Watt Hour Calculator

Watt-hours are what a load consumes; amp-hours are what a battery label advertises. This converts between them in both directions, at your system voltage, because the voltage is the only thing that joins the two and it is where most off-grid arithmetic goes wrong. Multiply watts by hours for a load, or enter a battery's amp-hours to see the energy it actually holds.

The bank voltage, not the appliance voltage. 12, 24 or 48 on most systems.

Results update as you type.

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DC cable ampacity with the ambient and bundling corrections, how far each size runs at 12, 24 and 48 V, PV string fuse sizes from Isc, and battery-to-inverter current by inverter size. Three pages, made to pin by the battery bank.

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How It Works

The two formulas

Wh = W × h × quantity
kWh = Wh ÷ 1000
Ah = Wh ÷ V      Wh = Ah × V

A watt-hour is a watt sustained for an hour, and it is a unit of energy. An amp-hour is an amp sustained for an hour, and it is a unit of charge, which only becomes energy once you know the voltage it sits at. That is why a battery's amp-hour rating means nothing on its own and why comparing a 100 Ah 12 V battery with a 100 Ah 48 V one by the number on the label is the commonest sizing error in off-grid work: the second holds four times the energy.

Worked example

A 12 V compressor fridge drawing 45 W, running about 10 hours in every 24 because the compressor cycles:

  • Energy = 45 × 10 = 450 Wh a day, which is 0.45 kWh
  • At 12 V that is 450 ÷ 12 = 37.5 Ah a day out of the bank
  • At 24 V the same fridge is 18.8 Ah a day, and at 48 V, 9.4 Ah

The 10 hours is the point worth dwelling on. A fridge is on the shelf for 24 hours a day and running for perhaps 10 of them, so its nameplate wattage times 24 overstates the load by more than a factor of two. Duty cycle, not connected hours, is what goes in the hours box.

What to do with the number

A single appliance is rarely the question. Add up every load and you have the daily watt-hours a battery bank has to deliver, which is where the off-grid load calculator takes over: it runs the same arithmetic per row, then charges AC loads the inverter's conversion loss and DC loads nothing, and returns the watt-hours at the battery rather than at the socket. Its worked campervan example comes to 1,055 Wh a day, which is 88 Ah at 12 V, and that figure is what the battery bank size calculator and the solar system size calculator both start from.

Two adjustments this page deliberately does not make, because they belong further down the chain. It does not apply inverter or wiring efficiency, so an AC load's true draw on the bank is a little higher than the Wh here; and it does not apply depth of discharge, so a bank has to be considerably larger than the Wh you need out of it. Both are handled by the tools above.

Typical Appliance Loads in Watt-Hours

Running watts and realistic daily hours for the loads an off-grid or van system usually carries, with the watt-hours and the amp-hours they come to at 12, 24 and 48 V. These are the starting figures the off-grid load calculator offers in its appliance picker, so the two pages cannot drift apart. They are typical values to begin from, not a substitute for the rating plate on your own kit.

Typical off-grid appliance loads with daily watt-hours and the amp-hours they draw at 12, 24 and 48 volts
Appliance Running watts Hours a day AC/DC Wh a day Ah at 12V Ah at 24V Ah at 48V
LED lights (per fitting) 8 W 5 DC 40 3.3 1.7 0.8
Compressor fridge (12V) 45 W 10 DC 450 37.5 18.8 9.4
Chest freezer 90 W 8 AC 720 60.0 30.0 15.0
Water pump 60 W 0.5 DC 30 2.5 1.3 0.6
Roof / extractor fan 25 W 6 DC 150 12.5 6.3 3.1
Diesel or gas heater fan 30 W 6 DC 180 15.0 7.5 3.8
Laptop 60 W 4 AC 240 20.0 10.0 5.0
Phone or tablet charger 10 W 3 AC 30 2.5 1.3 0.6
Starlink / router 45 W 8 AC 360 30.0 15.0 7.5
Television 80 W 3 AC 240 20.0 10.0 5.0
Microwave 1,000 W 0.2 AC 200 16.7 8.3 4.2
Induction hob (one ring) 1,500 W 0.5 AC 750 62.5 31.3 15.6
Electric kettle 1,800 W 0.15 AC 270 22.5 11.3 5.6
Washing machine (cycle) 500 W 0.5 AC 250 20.8 10.4 5.2
Well pump (1/2 hp) 750 W 1 AC 750 62.5 31.3 15.6
Air conditioner (small) 900 W 6 AC 5,400 450.0 225.0 112.5
Power tool (occasional) 800 W 0.5 AC 400 33.3 16.7 8.3

The Ah columns are the energy divided by the bank voltage, with no allowance for inverter efficiency or depth of discharge. An AC appliance costs the bank more than the figure shown, because the inverter takes its cut; the off-grid load calculator applies that and returns the draw at the battery.

Frequently Asked Questions

How do I calculate watt-hours?

Multiply the power in watts by the hours it runs. A 45 W fridge running 10 hours a day uses 450 Wh, or 0.45 kWh. For several of the same thing, multiply by the quantity as well. The only judgement involved is the hours: use the time the appliance is actually drawing power, not the time it is switched on, which for anything thermostatic is a fraction of the day.

How do I convert amp-hours to watt-hours?

Multiply the amp-hours by the voltage. A 100 Ah battery at 12 V holds 1,200 Wh; the same 100 Ah at 24 V holds 2,400 Wh, and at 48 V, 4,800 Wh. Going the other way, divide watt-hours by the voltage. Always use the system voltage of the bank, not the voltage of the appliance running off the inverter.

Is a 200 Ah battery bigger than a 100 Ah one?

Only at the same voltage. 200 Ah at 12 V is 2,400 Wh and 100 Ah at 48 V is 4,800 Wh, so the smaller-sounding battery holds twice the energy. Compare banks in watt-hours or kilowatt-hours and the question answers itself, which is why every sizing tool on this site works in Wh and converts to Ah only at the end.

How many watt-hours do I need for a night off grid?

Add up each load's watts times its hours, which is what the table above and the off-grid load calculator do. The campervan example there comes to 1,055 Wh a day, or 88 Ah at 12 V. Then size the bank well above it: usable capacity is the nameplate times the depth of discharge, so a lithium bank at 0.8 DoD needs about 1,320 Wh of nameplate to deliver 1,055 Wh, and lead-acid at 0.5 DoD needs over 2,100 Wh.

Why is my real runtime shorter than the watt-hours suggest?

Three reasons, none of them in this calculator by design. AC loads pass through the inverter, which takes roughly 8 to 10%. You cannot use the whole bank: depth of discharge caps it at half on lead-acid and about 80% on lithium. And on lead-acid, discharging faster than the 20-hour rate returns fewer amp-hours than the label promises, which is Peukert's effect. The battery runtime calculator models all three.

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