Off-Grid Solar Calculator

An off-grid system has no utility to fill an energy shortfall, so it must be sized around daily loads, seasonal sun, storage, and a backup plan. This calculator gives a transparent first pass for cabins and RVs using the critical design-day assumptions you enter.

Daily energy is the sum of appliance watts multiplied by hours, divided by 1,000. Array size divides that load by peak sun hours and system efficiency to allow for inverter, wiring, temperature, and charging losses.

Battery energy multiplies daily demand by autonomy days and divides by usable depth of discharge. Peak inverter power is a separate constraint: motors and compressors can surge above their running watts.

Use the lowest realistic seasonal sun-hours value, not an annual average, when the system must work year-round. Generator backup or load shedding can be more economical than oversizing for a few dark days.

Assumptions and edge cases

Peak sun hours is a normalization, not a clock reading: it expresses daily irradiation in kilowatt-hours per square meter, so 4 peak sun hours means the array gathers what it would in 4 hours at the 1,000 W per square meter test condition. The efficiency percentage is one lumped derate covering inverter and charge-controller conversion, round-trip battery losses, wiring, soiling, and high-temperature output reduction, so derating the battery again double-counts. The model assumes daily energy balance on the design day and cannot represent a system that leans on a generator in the worst week. Zero sun hours is rejected rather than returning an infinite array.

Worked example

A cabin using 5 kWh/day with 4 peak sun hours and 80% system efficiency needs about 1.56 kW of array. Two days of autonomy at 80% usable depth requires 12.5 kWh nominal storage.

Second worked example

A second case sizes an RV for winter travel. The rig draws 1.8 kWh a day: 0.9 kWh for a 12 V refrigerator, 0.4 for lighting and electronics, 0.5 for a pump and fan. In December near 35° north, 3 peak sun hours is realistic for a flat roof. Array size is 1.8 ÷ (3 × 0.8) = 0.75 kW, or two 400 W panels. One day of autonomy at 80 percent usable depth needs 2.25 kWh nominal, about 190 Ah at 12 V. Sized on a 5.5-hour annual average the same rig carries 0.41 kW and runs short every winter.

How to read the result

Round up to available module and battery sizes, then have overcurrent protection, grounding, conductor ampacity, and equipment listings reviewed for the installation.

Test the array against available space: at roughly 400 W per 21 square feet, 0.75 kW needs about 40 square feet and 5 kW about 260. If the array exceeds the area you have, the load list must shrink first. Autonomy beyond three days usually costs more than a small generator and fuel.

Planning inputs are deliberately editable because two homes in the same state can have different tariffs, roof conditions, equipment, and operating schedules. Save a scenario with “Copy link,” then change one assumption at a time. This makes the result useful for comparing decisions without implying false precision.

Formulas and defaults are documented in our data method. The principal references for this page are:

1. NREL — PVWatts Calculator documentation (2025) — Production modeling, array orientation, and system-loss context.
2. EIA — Electric Power Monthly, Table 5.6.A (2024 annual) — Residential electricity price benchmarks by state.
3. U.S. Department of Energy — Homeowner’s Guide to the Federal Tax Credit for Solar Photovoltaics (accessed methodology, 2025) — Background only; 2026 eligibility is never assumed.
4. NREL — PVWatts Version 8 documentation and the National Solar Radiation Database (2025) — Monthly peak sun hours and the losses folded into system efficiency.
5. NFPA 70, National Electrical Code (2023 edition) — Article 690 and Article 710, Stand-Alone Systems — Stand-alone requirements, overcurrent protection, and listing.

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