Watts to kWh Calculator

A 1,000W appliance running for one hour uses exactly 1 kWh. To convert watts to kWh, multiply the wattage by the hours it runs and divide by 1,000, so a 1,500W heater over 3 hours comes to 4.5 kWh. Watts are a rate of draw; kilowatt hours are the total your meter counts.

The watts to kWh formula, explained

Three terms, and no voltage, amperage or power factor anywhere in them. The answer is exact, so any error you end up with came from the inputs rather than the arithmetic.

• Watts (W) are a rate. They describe how fast a device is consuming energy at this instant, the way miles per hour describes a car. A watt figure on its own is not an amount of anything.
• Hours (h) are how long that rate is sustained. This is the term people estimate, and almost every wrong answer traces back to it rather than to the formula.
• Kilowatt hours (kWh) are the total. One kilowatt hour is 1,000 watts held for one hour. It is the unit utilities bill in, and the only one of the three that describes a quantity of energy.

The division by 1,000 is a unit change and nothing more. Leave it out and you have watt hours (Wh), the same quantity in smaller units: 4,500Wh and 4.5 kWh are the same measurement.

The trip back is the same arithmetic reversed: multiply the kilowatt hours by 1,000 and divide by the hours. 4.5 kWh over 3 hours is 1,500W.

Conversion table: wattage against run time

Every cell below is watts × hours ÷ 1,000. No voltage is assumed and none is needed, so these figures hold whether the load is 120V AC, 240V AC or 12V DC straight off a battery. Find your wattage on the left and read across.

The highlighted row is the anchor: one kilowatt for one hour is one kilowatt hour, and every other line is that relationship scaled. The bottom of the 24-hour column looks absurd on purpose. 5,000W held for a full day really is 120 kWh, about four days of electricity for a typical US home, and a number that size means a load problem rather than a battery problem.

Why this conversion decides which battery you buy

Appliances are labelled in watts. Batteries are sold in watt hours or kilowatt hours. This conversion is the bridge between them, and it is where a power station purchase is either right or quietly returned three weeks later.

Sizing only works in one direction. List the loads you actually intend to run, convert each to kWh for the hours you need it, add them up, and only then look at capacities. Picking a capacity because the number sounds reassuring is how people end up with a 1,000Wh unit and a kettle.

1. Convert the load, then add losses. Anything plugged into a household socket runs through an inverter, costing roughly 10–15%. A genuine 2 kWh of AC work draws about 2.2–2.35 kWh out of the cells, so a nominal 2,000Wh unit will not quite finish the job.
2. Keep a reserve. Sizing to about 80% of rated capacity is the sensible compromise. Emptying a pack to zero every cycle shortens its life, and cold weather takes another bite.
3. Check the output rating separately. Capacity in kWh tells you how long; continuous watts tells you whether the appliance starts at all. A 3 kWh battery behind a 600W inverter still cannot run a microwave for one second.

The solar side runs on the same conversion. Panels are rated in watts and what you need from them is a daily kWh figure: a 400W array averaging 4.5 usable sun hours makes roughly 1.8 kWh on a good day, and less once heat, dust and shading are counted. If your loads total 3 kWh a day, 400W of panel is a trickle rather than a system. Our solar sizing guide works through the array arithmetic, and what size power station you need covers the storage half.

If the battery you are comparing is quoted in amp hours instead, amp hours to kWh converts it once you know the nominal voltage. With a target figure in hand, the power station catalogue is listed by capacity and home battery backup covers the larger kWh brackets.

Running hours, not elapsed hours

Nearly all the error in this conversion lives in the hours. Very few appliances draw their rated wattage continuously; most are thermostatic, switched off for a large share of the time while still counting, in your head, as running. Use running hours, not elapsed hours.

• Refrigerator. A 150W compressor at a 35% duty cycle runs about 8.4 hours out of 24, which is 150 × 8.4 ÷ 1,000 = 1.26 kWh per day. The naive version, 150W for a full 24 hours, gives 3.6 kWh and would have you buy nearly three times the battery. Our breakdown of what a refrigerator actually draws goes through the duty cycle properly.
• Space heater. A 1,500W heater on a thermostat in a reasonably insulated room cycles at roughly half duty, so an eight-hour night is about four running hours and 6 kWh rather than 12. In a cold garage with no insulation, it is closer to the full 12.
• Well pump. A 1,000W pump running half an hour a day uses 0.5 kWh, which is trivial. Its difficulty is never the energy, it is the starting surge — an inverter question rather than a capacity one.

Resistive loads are the honest ones. Kettles, toasters, hair dryers and ovens at full heat draw their rating for as long as they are on, so elapsed and running time are the same and the nameplate figure holds exactly. Anything with a compressor, a thermostat or a variable-speed motor does not.

If the answer matters enough to spend money on, measure it. A plug-in energy monitor costs about $25 and reads cumulative kilowatt hours straight off the socket, and one day of real data beats every table on the internet, including this one.

AC, DC and the kilowatt hours you never see

A kilowatt hour delivered to an appliance is not the same as a kilowatt hour taken out of a battery, and the gap decides how much battery you need.

Running an AC appliance from a battery means converting DC to 120V AC, and that costs about 10–15%, with the worst efficiency at very small loads. A 60W load left on a 2,000W inverter overnight loses a real share of the pack to idle draw alone. A DC appliance wired straight to the battery — a 12V compressor fridge, USB charging, LED lighting — skips that stage entirely, which is why van and RV builds keep as much as possible on DC. Grid-side the loss does not apply: your meter counts what the appliance uses, with no inverter in the path.

Power factor is the other place watts and apparent power separate. Motors and some inverter-driven appliances draw more current than their wattage implies, and the difference is quoted in volt-amps. A device pulling 1,200VA at a power factor of 0.8 consumes 960W of real power, so five hours of it is 4.8 kWh rather than 6. Residential meters bill real energy, so the watt figure is the one to multiply; VA matters only when sizing a generator or an inverter, which is what the VA to watts calculator is for.

Turning kilowatt hours into money

Once a load is expressed in kWh, the running cost is one multiplication: kilowatt hours × your rate. The US average sits around 17 cents per kilowatt hour, but state rates run from roughly 11 cents to well over 40, so take the number off your own bill. The table below uses 17 cents and a 30-day month.

The shape of that table is the useful part. Small always-on loads feel wasteful and cost almost nothing; large intermittent loads feel harmless and dominate the bill. Unplugging phone chargers is pointless next to one space heater running four hours a night, which is why our guide to cutting an electricity bill starts with heating and cooling. Off-grid, that heater empties a 2 kWh pack in about 80 minutes, which is why resistive heating and portable batteries are a poor match.

Frequently asked questions