Power-use calculation result
Daily energy use
520.00Wh
Using 65W for 8 hours equals 520Wh, or 0.520kWh.
0.520 kWh1,872,000 J65W × 8 hours
Wh (Watt-hour) Calculator
Calculate Wh, kWh, and joules from power draw and runtime, convert battery mAh↔Wh, and estimate battery runtime, monthly kWh, and simple energy cost.
Power · battery · energy unit calculator
Wh (Watt-hour) Calculator
Calculate Wh from power draw and runtime, convert battery mAh↔Wh, switch Wh↔kWh/J, and estimate monthly energy use and battery runtime in one place.
Calculate Wh from power use
W × hours = WhQuick examples
Battery mAh↔Wh
mAh × V ÷ 1000Wh conversion base
Wh↔kWh↔JBy default this follows the power-use result above. Enter another Wh value only when you want a separate conversion.
Please check your inputs.
Based on 20 days
Simple $0.18/kWh estimate
10,000mAh · 3.7V basis
Reference value with 85% efficiency
Unit conversion table
520Wh base| Wh → kWh | 0.520 kWh | 520 ÷ 1000 |
|---|---|---|
| Wh → J | 1,872,000 J | 520 × 3600 |
| Wh → mAh | 140,541 mAh | Wh × 1000 ÷ V |
| mAh → Wh | 37.00 Wh | 10000 × 3.7 ÷ 1000 |
Formula check
For verificationWh calculation
65W × 8 hours = 520WhMonthly kWh
520Wh × 20 days ÷ 1000 = 10.40kWhUsable battery energy
37Wh × 85% = 31.45WhEstimated runtime
31.45Wh ÷ 65W = 0.48 hoursA battery’s mAh rating needs a voltage before it can be converted to Wh. Power banks may show a 3.7V cell capacity and a different 5V output capacity, so check the voltage printed on the product.
What is a Wh (watt-hour) calculator?
A Wh calculator multiplies power draw by time to estimate electrical energy use. For example, a 65W laptop used for 8 hours consumes 65 × 8 = 520Wh, which is 0.52kWh. It is useful when you need to compare device specs, electricity bills, battery capacity, portable power stations, or camping power plans on the same energy basis.
This calculator does more than return a single Wh number. It also shows kWh, joules, monthly energy use, an estimated energy cost, battery mAh↔Wh conversion, and estimated battery runtime so you can compare use and storage in one flow.
When to use it
Power-related values often appear as W, Wh, kWh, mAh, and J. Device labels usually show W, utility bills use kWh, and batteries often use mAh. This tool brings those values back to a Wh-centered view so you can compare device consumption and stored energy without switching calculators.
- Estimate Wh for a laptop, lamp, router, heater, or other device over a set runtime
- Convert a power bank or portable battery from mAh to Wh for capacity planning
- Convert Wh to kWh or joules for bill estimates, physics work, or product comparison
- Estimate monthly kWh and a simple cost for devices you use every day
Key features
The input area separates power and runtime, battery capacity and voltage, and a standalone Wh conversion base. The result area highlights Wh and kWh first, then shows monthly use, cost estimate, battery Wh, and estimated runtime in compact cards. The conversion table and formula cards make the result easy to verify.
- Power-to-Wh calculation – Enter W or kW and runtime to get Wh, kWh, and J
- Monthly energy estimate – Combine daily energy with days per month and a kWh rate
- Battery mAh↔Wh conversion – Convert mAh with voltage, and convert Wh back to mAh
- Runtime estimate – Use battery Wh, device power draw, and efficiency to estimate operating time
- Copyable result – Copy a clean summary for notes, messages, or planning documents
How to use
Enter the device power draw and the runtime. If the device is labeled in kW, switch the unit to kW. Add days per month if you want a monthly energy estimate. For battery planning, enter mAh, voltage, and a realistic efficiency value. Lower efficiency gives a more conservative runtime estimate.
- Enter power draw and select W or kW.
- Enter the runtime in hours and minutes.
- Enter days per month and a kWh rate to estimate monthly energy and cost.
- Enter battery capacity, voltage, and efficiency to see Wh and estimated runtime.
- Enter a separate Wh value in the conversion base field if you want to convert another amount.
Wh formulas and unit interpretation
A watt-hour is energy: Wh = W × h. A 100W bulb used for 2 hours consumes 200Wh, or 0.2kWh. To convert watt-hours to joules, use 1Wh = 3600J.
Battery capacity cannot be understood from mAh alone. Voltage is required: Wh = mAh × V ÷ 1000. To go the other way, use mAh = Wh × 1000 ÷ V. Real runtime may be shorter than the theoretical result because of conversion losses, protection circuits, discharge behavior, cable loss, and changing device load. For power-to-dBm work, use the Power ↔ dBm Converter.
- Wh – Energy used by a power draw over time
- kWh – Wh ÷ 1000, the common billing unit
- J – Wh × 3600, the SI energy unit used in physics
- mAh↔Wh – Battery conversion that must include voltage
- Runtime – Usable Wh ÷ device W, shown as a planning estimate
FAQ
What is the difference between W and Wh?
W is power at a moment in time. Wh is the total energy used over time. A 50W device used for 2 hours consumes 100Wh.
How do I convert Wh to kWh?
Divide Wh by 1000. For example, 520Wh is 0.520kWh. This is the unit most electricity bills use.
Why do I need voltage to convert mAh to Wh?
mAh is closer to charge capacity, not energy. The same 10,000mAh battery equals 37Wh at 3.7V but 50Wh at 5V, so voltage must be included.
Why can real battery runtime be shorter?
Voltage conversion, cable loss, protection circuits, device load changes, and battery age reduce usable energy. Use an efficiency such as 80–90% for a more realistic estimate.
Is the energy cost estimate the same as my bill?
No. It is a simple monthly kWh × rate estimate. Actual bills may include base charges, tiered pricing, taxes, discounts, and plan-specific adjustments.
Can I use this for airline power bank limits?
It helps you understand the Wh value, but airline and country rules differ. Check your airline’s latest carry-on battery guidance before travel.
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