Watt Hours Calculator
Estimate energy use in Wh and kWh, then optionally calculate electricity cost.
Battery Runtime Calculator
Use battery amp-hours and voltage to estimate how long a load can run.
What is a watt-hour?
A watt-hour (Wh) is a unit of energy. It tells you how much electricity a device uses over time. A watt (W) measures power at one instant, while a watt-hour measures accumulated energy usage.
Think of it this way: power is the speed, energy is the distance traveled. If you know device watts and runtime, you can quickly estimate battery needs, solar panel sizing, and electric bill impact.
Core watt-hour formula
Basic equation
Watt-hours (Wh) = Watts (W) × Hours (h)
- 100W device running for 2 hours = 200Wh
- 60W light running for 8 hours = 480Wh
- 1500W space heater running for 1.5 hours = 2250Wh
Reverse formulas
- Hours = Wh ÷ W (find runtime)
- Watts = Wh ÷ Hours (find average power)
- kWh = Wh ÷ 1000 (convert to utility billing unit)
How to use this calculator
- Enter your appliance power in watts.
- Enter daily runtime in hours and minutes.
- Set quantity if you have more than one identical device.
- Set number of days for the period you want to analyze.
- Add your electricity rate per kWh (optional) to estimate cost.
The calculator returns total Wh, total kWh, average daily use, and optional cost estimates.
Why watt-hours matter in real life
1) Managing your electric bill
Most electric companies bill in kilowatt-hours. Converting appliance use into kWh helps identify high-cost devices and habits. Small changes (shorter runtime, efficient devices, timers) can reduce annual costs.
2) Sizing batteries and portable power stations
If your laptop and router need 400Wh overnight, you know your battery must provide at least that much usable energy, plus margin for inefficiency.
3) Planning solar systems
Solar design starts with daily energy demand. Once you know your daily Wh load, you can estimate panel size and battery requirements more accurately.
Common conversion shortcuts
- 1 kWh = 1000 Wh
- Wh = Ah × V (ideal battery energy before losses)
- mAh to Wh: (mAh × V) ÷ 1000
- Ah from Wh: Wh ÷ V
Example: A 20,000mAh power bank at 3.7V has about 74Wh nominal energy (20,000 × 3.7 ÷ 1000).
Typical daily energy use by appliance
| Appliance | Typical Power | Runtime / Day | Daily Use (Wh) |
|---|---|---|---|
| LED Bulb | 10W | 5 h | 50 Wh |
| Laptop | 60W | 6 h | 360 Wh |
| Wi-Fi Router | 12W | 24 h | 288 Wh |
| TV (LED) | 100W | 4 h | 400 Wh |
| Mini Fridge (avg) | 70W | 24 h cycling | 800-1200 Wh |
Frequent mistakes to avoid
- Confusing watts and watt-hours: watts are instant power, watt-hours are total energy.
- Ignoring runtime: a high-watt device used briefly may consume less than a low-watt device used all day.
- Forgetting efficiency losses: inverters, battery chemistry, and converters reduce usable energy.
- Not using average power: many appliances cycle on and off, so nameplate watts may overstate real use.
Battery runtime example
Suppose you have a 12V, 100Ah battery and a 60W load. Battery energy is 12 × 100 = 1200Wh nominal. If system efficiency is 85%, usable energy is about 1020Wh.
Estimated runtime = 1020Wh ÷ 60W = 17 hours (approximate). Real-world values vary with temperature, age, depth of discharge, and inverter behavior.
Quick FAQ
Is watt-hour the same as amp-hour?
No. Amp-hours measure charge capacity, while watt-hours measure energy. Voltage is needed to convert between them.
Why does my power station deliver less than listed Wh?
Because of inverter losses, voltage conversion, battery protection limits, and manufacturer test conditions.
Can this calculator estimate monthly cost?
Yes. Enter your electricity rate and set days to the billing period (for example, 30 days).
Final takeaway
A good watt hours calculator is one of the simplest tools for understanding energy consumption. Use it to compare appliances, estimate costs, size battery backup, and make smarter efficiency decisions.