People mix up watts and kWh all the time. The short version is this: watts are a rate, while watt-hours and kWh are total energy used over time.
For quick system sizing, use the WattSizing Calculator.
Quick Answer
- Watt (W): how fast electricity is being used right now.
- Watt-hour (Wh): how much electricity was used over time.
- Kilowatt-hour (kWh): 1,000 Wh (the unit on your utility bill).
Core Equations
Use these three relationships:
Power (W) = Energy (Wh) / Time (h)Energy (Wh) = Power (W) x Time (h)Energy (kWh) = Power (W) x Time (h) / 1000
If your appliance uses 500 W for 3 hours:
Wh = 500 x 3 = 1,500 WhkWh = 1,500 / 1000 = 1.5 kWh
Practical Examples
Example 1: Laptop Charger
A 65 W charger used for 4 hours:
Wh = 65 x 4 = 260 WhkWh = 0.26 kWh
Example 2: Space Heater
A 1,500 W heater used for 2 hours:
Wh = 1,500 x 2 = 3,000 WhkWh = 3.0 kWh
Example 3: Refrigerator (average draw)
If average draw is 150 W over 24 hours:
Wh = 150 x 24 = 3,600 WhkWh = 3.6 kWh/day
Conversion Table
| Quantity | Meaning | Typical Use |
|---|---|---|
| Watt (W) | Instantaneous power | Appliance label, inverter limits |
| Kilowatt (kW) | 1,000 watts | Generator/inverter nameplate |
| Watt-hour (Wh) | Energy over time | Battery capacity (small systems) |
| Kilowatt-hour (kWh) | 1,000 Wh | Utility bills, home daily usage |
Common Mistakes to Avoid
- Comparing a battery in Wh directly to an appliance in W without including runtime.
- Treating kW (power) and kWh (energy) as the same thing.
- Forgetting duty cycle for cycling loads (fridges, pumps, AC systems).
For load-specific estimates, see How Many Watts Does a Refrigerator Use and How Many Watts Does a Well Pump Use.
FAQs
Is kWh the same as kW per hour?
No. kWh is a unit of energy. kW is a unit of power.
Why does my bill use kWh instead of watts?
Utilities charge for total energy consumed over time, not just instantaneous power.
Can two devices with the same watts use different daily kWh?
Yes. Runtime is the deciding factor. Same watts x different hours = different kWh.
What should I use for battery sizing: W or Wh?
Use both: watts for inverter power capability, and Wh (or kWh) for stored energy/runtime.
CTA
Need to turn appliance watts into daily energy and backup runtime? Use the WattSizing Calculator to estimate load, battery, inverter, and generator requirements.
