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What Are Watt Hours?
A watt-hour (Wh) is a unit of energy equal to one watt of power sustained for one hour. It measures the total amount of electrical energy consumed or produced over time. Your electricity bill is measured in kilowatt-hours (kWh), where 1 kWh = 1,000 Wh.
Understanding watt-hours helps you estimate running costs for appliances, size battery systems, and compare energy efficiency between devices. A device's wattage tells you how fast it uses energy, while watt-hours tell you how much total energy it uses over a period.
The Formula
Device Energy Consumption
| Device | Watts | Daily Use (h) | Monthly kWh | Monthly Cost |
|---|---|---|---|---|
| LED Bulb | 10 | 8 | 2.4 | $0.29 |
| Desktop PC | 200 | 8 | 48.0 | $5.76 |
| Refrigerator | 150 | 24 | 108.0 | $12.96 |
| AC Unit | 3500 | 8 | 840.0 | $100.80 |
| EV Charger | 7200 | 4 | 864.0 | $103.68 |
Energy Saving Tips
- Replace incandescent bulbs with LEDs to reduce lighting energy by 75-80%.
- Use a smart power strip to eliminate standby power draw (phantom loads).
- Run dishwashers and laundry during off-peak hours for lower rates.
- Set your thermostat 2 degrees higher in summer and lower in winter.
- Enable sleep mode on computers; a sleeping PC uses 3-5W vs 200W active.
Frequently Asked Questions
How do I read my electricity meter?
Modern digital meters display cumulative kWh directly. Subtract last month's reading from this month's to get your consumption. Smart meters transmit data automatically to your utility company.
What is a kilowatt-hour?
One kilowatt-hour equals 1,000 watt-hours. It is the standard billing unit for electricity. Using a 1,000W appliance for one hour, or a 100W appliance for 10 hours, both consume 1 kWh.
How do I calculate battery life in watt-hours?
For batteries, Wh = Voltage × Amp-hours. A 12V, 100Ah battery stores 1,200 Wh. Divide battery Wh by device wattage to get approximate run time in hours, accounting for efficiency losses of 10-20%.