Use this Watt Hour Calculator to convert power and time into energy measured in Wh. Enter watts (or volts and amps) and hours, and it returns watt-hours, kilowatt-hours, and common battery/runtime estimates.
It also helps you compare devices and understand how quickly energy is used, which is essential for sizing batteries and estimating electricity cost.
What Is a Watt Hour (Wh)?
A watt-hourenergy. It tells you how much electrical energy a device uses when it runs for a certain amount of time.
Power (watts) is the rate of energy use, and time (hours) is how long the energy is used. Multiply them to get watt-hours.
The Core Formula (The One You Actually Use)
The basic relationship is:
- Wh = W × h
Where:
- Wh = watt-hours of energy
- W = power in watts
- h = time in hours
If your device is rated in watts, this is the fastest path.
Converting Volts and Amps to Watts
If you have volts and amps, you can find watts first.
- W = V × A
Where:
- V = volts
- A = amps
This is the most common DC relationship for many electronics and battery systems. For AC systems, real-world power can differ due to power factor, but the calculator targets the common “nameplate” estimate.
From Watt Hours to Kilowatt Hours (kWh)
Electric bills are typically in kilowatt-hours (kWh). Convert by dividing by 1,000:
- kWh = Wh ÷ 1000
This lets you estimate cost using your electricity rate.
How the Calculator Handles Different Inputs
The Watt Hour Calculator uses your inputs to compute energy. You can provide power directly in watts, or compute it from volts and amps.
- If you enter watts, it uses Wh = W × time.
- If you enter volts and amps (and leave watts blank), it uses W = V × A first, then multiplies by time.
- It always converts results to kWh for cost and comparison.
Common Use Cases and What to Watch For
Energy estimates are most accurate when the device’s power rating matches your actual operating mode.
- Constant load devices (like many LED drivers at a fixed setting) are straightforward.
- Variable load devices (like compressors, heaters with cycling, or phones with fluctuating draw) require an average power.
- Battery runtime depends on battery capacity, voltage, and efficiency losses. Use the estimate as a planning tool, not a guarantee.
Example 1: Estimate Daily Energy Use
Suppose a router uses 12 W and runs 24 hours per day.
| Step | Calculation | Result |
|---|---|---|
| Energy | Wh = 12 W × 24 h | 288 Wh/day |
| Bill unit | kWh = 288 ÷ 1000 | 0.288 kWh/day |
If your electricity rate is $0.20/kWh, daily cost is about $0.058 (0.288 × 0.20).
Example 2: Estimate Battery Runtime
Imagine you have a battery with 500 Wh of capacity and want to run a device that draws 100 W for planning purposes.
- Runtime (hours) = Battery Wh ÷ Load W
Runtime ≈ 500 ÷ 100 = 5 hours. If you include an efficiency factor (in real systems), actual runtime may be lower.
Frequently Asked Questions
How do I calculate watt-hours from watts and hours?
Multiply the device power in watts by the time in hours. The result is energy in watt-hours. Example: a 60 W device running for 3 hours uses 60 × 3 = 180 Wh. This is the most direct method when watt ratings are available.
What if I only know volts and amps?
First convert volts and amps to watts using W = V × A. Then compute watt-hours by multiplying watts by hours: Wh = (V × A) × h. This works for many DC systems and many nameplate ratings, giving a practical energy estimate.
How do I convert watt-hours to kilowatt-hours for my electric bill?
Divide watt-hours by 1000. For example, 1500 Wh equals 1.5 kWh. Utility bills usually charge per kWh, so you can multiply kWh by your electricity rate to estimate cost. Keep units consistent to avoid errors.
Can I use watt-hours to estimate battery runtime?
Yes, as a planning estimate. Runtime in hours is roughly Battery Wh divided by device watts. If the battery capacity is given in amp-hours, convert to watt-hours first using Wh ≈ V × Ah, then divide by load watts.
Why might my real energy use differ from the calculator?
Real-world power changes with load, temperature, and efficiency. Devices may not run at their maximum rating continuously, and battery systems lose energy through conversion and heat. If you need accuracy, use measured average watts or a conservative power estimate.
Quick Checklist Before You Calculate
- Identify power: watts (W) or compute watts from V and A.
- Use correct time units: hours for Wh.
- Check whether you need kWh for billing.
- For batteries, compare battery Wh to load watts for runtime planning.
Final Takeaway
The Watt Hour Calculator turns power and time into energy in Wh and kWh. That gives you a clear way to estimate electricity cost, compare devices, and plan battery runtime with confidence.