Enter both values to calculate kilowatt hours
Watts (W)
Hours (h)
Kilowatt Hours (kWh)
Result will appear here
Understanding Energy vs Power
The relationship between kilowatt hours and watts involves time:
- Energy (kWh) = Power (W) × Time (h) ÷ 1000
- Power (W) = Energy (kWh) × 1000 ÷ Time (h)
For Your Electric Bill
Understanding these relationships helps you:
- Estimate costs - Know how much devices cost to run
- Size solar panels - Match production to consumption
- Compare appliances - Understand energy efficiency ratings
- Track usage - Break down your monthly bill
Real-World Solar Examples
Daily Output of a 400W Panel
A 400W panel with 5 peak sun hours produces 2.0 kWh per day. Over a year: 730 kWh. At $0.16/kWh, that is $116.80 in electricity from one panel.
10 kW Rooftop Array
A 10 kW array with 5 peak sun hours and 80% system efficiency produces 40.0 kWh per day — exceeding the average US home's 30 kWh/day usage.
Space Heater on Battery Backup
A 1,500W space heater running for 3 hours consumes 4.5 kWh — one-third of a Tesla Powerwall's capacity for just one appliance. This shows why electric heating on battery backup is impractical.
When You'll Need This Conversion
- Estimating Monthly Solar Production — A 7,600W array with 4.8 peak sun hours and 82% performance ratio produces about 899 kWh/month. If usage is 850 kWh/month, this array slightly overproduces — ideal for net metering.
- Calculating Battery Runtime Under Load — Running 450W of loads (fridge, lights, router) on a 48V 100Ah bank (4,800Wh) lasts 10.67 hours. With 90% inverter efficiency: 9.6 hours of actual runtime.
- Comparing Appliance Energy for Solar Sizing — Daily usage: microwave 1,000W × 0.5h = 0.5 kWh, laptop 65W × 8h = 0.52 kWh, LED lights 100W × 6h = 0.6 kWh, mini-fridge 60W × 24h = 1.44 kWh. Total: 3.06 kWh/day.
Solar Tips & Common Mistakes
STC Ratings Overstate Real Output: A "400W" panel produces 400W only under Standard Test Conditions. Real-world output is typically 75–85% due to heat, angle, shading, and soiling. Use 320–340W as a realistic figure.
Do Not Forget Phantom Loads: Inverters, charge controllers, and appliance standby modes draw power 24/7. A 10W standby draw adds 0.24 kWh/day — equivalent to nearly one solar panel's output over a month.
Design for Worst-Case Seasonal Usage: A 5,000W AC running 4h/day uses 20 kWh. Running 8h/day in a heat wave uses 40 kWh. Always design for worst-case, not average conditions.
Solar Calculators
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Last updated: January 5, 2026
