Watts (W)
Power: how fast energy is being transferred or used at a moment.
Watt Hours
A watt-hour is the energy used by one watt over one hour. Calculate Wh or estimate battery runtime.
Nominal
Usable
After efficiency
Runtime
Compare another setup
Setup A stays fixed from the estimate above. Change one or more fields for Setup B.
Setup A
Setup B
Watts, watt-hours, and amp-hours are different
Watt-hours (Wh)
Energy: power multiplied by time. One Wh equals 3,600 joules.
Amp-hours (Ah)
Charge: useful for batteries, but it needs nominal voltage before it can be compared as energy.
Three calculations in one path
For a device running at steady average power, use Wh = watts × hours. For a battery labeled in volts and amp-hours, use Wh = nominal volts × Ah; divide mAh by 1,000 first. Either Wh result can be carried directly into the runtime tab without retyping the capacity.
What the runtime estimate shows
The runtime result keeps every assumption visible: nominal Wh, the usable share, energy remaining after end-to-end efficiency, and estimated hours at the entered average load. Defaults are 100% usable and 100% efficiency, so the first result is explicitly labeled ideal rather than pretending hidden loss assumptions are universal.
The A/B comparison holds Setup A fixed and tells you exactly which inputs changed in Setup B. It is useful for comparing capacity, average load, usable percentage, or efficiency without relying on a product database.
Why real battery runtime can be shorter
Battery chemistry, age, temperature, discharge rate, manufacturer reserve, battery-management cutoff, inverter idle draw, conversion efficiency, and changing loads can all move the result. High start-up surge is an equipment power-rating question, not something an energy-only Wh formula can approve.
Runtime is an estimate, not a safety or equipment-rating check. Verify continuous and surge ratings in the battery, inverter, and device manuals. Do not rely on this calculator for medical, emergency, life-safety, transport, or code-compliance decisions.
Worked examples
- Power and time: 100 W × 2 h = 200 Wh = 0.2 kWh.
- Battery label: 12 V × 8 Ah = 96 Wh.
- Adjusted runtime: 1,000 Wh × 80% usable × 90% efficiency ÷ 100 W = 7.2 hours, or about 7 hr 12 min.
Frequently asked questions
What is a watt-hour?
A watt-hour (Wh) is energy equal to one watt of power used for one hour. It is not the same as a watt, which measures the rate of energy transfer. One watt-hour equals 3,600 joules.
Are watt-hours the same as watts per hour?
No. Watt-hours are watts multiplied by hours, so Wh measures energy. Watts per hour would describe how quickly power changes over time, which is a different quantity.
How do I calculate watt-hours?
Multiply average power in watts by time in hours. If the time is in minutes, divide it by 60 first. For example, 60 watts for 30 minutes uses 30 Wh.
How long will a battery last?
Divide energy available after usable-capacity and efficiency losses by the average load in watts. The result is an estimate because temperature, battery age, discharge rate, idle draw, cutoff behavior, and changing loads affect real runtime.
How many watt-hours is a 20,000 mAh battery?
You also need nominal voltage. At 3.7 V, 20,000 mAh is 20 Ah, so 3.7 × 20 = 74 Wh. A different nominal voltage produces a different energy value.
What is the difference between Ah and Wh?
Amp-hours measure electric charge; watt-hours measure energy. Batteries with the same Ah but different voltage do not store the same Wh, so multiply nominal volts by Ah before comparing energy.
How many watt-hours are in a kilowatt-hour?
One kilowatt-hour is 1,000 watt-hours. Divide Wh by 1,000 to get kWh.