Watts to Watt-Hours: How to Convert

Watts measure power (the rate of energy use at a given moment), while watt-hours measure energy (the total amount used over time). To convert watts to watt-hours, multiply the power in watts by the time in hours: watt-hours = watts × hours.

Formula

The conversion is straightforward:

E (Wh) = P (W) × t (h)

• E = energy in watt-hours (Wh)
• P = power in watts (W)
• t = time in hours (h)

If your time is in minutes, convert it first by dividing by 60. For example, 30 minutes becomes 0.5 hours. If your power is in kilowatts (kW), multiply by 1000 to get watts before using the formula.

Worked Example

Using the default values from the calculator: a device draws 100 watts and runs for 3 hours.

1. Write down the formula: E = P × t
2. Plug in the values: E = 100 W × 3 h
3. Multiply: E = 300 Wh

The device consumes 300 watt-hours of energy. The calculator above gives the same result: 300.00 Wh.

Why the Distinction Matters

Watts and watt-hours answer different questions. Watts tell you how fast energy is being used—like the speedometer in a car. Watt-hours tell you how much total energy was used—like the odometer. A 60 W light bulb uses 60 watts at any instant, but over 10 hours it consumes 600 watt-hours. Confusing the two leads to errors when sizing batteries, solar panels, or estimating electricity costs.

Electric utilities bill in kilowatt-hours (kWh), which are simply 1,000 watt-hours. A 1,000 W space heater running for 1 hour uses 1 kWh. Running it for 5 hours uses 5 kWh. Understanding this conversion helps you estimate the cost of running appliances: multiply the kWh by your electricity rate.

How to Convert in Practice

To convert watts to watt-hours, you only need two pieces of information: the power rating of the device (usually printed on its label) and the duration it runs. Here is the process:

1. Find the power rating in watts (W) on the device label or manual.
2. Determine the runtime in hours (convert minutes to hours by dividing by 60).
3. Multiply the watts by the hours to get watt-hours.
4. If you need kilowatt-hours, divide the watt-hours by 1,000.

For example, a laptop charger rated at 65 W used for 2 hours consumes 130 Wh. A refrigerator that draws 150 W on average and runs 24 hours per day uses 3,600 Wh, or 3.6 kWh, per day.

Common Mistakes and Misconceptions

One frequent error is treating watts and watt-hours as interchangeable. Saying “a battery stores 100 watts� is incorrect; it stores watt-hours. Another mistake is forgetting to convert minutes to hours. A 500 W device running for 30 minutes uses 250 Wh, not 15,000 Wh.

A related misconception involves “watt-hours per day� versus “watts.� A solar panel rated at 300 W produces 300 watts under ideal conditions, but over a day with 5 hours of peak sun it generates about 1,500 Wh (1.5 kWh). The rating alone doesn’t tell you daily energy output—time matters.

Also, be aware that power ratings on devices are often maximum values. A device may draw less in practice. For accurate energy estimates, use a plug-in power meter to measure real-time watts over a period.

When to Use This Conversion

You’ll need to convert watts to watt-hours in several everyday situations:

• Sizing batteries: If a device uses 50 W for 4 hours, you need a battery that can supply at least 200 Wh (plus some margin for losses).
• Estimating solar needs: Calculate your daily watt-hour consumption, then divide by peak sun hours to find the required solar panel wattage.
• Understanding electricity bills: Convert appliance wattage and usage time to kWh to see which devices cost the most.
• Choosing a portable power station: Compare its capacity in watt-hours to the watt-hours your devices need.

For example, a camping trip with a 40 W fan running 6 hours per night requires 240 Wh. A power station with a 300 Wh capacity would handle it, but one with 200 Wh would not.

Limitations of the Simple Formula

The basic formula assumes constant power draw. Many devices cycle on and off (refrigerators, air conditioners) or have variable power (power tools, laptops). In those cases, use an average power value over the time period. Also, the formula doesn’t account for inverter losses, battery inefficiency, or temperature effects. When sizing a battery or solar system, add a safety margin of 20–30% to cover these losses.

For AC devices running on battery power through an inverter, the inverter draws extra power. A 100 W AC device might consume 110–120 W from the battery due to inverter inefficiency. Always factor that into your watt-hour calculations.

Frequently Asked Questions

How many watt-hours is 100 watts?

It depends on time. 100 watts for 1 hour equals 100 watt-hours. For 2 hours, it’s 200 watt-hours. The watt-hour value only makes sense when you specify a duration.

What’s the difference between a watt and a watt-hour?

A watt is a unit of power—the rate of energy transfer. A watt-hour is a unit of energy—the total amount transferred. Think of watts as speed and watt-hours as distance.

How do I convert watt-hours back to watts?

Divide watt-hours by hours. For example, 300 Wh over 3 hours equals 100 W. This gives the average power draw.

Is 1 watt-hour the same as 1 volt-amp-hour?

No. For DC circuits they can be equal, but for AC circuits with a power factor less than 1, volt-amp-hours are higher than watt-hours. Watt-hours represent real power, while volt-amp-hours represent apparent power.

How many watt-hours are in a kilowatt-hour?

1 kilowatt-hour (kWh) equals 1,000 watt-hours. Electricity bills usually show usage in kWh.

Can I use this formula for batteries?

Yes, but check the battery’s rated capacity in watt-hours. Many batteries list milliamp-hours (mAh) and voltage; multiply those and divide by 1,000 to get watt-hours. For example, a 10,000 mAh, 3.7 V battery has 37 Wh.

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in p | Power | W | 100 | 0
in h | Time | hours | 3 | 0
out wh | Energy | Wh | p * h | 2