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W
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h
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30 min = 0.5 h • 90 min = 1.5 h
$/kWh
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US avg ~$0.15 • UK ~$0.28 • EU ~$0.30
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Sources & Methodology
Formula Wh = W × h from NIST SP 811. US electricity rate reference from EIA.
US Energy Information Administration — Electricity Prices
EIA reference for US average residential electricity retail price used as the default rate ($0.15/kWh) for cost calculations in this calculator.
NIST SP 811 — Guide for the Use of SI
Official NIST definition of the watt-hour (1 Wh = 3600 J) and kilowatt-hour (1 kWh = 3.6 MJ) as units of energy derived from the watt and the hour.
Methodology: Wh = W × h. kWh = Wh / 1000. Runtime (h) = Wh / W. Cost = (W × h / 1000) × rate. Joules = Wh × 3600. Amp-hours at voltage V: Ah = Wh / V. Daily cost = (W × h_per_day / 1000) × rate. Monthly = daily × 30. Annual = daily × 365.
⏱ Last reviewed: April 2026
How to Calculate Watt-Hours, kWh, and Electricity Cost
A watt-hour (Wh) is the energy consumed by a one-watt device running for one hour. Your electricity bill is denominated in kilowatt-hours (kWh) — thousands of watt-hours. Understanding how to calculate watt-hours lets you estimate running costs, size battery systems, and plan solar installations with confidence.
The Core Formulas
Wh = Watts × Hours kWh = Wh / 1000
Runtime (h) = Wh / Watts Cost = kWh × rate
Example 1: 100W bulb, 8 hours: Wh = 100 × 8 = 800 Wh = 0.8 kWh = $0.12 at $0.15/kWh
Example 2: 1200Wh battery at 100W load: Runtime = 1200/100 = 12 hours
Example 3: Monthly cost of a 2500W avg home: 2500 × 24/1000 = 60 kWh/day × 30 = 1800 kWh = $270/month
Example 2: 1200Wh battery at 100W load: Runtime = 1200/100 = 12 hours
Example 3: Monthly cost of a 2500W avg home: 2500 × 24/1000 = 60 kWh/day × 30 = 1800 kWh = $270/month
Common Appliance Energy Use and Monthly Cost
| Appliance | Power | Hours/Day | kWh/Month | Cost/Month ($0.15) |
|---|---|---|---|---|
| LED bulb (10W) | 10W | 6h | 1.8 | $0.27 |
| Laptop | 65W | 8h | 15.6 | $2.34 |
| Refrigerator | 150W avg | 24h | 108 | $16.20 |
| Washing machine | 1200W | 1h | 36 | $5.40 |
| Air conditioner (2T) | 2000W | 8h | 480 | $72.00 |
| Electric dryer | 5000W | 1h | 150 | $22.50 |
| EV charging (7kW) | 7000W | 3h | 630 | $94.50 |
Battery Energy and Runtime Reference
| Battery | Total Wh | Usable Wh | Runtime @ 100W |
|---|---|---|---|
| Phone (3.8V, 4000mAh) | 15 Wh | 13 Wh | 8 min |
| Laptop (11.1V, 55Wh) | 55 Wh | 50 Wh | 30 min |
| 12V 100Ah lead-acid | 1,200 Wh | 600 Wh (50%) | 6 hours |
| 12V 100Ah lithium | 1,200 Wh | 960 Wh (80%) | 9.6 hours |
| Tesla Powerwall 2 | 14,000 Wh | 13,500 Wh | 135 hours |
| Tesla Model 3 LR | 82,000 Wh | 75,000 Wh | 750 hours |
💡 Key insight: Electricity bills are in kWh. Divide appliance watts by 1000 to get kW, multiply by hours used to get kWh. A 2000W heater for 3 hours = 2 kW × 3 h = 6 kWh = $0.90 at $0.15/kWh. High-power heating appliances (water heater, HVAC, dryer) dominate electricity bills because watts × hours is large. Reducing use time or switching to heat pumps has the biggest impact on costs.
Frequently Asked Questions
Wh = Watts × Hours. A 100W device running 5 hours = 500 Wh = 0.5 kWh. To convert to kWh, divide by 1000. kWh is how your electricity meter and bill measure energy. A 1 kW (1000W) appliance running for exactly 1 hour uses 1 kWh.
100Ah × 12V = 1200 Wh total energy. Lead-acid: use 50% DoD = 600 Wh usable → at 100W = 6 hours. Lithium: 80% DoD = 960 Wh usable → at 100W = 9.6 hours. At 50W load: lead-acid 12 hours, lithium 19.2 hours. Divide usable Wh by your actual load watts to find runtime.
Watts = rate of power (like speed). Watt-hours = total energy used (like distance). A 100W bulb uses energy at 100 watts. Run it for 3 hours and it has consumed 300 Wh = 0.3 kWh of energy. Power (watts) tells you how fast energy flows; energy (watt-hours) tells you how much total energy was used.
Cost = Watts × Hours / 1000 × rate per kWh. For a 1500W kettle boiling 3 times daily for 4 minutes each (0.2 h): kWh/day = 1500 × 0.2/1000 = 0.3 kWh. At $0.15/kWh = $0.045/day = $1.35/month. Annual = $16.43. For large appliances running many hours, costs add up quickly.
Ah = Wh / Voltage. 1200 Wh at 12V = 100 Ah. At 24V = 50 Ah. At 48V = 25 Ah. Higher system voltage means fewer Ah needed for the same stored energy. This is why solar and EV systems use 48V or higher — lower amperage reduces wire sizing requirements and resistive losses.
US average: ~900 kWh/month ($135/month at $0.15/kWh). UK average: ~250 kWh/month. Australia: ~600 kWh/month. Homes with electric heating, large HVAC, or EV charging may use 1500–2500 kWh/month. High-efficiency homes may use under 400 kWh/month. Knowing daily kWh helps size solar panels: 900 kWh/month = 30 kWh/day → 10+ kW solar system in sunny climates.
Step 1: Find your daily kWh use. Step 2: Divide by peak sun hours at your location (4–6 typical). Step 3: Divide by panel wattage (400W typical). For 30 kWh/day in a 5 sun-hour location: 30/5 = 6 kW system needed = 6000/400 = 15 panels. In a 4 sun-hour location: 30/4 = 7.5 kW = 19 panels. Add 15–20% for system losses.
Most EVs use 200–350 Wh per mile. Tesla Model 3: ~250 Wh/mile. Nissan Leaf: ~290 Wh/mile. For 100 miles in a Model 3: 25,000 Wh = 25 kWh. At $0.15/kWh = $3.75. A gasoline car at 30 mpg and $3.50/gal costs $11.67 per 100 miles. EVs use about 70% less energy per mile.
1 kWh = 3,600,000 joules = 3.6 MJ. 1 Wh = 3600 J. A kettle boiling 1L of water uses about 0.1 kWh = 360,000 J. This is why kWh is preferred over joules for household energy — it produces manageable numbers. Your electricity meter records kWh accumulated over time.
Calculate each major load: kWh/month = Watts × h/day × 30 / 1000. Focus on high-kWh items: HVAC (2000W, 8h = 480 kWh/month = $72), water heater (4000W, 3h = 360 kWh = $54), dryer (5000W, 1h = 150 kWh = $22.50). Switching HVAC to a heat pump (COP 3) cuts that 480 kWh to 160 kWh, saving $48/month.
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