Power Load Calculator: Generator, Battery & Solar Sizing

Every appliance you list contributes two different numbers: its running watts (how much power it draws while it's on) and its daily watt-hours (running watts × hours used each day). The first number matters for sizing something that has to supply everything at once, like a generator. The second matters for sizing something that has to supply everything over a whole day, like a battery bank or a solar array.

That's why this calculator asks for both — quantity and hours per day per appliance — rather than just a single wattage figure, and why the recommendations below use different combinations of your appliance list for each purpose.

Motors and compressors — in fridges, well pumps, air conditioners and power tools — briefly draw two to four times their running wattage for a fraction of a second when they switch on, before settling down to their normal running watts. A generator has to survive that spike without stalling, so its rating needs to cover more than just the sum of everyone's running watts.

The standard approach, used here, is to add up everyone's running watts, then add just the extra surge headroom for the single largest motor — not every motor's surge stacked together, since in real use they very rarely all start at the exact same instant. Ticking "Motor" on an appliance applies a common 3× rule-of-thumb surge multiplier to it for this calculation.

The same daily watt-hours that size a battery or solar array also translate into a monthly electricity cost: multiplied by 30 for a monthly total, then by your electricity rate. That figure works two ways depending on why you're running the numbers. If these appliances are already on the grid, it's roughly your expected bill for running them. If you're planning to cover this load with a generator, battery bank or solar array instead, that same figure is roughly what you'd save on your electricity bill each month by doing so.

It won't match a real bill exactly — utility pricing often includes fixed charges, tiered rates or time-of-use pricing this calculator doesn't model — but it's a solid estimate for comparing the ongoing cost of one option against another.

A battery bank is sized from total energy, not peak power: your daily watt-hours, multiplied by how many days of backup you want, adjusted for how deep the battery can safely discharge and for the losses an inverter and wiring introduce along the way. The result converts to amp-hours at whichever system voltage (12 V, 24 V or 48 V) your setup uses — higher voltages need fewer amp-hours for the same energy, which is why larger off-grid systems often run at 24 V or 48 V.

A solar array is sized similarly, but against "peak sun hours" instead of a full day — the equivalent number of hours of full-strength sunlight a location gets on average, which is usually well under the number of daylight hours. A generous derating accounts for real-world losses from wiring, heat, dust, panel angle and charge-controller inefficiency, so the recommended array is comfortably larger than a naive full-sun calculation would suggest.