Why you get less than the rated capacity
Updated
Where the energy goes
The battery supplies direct current at its own voltage. The AC outlets supply 120 V alternating current, so an inverter converts one into the other, and each conversion turns part of the energy into heat. The station also powers its own electronics from the same battery: the battery management system, the display and, under load, the cooling fans.
Rated capacity counts none of those losses. It describes what the cells hold. Usable energy is what reaches the plug of your appliance.
Why the default is 85%
85% is the site's own assumption. It covers the losses in the inverter and in the conversion from battery to outlet, and it gives a runtime of capacity × 0.85 ÷ the watts of the device. Enough Watts uses it for every station, so stations compare on equal terms.
The real share varies with the station and the load, and we have not measured it for any station. Treat 85% as a planning figure. Under Assumptions and solar in the runtime calculator you can set it anywhere from 75% to 95%.
What the setting does to the numbers
| Rated capacity | At 75% | At 85% | At 95% |
|---|---|---|---|
| 300 Wh | 225 Wh | 255 Wh | 285 Wh |
| 500 Wh | 375 Wh | 425 Wh | 475 Wh |
| 1,000 Wh | 750 Wh | 850 Wh | 950 Wh |
| 2,000 Wh | 1,500 Wh | 1,700 Wh | 1,900 Wh |
| 3,600 Wh | 2,700 Wh | 3,060 Wh | 3,420 Wh |
From one end of the range to the other, a 2,000 Wh station gives 1,500 Wh or 1,900 Wh. The gap of 400 Wh would run a 12 W router for 33 hours. For a list that sits close to a station's limit, the setting decides the verdict.
One list, three settings
Take a refrigerator, a Wi-Fi router and a CPAP: 1,808 Wh a day, as worked out in watts vs watt hours. Put them on a 2,042 Wh station with a goal of 24 hours.
| Setting | Usable | Runtime | Verdict at 24 h |
|---|---|---|---|
| 75% | 1,532 Wh | 20.3 h | Runs it, but not long enough |
| 85% | 1,736 Wh | 23.0 h | Runs it, but not long enough |
| 95% | 1,940 Wh | 25.8 h | Enough |
The station and the list stay the same, and only the setting moves the verdict. When a result sits this close to your goal, read it at the lower setting, or look one size up.
When to plan with less than 85%
- Cold. Makers set temperature limits for their batteries. Of the 54 stations in the data set, 48 publish a lowest charging temperature, from -4 °F (-20 °C) to 32 °F (0 °C), and 48 a lowest discharging temperature, from -4 °F (-20 °C) to 14 °F (-10 °C). A station kept in a cold garage or a winter camp is a case for a lower setting and a look at its spec sheet.
- An older battery. A cycle life rating states how much capacity is left after a number of full cycles. After the cycles of a rating that ends at 80%, a 1,000 Wh station holds 800 Wh by the maker's own figure, and 800 × 0.85 = 680 Wh at the outlets. LiFePO4 vs NMC explains how to read those ratings.
- Small loads on a big inverter. The inverter uses some power of its own whenever it is on. Next to a 10 W load that share is large; next to a 1,000 W load it is small.
Phones, lights and fans on the USB or 12 V ports skip the AC inverter, so the inverter part of the loss does not apply to them. The calculator models the AC outlets, which makes the 85% default cautious for a list of USB devices.
How the calculator uses the share
- Usable watt hours = rated capacity × the share you set (85% by default).
- Runtime = usable watt hours ÷ the watt hours your list uses in a day × 24. This is calendar time while you follow your routine, and it is the number compared with your goal.
- Nonstop runtime = usable watt hours ÷ running watts, the time with everything on at once. It is shown for reference and never decides the verdict.
The share affects every station the same way, so the order of stations by rated capacity stays the same. It matters when you turn watt hours into hours for your own list. The methodology states every formula, and watts vs watt hours works through a full day.
Sources
- Power station and appliance figures: the Enough Watts data set, each record with its own sources and dates.