Watts to Energy: How to Convert Power to kWh

This calculator converts power in watts and time in hours into energy in watt-hours (Wh) and kilowatt-hours (kWh). You enter the power draw of a device and how long it runs, and you get back the total energy consumed, which is what your electricity bill charges for.

Formula

The relationship between power and energy is simple: Energy (E) = Power (P) × Time (t).

• E = energy in watt-hours (Wh) or kilowatt-hours (kWh)
• P = power in watts (W) or kilowatts (kW)
• t = time in hours (h)

If you use watts and hours, the result is watt-hours. Divide by 1000 to get kilowatt-hours, the standard unit on electricity bills.

Worked Example

Using the calculator defaults: a 1000 W (1 kW) appliance runs for 5 hours.

1. Energy in watt-hours: 1000 W × 5 h = 5000 Wh
2. Convert to kilowatt-hours: 5000 Wh ÷ 1000 = 5 kWh

The calculator shows 5000 Wh and 5.00 kWh, matching the formula.

Why Watts and Energy Are Different

Watts measure the rate of energy use at an instant. Energy is the total amount used over time. A 100 W light bulb uses energy at a rate of 100 joules per second. If it runs for 10 hours, it consumes 1000 Wh (1 kWh) of energy. The same energy could come from a 1000 W device running for 1 hour. Power tells you how fast, energy tells you how much.

How to Read the Result

Your electricity bill charges per kilowatt-hour. If you know your rate (e.g., $0.15 per kWh), multiply the kWh result by that rate to estimate cost. For example, 5 kWh × $0.15 = $0.75. The calculator gives energy, not cost, because rates vary widely.

For appliances that cycle on and off (like refrigerators), use the average running power, not the nameplate wattage. A fridge might draw 150 W when running but only run 30% of the time, so average power is about 45 W.

Common Mistakes and Unit Mix-Ups

• Confusing watts with watt-hours: A 100 W bulb is not “100 watts per hour.” It uses 100 watt-hours for each hour it runs.
• Forgetting to convert minutes to hours: If a device runs 30 minutes, use 0.5 hours, not 30.
• Using kilowatts without adjusting: 1 kW × 5 h = 5 kWh directly. If you use watts, divide by 1000 after multiplying.
• Ignoring standby power: Many devices draw power even when off. A TV on standby might use 5 W, which over a month is 3.6 kWh.

Real-World Examples

• A 1500 W space heater running 3 hours: 1500 × 3 = 4500 Wh = 4.5 kWh.
• A 60 W LED bulb for 8 hours: 60 × 8 = 480 Wh = 0.48 kWh.
• A 50 W laptop charger for 4 hours: 50 × 4 = 200 Wh = 0.2 kWh.

These examples show how the same formula applies to any device, from tiny chargers to large heaters.

Limitations of This Calculation

This calculator assumes constant power over the entire time. Real devices often vary: a washing machine uses more during spin, a computer fluctuates with workload. For accurate energy estimates, use an average or measure with a plug-in power meter. Also, the calculator gives energy consumed, not the heat or light produced—some energy is always wasted as heat.

Frequently Asked Questions

How many watts are in a kilowatt-hour?

There are no watts in a kilowatt-hour, because they measure different things. A kilowatt-hour is energy, equal to 1000 watt-hours. For example, a 1000 W device running for 1 hour uses 1 kWh.

How do I convert watts to kWh?

Multiply watts by hours to get watt-hours, then divide by 1000. For example, 500 W for 2 hours = 1000 Wh = 1 kWh.

What is the difference between a watt and a watt-hour?

A watt is a unit of power (rate of energy use). A watt-hour is a unit of energy (total amount). A 60 W bulb uses 60 watt-hours in one hour, but it is always a 60 W bulb regardless of time.

How many kWh does a typical house use per day?

In the US, average residential electricity use is about 30 kWh per day (roughly 900 kWh per month), but this varies widely with climate, home size, and appliances.

Why do electricity bills use kWh instead of watts?

Bills charge for total energy consumed, not the rate. kWh is a convenient unit: a 1000 W appliance running for 1 hour uses 1 kWh, which is easy to understand and bill.

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in p | Power | W | 1000 | 0
in t | Time | hours | 5 | 0
out e_wh | Energy | Wh | p * t | 0
out e_kwh | Energy | kWh | (p * t) / 1000 | 2