How to Calculate Watt‑Hours – Simple Formula & Real Examples
Watt‑hours (Wh) tell you how much energy a device uses over time. If watts measure power, watt‑hours measure energy. This is the number you need when sizing a solar generator, battery bank, or backup power system.
The good news: calculating watt‑hours is extremely simple. You only need two numbers — watts and hours.
The Watt‑Hour Formula
Watt‑Hours (Wh) = Watts (W) × Hours (h)
That’s it. One multiplication gives you the energy used.
Example Calculations
| Device | Watts | Hours | Watt‑Hours (Wh) |
|---|---|---|---|
| LED Light | 10W | 5h | 50Wh |
| Laptop | 60W | 3h | 180Wh |
| Mini Fridge (average) | 60W | 24h (cycling) | 1440Wh |
| CPAP Machine | 40W | 8h | 320Wh |
How to Calculate Daily Energy Use
To size a battery or solar generator, calculate the total daily watt‑hours of everything you want to run.
Example load list:
| Device | Watts | Hours | Daily Wh |
|---|---|---|---|
| Lights | 40W | 5h | 200Wh |
| Laptop | 60W | 4h | 240Wh |
| Router | 15W | 8h | 120Wh |
| Mini Fridge | 60W | 24h | 1440Wh |
| Total Daily Energy | 2000Wh | ||
This home needs about 2000Wh per day.
How to Convert Amp‑Hours (Ah) to Watt‑Hours (Wh)
If your battery is labeled in amp‑hours instead of watt‑hours, use this formula:
Watt‑Hours (Wh) = Amp‑Hours (Ah) × Volts (V)
Example:
- 12V battery
- 100Ah capacity
100Ah × 12V = 1200Wh
A 12V 100Ah battery stores about 1200Wh.
How to Estimate Runtime from Watt‑Hours
Once you know watt‑hours, you can estimate runtime:
Runtime (hours) = Battery Wh ÷ Device Watts
Example:
- Battery: 1000Wh
- Device: 100W
1000Wh ÷ 100W = 10 hours
Real‑world runtime is usually 10–15% lower due to inverter losses.
Watt‑Hour Cheat Sheet
| Device | Daily Wh |
|---|---|
| Phone charging | 20–40Wh |
| LED bulb | 40–80Wh |
| Laptop | 150–300Wh |
| CPAP | 300–500Wh |
| Mini fridge | 800–1500Wh |
| Full‑size fridge | 1000–2000Wh |
Common Mistakes When Calculating Watt‑Hours
- Ignoring inverter losses: subtract 10–15% for AC loads
- Using peak watts instead of average watts: fridges cycle
- Not counting all devices: routers, lights, chargers add up
- Forgetting 24‑hour loads: fridges and routers run all day
FAQ: Watt‑Hours Explained
What’s the difference between watts and watt‑hours
Watts measure power. Watt‑hours measure energy used over time.
How many watt‑hours is 1 kWh
1 kWh = 1000Wh.
How many watt‑hours do I need per day
Most homes need 2000–5000Wh for essentials; off‑grid cabins vary widely.
How do I size a battery using watt‑hours
Multiply your daily Wh by 1.5–2× for comfortable capacity.
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