Land, water & building research
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Off-grid solar calculator

By Published October 10, 2026

Turn a household load list into first-pass numbers for daily energy, battery storage and array size. Every assumption is visible and editable, so you can test what really drives the design.

This calculator uses the same arithmetic as our off-grid solar planning guide. It is a planning aid for a conversation with a qualified designer, not an equipment specification. The example rows reproduce the guide’s illustrative load list; replace them with your own measurements.

1. List what the home must run

Enter the operating watts, hours used per day and quantity for each load. For a refrigerator or other cycling appliance, enter its measured daily energy as the watts with 1 hour, rather than multiplying the nameplate rating by 24.

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2. Set the design assumptions

Daily energy–Sum of the load list
If everything ran at once–Watts × qty. Overstated for loads entered as daily energy; excludes motor surge.
Nominal battery capacity––
Array size–To replace one day’s use in the worst month

The formulas behind each number

Daily energy (Wh) = Σ watts × hours × quantity. Divide by 1,000 for kWh. This is the figure batteries and panels are sized against, so the largest errors in any design usually start here: a forgotten well pump, an electric water heater or a winter heating fan can dwarf every other row.

Simultaneous load (W) = Σ watts × quantity. It is a rough ceiling for inverter discussion, not a selection. Motors, compressors and pumps draw several times their running power when starting, and a designer will ask which loads can coincide.

Nominal battery capacity (Wh) = daily energy × days of autonomy ÷ (usable fraction × discharge efficiency). With the guide’s example of 3 kWh for two days at 80% usable and 90% efficiency, that is 6 ÷ 0.72 = 8.33 kWh. Amp-hours are the same capacity divided by bank voltage.

Array size (W) = daily energy ÷ (peak sun hours × delivery factor). Three equivalent full-sun hours at a 75% delivery factor turn each installed watt into about 2.25 Wh a day. This replaces one day of use; it does not add the surplus needed to recharge a depleted battery after a cloudy spell.

Finding peak sun hours for your site

Peak sun hours describe the solar energy reaching a panel each day, expressed as hours at 1,000 watts per square metre. They vary by latitude, season, tilt, local weather and shading. The National Renewable Energy Laboratory’s PVWatts application reports monthly solar radiation for an address and array orientation. Take the lowest month in which you intend to rely on solar, then adjust for trees, ridgelines and snow that a regional model cannot see.

PVWatts was built for grid-connected systems. Its annual production estimate does not show whether an independent home stays powered through a run of short, overcast days. Use its monthly figures as an input to this calculator and to the designer’s seasonal model, not as proof of reliability.

What this calculator leaves out

  • Recovery after bad weather. Recharging a battery bank while still supplying the household requires extra array capacity, a generator or reduced demand.
  • Battery chemistry and temperature. Usable capacity, charging limits and cold-weather behaviour differ by chemistry and manufacturer settings.
  • Surge and power limits. An inverter can have enough energy available and still be unable to start a well pump with the kettle running.
  • Codes and approvals. Electrical permits, inspections and installer licensing are set by the state and local authority. Check the building and electrical rules in your state guide.

Bring the load list and these numbers to designers and ask each to state their own assumptions in writing. Our guide explains how to compare proposals on the same basis.

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