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How we calculate

The calculator uses standard sizing rules that installers use, with every assumption stated here. It gives a sound starting point, not a final design: a qualified installer should confirm the sizes for your home and wiring.

1. Your load

For each appliance we multiply its watts by how many you have. Adding them up gives the running load: the power drawn if everything is on at once. Multiplying each by its hours a day gives the daily energy in watt-hours.

Appliance watts start from typical values, in line with published tables such as theUS Department of Energy's. The real figure is on each appliance's label, and you can change every number.

2. The inverter

• Headroom: an inverter should not run flat out for long, so we size it at 1.25× the running load (the load uses at most 80% of its rating).
• Starting surge: motors and compressors (fridges, freezers, pumps, air conditioners) draw about 3× their running power for a moment when they start. We add that for the largest motor, assuming two motors rarely start at the same instant. Inverters typically handle about 2× their rating for a few seconds, so the inverter must be at least half the starting load.
• Sizes: we round up to a common size (0.5 kW, 1 kW, 1.5 kW, 2 kW, 3 kW, 3.6 kW, 4 kW, 5 kW, 6 kW, 8 kW, 10 kW, 12 kW, 15 kW, 20 kW). Many inverters are labelled in kVA; we show kVA as kW ÷ 0.8, the power factor most of them use.
• Battery voltage: 12 V up to 1.5 kW, 24 V up to 3.6 kW, 48 V above that, which keeps cable currents sensible.

3. The battery

Energy needed during outages = daily energy × hours without power ÷ 24. This assumes use is spread through the day; if your outages are mostly in the evening, when lights and TV are on, you may need a little more.

• We divide by 0.9 for the energy the inverter loses converting battery power (10%).
• We then divide by the share of the battery you can use daily while keeping it healthy: 80% for lithium (LiFePO4) and 50% for lead-acid, tubular and AGM batteries. That is why lead-acid banks need to be bigger.
• Amp-hours = watt-hours ÷ battery voltage.

4. Solar panels

Solar has to produce the energy the battery gave out (or, if you choose, everything you use in a day), plus inverter losses.

• Peak sun hours are the long-term average of daily sunlight energy, in kWh per square metre, fromNASA POWER (all-sky surface irradiance, monthly climatology from satellite data since 2001). One peak sun hour is one hour of full-strength sunlight.
• Real-world output: panels rarely deliver their label rating. Heat, dust, wiring and charge-controller losses typically cost about a quarter, so we count 75% of the rating.
• Panel watts needed = daily solar energy ÷ (peak sun hours × 0.75). We round up to whole panels.
• By default we use the month with least sun, so the system keeps up all year. You can switch to the yearly average, which gives a smaller system that falls short in the dullest months.
• Charge controller current = total panel watts ÷ battery voltage × 1.25, rounded up to the next 5 A.

What the calculator doesn't cover

• Cable sizes, fuses, breakers, earthing and changeover switches: these need an electrician.
• Roof angle, direction and shade, which can change output a lot. NASA figures are for flat, unshaded ground.
• Several days of cloud in a row. Add battery capacity if you need to ride through those.
• Prices, which change quickly and vary by country and seller.

Spotted a mistake or have a better figure? Tell us.