Off grid solar calculator
Size the array on your dimmest month and the battery for days without sun. Your bill is read in the browser and never uploaded.
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Why off-grid is a different calculation
A grid-tied system can average out the year because the grid covers winter shortfalls and absorbs summer surplus. An off-grid system has no such partner. Every day has to work, so the sizing starts from the worst month of sun and from a battery that bridges cloudy days.
The formulas, with the constants
- array kWdc = annual kWh / 365 / (worst-month peak sun hours x 0.75 derate)
- battery kWh = daily kWh x 2 days of autonomy / 0.9 depth of discharge
Worst-month peak sun hours per day come from NREL PVWatts v8 runs, compiled by TheGreenWatt and checked on 2026-11-03. The 0.75 derate allows for wiring, charge controller and battery losses. These are planning values; a designer will refine them for your exact location, tilt and loads.
A worked example
The average Montana home uses 852 kWh a month (EIA, 2024 residential average), or 28 kWh a day. Montana's worst month gives 2.18 peak sun hours a day in our table. Sized off-grid, that is 17.1 kWdc of panels and 62 kWh of battery. The same usage on a grid-tied system would need only 7.6 kWdc. The difference is the price of independence.
Cut your load before you size
Off-grid owners almost always reduce demand first, because each kilowatt-hour avoided saves array, battery and money. Swap resistance heating for a heat pump or propane, use efficient appliances and shift heavy loads to daytime. Even a cabin that uses a tenth of an average home's power needs a very different, much smaller system.
What this does not cover
The tool does not size the charge controller, wire gauge, fuses or inverter surge rating, and it does not check that the battery can deliver your peak power. Those are safety-critical choices that depend on voltage and equipment. Use this to scope a budget and a parts shortlist, then have a qualified designer or electrician check the system. We may earn a commission on parts links; see the affiliate disclosure.
Planning estimate. Permits, interconnection approval and a licensed electrician are required for grid-tied systems.
Questions
Why does off-grid size to the worst month?
With no grid to fall back on, the array has to cover the dimmest month. If you size for the yearly average, you run short every winter.
What is autonomy?
Autonomy is how many cloudy days the battery can carry you without any sun. The tool uses two days, which is a planning figure; stormy climates may need more.
Why is an off-grid array so much bigger than a grid-tied one?
It sizes to winter sun instead of the yearly average, and it applies a 0.75 derate for charging and conversion losses. In low-sun states the array can be several times larger.
Do I need a generator as well?
Many off-grid homes keep one as a backstop for long cloudy spells, because adding panels and batteries to cover the rare worst case costs more than a few hours of fuel.
Is off-grid legal everywhere?
Rules vary by county and state, including permits, inspections and sometimes whether a home can be fully disconnected from the grid. Check local requirements and use a licensed electrician.