Battery Runtime Calculator
Enter a battery’s watt-hour capacity and your device’s power draw to see a realistic estimate of how long it will run — including the conversion losses and safety reserve that advertised numbers usually skip. Check a battery you own, or sanity-check one before you buy.
Estimate your runtime
Results update as you type. Every assumption is adjustable.
Estimated runtime
7 hr 39 min
7.65 hours, with a 10% reserve kept back.
Before reserve: 8 hr 30 min (8.50 hours).
Usable energy
383 Wh
Of the 500 Wh on the label, this is what realistically reaches your device after conversion losses and reserve.
Capacity class
A 500Wh battery sits in our 500–1,000Wh class. See how it compares.
How this is calculated
Both runtime figures come from the same two-step formula in our shared calculation library:
usableWh = batteryWh * (efficiency / 100) * (1 - reserve / 100) runtimeHours = usableWh / deviceWatts In plain English: start with the capacity printed on the battery. Multiply by the efficiency percentage — converting DC battery power to AC wall power through an inverter typically consumes about 15% of the energy, so 85% survives. Then set aside the reserve, capacity you deliberately don’t plan to use. What remains is the usable energy; dividing it by your device’s steady draw in watts gives hours of runtime. The “before reserve” figure skips that last set-aside so you can see both.
Default assumptions
- 85% efficiency — a typical figure for AC output through an inverter. Direct USB-C or 12V DC output loses less; 90–95% is fair there.
- 10% reserve — a buffer for battery aging, cold weather, and estimate error. Set it to 0 to see the theoretical maximum.
- Label capacity — we take the watt-hour rating at face value; older or heavily cycled batteries store less than the label says.
The full method — and why we picked these defaults — is on How we estimate.
What to do with this number
Some links on this page may be paid links. If you buy through them, Cynosure LLC may earn a commission at no extra cost to you. We do not claim to have personally tested products unless clearly stated.
Example power stations by capacity class
Map your runtime result to a capacity class with these placeholder examples. The specs are illustrative category estimates — the point is the difference between classes, not any specific model.
Some links on this page may be paid links. If you buy through them, Cynosure LLC may earn a commission at no extra cost to you. We do not claim to have personally tested products unless clearly stated.
| Product | Capacity | Output | Ports | Weight | Est. price | Ideal for | Link |
|---|---|---|---|---|---|---|---|
| Example 300Wh Power Station Placeholder Brand | 300Wh | 300W AC | AC ×1, USB-C 100W, USB-A ×2, 12V car port | 7–10 lb | $150–$250 | Router and modem backup, Charging phones and tablets for days, A laptop for a few hours, Car trips and short outages | Link pending |
| Example 500Wh Power Station Placeholder Brand | 500Wh | 500W AC | AC ×2, USB-C 100W, USB-A ×2, 12V car port | 13–17 lb | $250–$450 | A full laptop workday, A day or more of router and modem backup, Weekend camping electronics, Fans, lights, and small electronics together | Link pending |
| Example 1,000Wh Power Station Placeholder Brand | 1,000Wh | 1,000W AC | AC ×3, USB-C 100W, USB-A ×2, 12V car port, DC5521 ×2 | 22–28 lb | $500–$900 | Multi-day phone and internet backup, A mini fridge through an outage, Family camping trips, Several devices running at once | Link pending |
| Example 2,000Wh Power Station Placeholder Brand | 2,000Wh | 2,000W AC | AC ×4, USB-C 100W ×2, USB-A ×2, 12V car port, DC5521 ×2 | 45–60 lb | $1,000–$1,900 | Days of essentials during long outages, A full-size refrigerator in duty cycles, High-draw devices up to 2,000W, Base camp or supplemental RV power | Link pending |
Placeholder examples with category-typical specs, not specific tested products. Verify capacity, continuous output, and surge ratings on the manufacturer’s spec sheet.
Frequently asked questions
Why is my real runtime lower than the number advertised on the box?
Advertised runtimes usually assume every watt-hour on the label reaches your device. In practice, the inverter that produces AC power consumes 10–20% of the energy, batteries lose capacity as they age, and cold weather cuts output further. That is why this calculator defaults to 85% efficiency and holds back a 10% reserve — the result is closer to what you will actually see.
Can this tell me whether a battery can run a specific device, like a mini fridge?
Runtime is only half the answer. The battery also needs a continuous AC output rating at or above the device’s running watts, plus surge headroom for anything with a compressor or motor — a fridge can briefly draw two to three times its running wattage at startup. Check the output and surge ratings on the spec sheet, not just the watt-hours.
What efficiency percentage should I enter?
85% is a reasonable default for devices plugged into a power station’s AC outlets. If you are charging USB devices directly from a USB-C port or running 12V devices from a DC port, losses are smaller — 90–95% is a fair estimate. For older batteries or cold conditions, use a lower number.
Does the reserve setting really matter?
The reserve is a planning buffer, not physics. Keeping 10% back covers estimate error, battery aging, and the device you forgot to count, and avoiding routine full discharge is gentler on most battery chemistries. You can set it to 0 to see the absolute maximum, but we would not plan an outage around that number.
Calculations are estimates only. Real runtime depends on battery age, inverter efficiency, device behavior, temperature, surge loads, manufacturer limits, and actual measured wattage. Always verify product specifications before buying or relying on a setup.
Affiliate disclosure
Some links on this page may be paid links. If you buy through them, Cynosure LLC may earn a commission at no extra cost to you. We do not claim to have personally tested products unless clearly stated.