How to size an off-grid solar system, step by step

01Build the load budget in watt-hours per day

List every device: its watts and its realistic hours per day. Multiply, then sum. This number — daily watt-hours — drives every other size in the system, so a bad guess here echoes through the whole build.

Load (Wh/day) = Σ (watts × hours), then multiply by ≈1.25 for inverter and wiring losses. A 60 W fridge compressor running 8 h is 480 Wh; a 10 W LED strip for 5 h is 50 Wh. Duty cycle matters more than nameplate.

02Decide days of autonomy

How many sunless days must the system ride through on battery alone? One is optimistic camping; two to three is the usual cabin answer; critical loads (a well pump, a freezer full of food) argue for more, or for a generator as the backstop.

Autonomy is a lifestyle-and-risk decision, not physics — but it multiplies straight into battery cost, so decide it consciously and write it in the spec with its reasoning.

03Size the battery bank through depth of discharge

Batteries can't give you their whole nameplate. Lithium (LiFePO4) comfortably yields ~80% of rated capacity; flooded lead-acid only ~50% if you want it to live a full life.

Battery (Wh) = daily load × autonomy days ÷ DoD. 3 kWh/day × 2 days ÷ 0.8 = 7.5 kWh of lithium — at 24 V that is ≈ 310 Ah.

04Size the array from your worst month's sun

“Peak sun hours” compresses your local solar resource into equivalent full-power hours per day. It ranges from ~2 (northern winter) to ~6+ (desert summer). Off-grid systems get sized for the worst month you intend to occupy — a system sized on July carries you into a dark October.

Array (W) = daily load ÷ (peak sun hours × derate), derate ≈ 0.75 for temperature, dust, and wiring. 3000 Wh ÷ (5.5 h × 0.75) ≈ 730 W — call it 800 W of panels.

05Pick the inverter for surge, not just running watts

Sum the loads that genuinely run at once for the continuous rating — then check surge. Anything with a motor (well pump, fridge compressor, power tools) draws roughly 3× its running watts for the first moments.

A 750 W well pump needs ~2250 W of surge headroom. An inverter that only clears the continuous number stalls the pump and trips — the classic “worked until the pump kicked in” failure.

06Choose the system voltage by power (this decides your copper)

Current = watts ÷ volts, and cable size follows current. At 12 V, 1500 W is 125 A — welding-cable territory, expensive and lossy. The working rule: up to ~800–1000 W, 12 V is fine; 1–3 kW wants 24 V; above ~3 kW, 48 V.

Higher system voltage also means cheaper charge controllers per watt (current is what they're rated in). This one choice, made early, sets the cost of every conductor in the system.

07Write it to code, and defer the rest by name

NEC Article 690 covers PV systems: disconnects, overcurrent protection, grounding, rapid shutdown. Even where no inspector will ever visit, your insurance carrier assumes a code-compliant install.

Leave the vendor/installer their craft — exact panel model, controller brand, combiner layout — as named, deferred items. The spec's job is the numbers above plus environment (temperature range matters: batteries hate the cold) and the code reference.

Worked example: the off-grid cabin

The whole chain, numbers shown — the same derivations the SpecBuildr interview performs and prints in your spec's math appendix.

Loadsfridge 480 Wh + well pump 400 Wh + lights 150 Wh + laptop/misc 370 Wh + losses → ≈ 3000 Wh/day

Autonomy2 days (freezer contents at risk → not less)

Battery3000 × 2 ÷ 0.8 = 7.5 kWh LiFePO4 → ≈ 310 Ah @ 24 V

Peak sun (worst occupied month)5.5 h (September, mid-latitude)

Array3000 ÷ (5.5 × 0.75) ≈ 730 W → spec 800 W

Inverter1200 W continuous, ≥ 2500 W surge (750 W pump × 3)

System voltage24 V (1.2 kW class — 12 V would mean >100 A cabling)

CodeNEC 690: DC disconnect, OCP at the battery, grounding per 690.43

The mistakes that make quotes expensive

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Sizing the array on average annual sun

The system dies in the worst month you actually occupy. Size for that month or plan the generator honestly.

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Counting nameplate battery capacity as usable

Divide by depth of discharge or buy the outage: 50% (lead) to 80% (lithium) is what you really get.

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Ignoring motor surge

The inverter that runs everything fine trips the moment the pump starts. Multiply motor loads by ~3 for surge.

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Building a big system at 12 V

Above ~1 kW, 12 V current makes cables the most expensive metal on site. Move up to 24/48 V.

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Cold batteries, warm assumptions

Capacity drops and charging rules change below freezing — the battery box needs the same design attention as the array.

Frequently asked questions

How many solar panels do I need for an off-grid cabin?

Work backwards from daily watt-hours: array watts = daily load ÷ (worst-month peak sun hours × 0.75). A 3 kWh/day cabin at 5.5 sun hours needs ~730 W — three to four 250 W panels, rounded up.

What battery size do I need for two days of autonomy?

Battery Wh = daily load × autonomy days ÷ depth of discharge. At 3 kWh/day and lithium's 0.8 DoD, two days needs 7.5 kWh — about 310 Ah at 24 V.

Should my off-grid system be 12 V, 24 V, or 48 V?

By power: under ~1 kW, 12 V works; 1–3 kW belongs at 24 V; above that, 48 V. Current (= watts ÷ volts) sets cable size and controller cost, so higher voltage saves real money as systems grow.

How big should the inverter be?

Continuous rating ≥ the loads that truly run simultaneously, and surge rating ≥ 3× the largest motor load. The surge number is the one people miss.

Does an off-grid system need to meet electrical code?

NEC Article 690 applies to PV installs whether or not an inspector visits, and insurers assume compliance. Disconnects, overcurrent protection, and grounding are the non-negotiable core.

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