If you know an appliance's wattage and how many hours you use it, you can calculate energy in seconds. That one number (kWh) is what your utility bill is based on.
For full system planning, use the WattSizing Calculator.
Quick Answer
Use this formula:
kWh = (Watts x Hours) / 1000
Example: a 120 W fan running 8 hours/day uses:
(120 x 8) / 1000 = 0.96 kWh/day
Why This Formula Works
- Watts (W) measure power at a moment in time.
- Watt-hours (Wh) measure energy over time.
- Kilowatt-hours (kWh) are simply 1,000 Wh.
So when you multiply watts by time (hours), you get Wh. Dividing by 1,000 converts Wh to kWh.
If you are building a full load list, pair this with How to Calculate Appliance Running Cost and Calculate Daily Energy Use Off-Grid Solar.
Watts to kWh Reference Table
| Appliance Load | Usage Time | Daily kWh |
|---|---|---|
| 60 W light bulb | 5 h/day | 0.30 kWh |
| 120 W fan | 8 h/day | 0.96 kWh |
| 300 W desktop setup | 6 h/day | 1.80 kWh |
| 1,200 W microwave | 0.5 h/day | 0.60 kWh |
| 1,500 W space heater | 4 h/day | 6.00 kWh |
Step-by-Step Example
Let's say you want to estimate monthly usage for a 200 W TV used 4.5 hours/day.
- Daily energy:
kWh/day = (200 x 4.5) / 1000 = 0.9 kWh/day - Monthly energy (30 days):
0.9 x 30 = 27 kWh/month - If your utility rate is $0.18/kWh:
27 x 0.18 = $4.86/month
This method is accurate enough for budgeting and generator/solar planning.
Common Mistakes to Avoid
- Mixing up kW and kWh (power vs energy).
- Using rated watts when real draw is much lower/higher in practice.
- Forgetting duty cycle for appliances that cycle on and off.
- Entering minutes as hours (20 minutes is 0.33 hours, not 20).
For cycling loads such as fridges, using measured average watts is better than nameplate guesses. See How Many Watts Does a Refrigerator Use.
FAQs
Can I calculate kWh from amps instead of watts?
Yes. First estimate watts using Watts = Volts x Amps (for simple loads), then use the kWh formula.
What if the appliance has multiple power modes?
Calculate each mode separately (high, eco, standby), then add them together for a realistic daily total.
Is this formula good for solar and battery sizing?
Yes, this is the first step. After kWh/day, include system losses and autonomy targets. A practical next read is Off-Grid Solar Sizing Calculator Guide.
How do I convert kWh back to watts?
Rearrange the formula: Watts = (kWh x 1000) / Hours.
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Want instant daily and monthly totals without manual math? Use the WattSizing Calculator to convert appliance watts and usage hours into actionable system sizing numbers.
