little, undersize a little, the utility nets it out over the year. Off-grid has no utility to lean on, so every step above is quietly answering one question: what happens on your worst stretch, not your best day.

And the sequence isn’t a one-way pipe. It’s a loop you run two or three times before anything gets ordered. If Step 3 prices the battery $9,000 over budget, the fix usually isn’t a smaller battery — it’s back to Step 1 to cut a load (electric water heater to propane, say) so every downstream number shrinks with it. Buy hardware before looping through once, and you’re the household whose battery costs more than its array.