Runtime estimates are simple until the fine print shows up. The useful version includes usable capacity, inverter losses, reserve, and load behavior.
The simple formula
usable watt-hours / load watts = runtime hours
If you have 1,000 usable Wh and a 100-watt load, the simple answer is 10 hours.
The real answer may be lower because of inverter losses, standby draw, cold temperatures, battery reserve settings, and loads that surge or cycle.
Better formula
Use:
usable Wh x efficiency factor / average watts = runtime hours
A conservative efficiency factor might be 0.8 to 0.9 for AC loads. Use manufacturer guidance where available.
Example load list
| Load | Average watts | Hours | Wh |
|---|---|---|---|
| Router | 25 | 12 | 300 |
| LED lamps | 20 | 6 | 120 |
| Phone charging | 20 | 3 | 60 |
| Laptop | 60 | 4 | 240 |
Total: 720 Wh before margin.
If the battery has 1,000 usable Wh, this plan may work. If you add a refrigerator, sump pump, or medical device, recalculate instead of assuming.
What people forget
- inverter idle draw
- battery reserve settings
- cold-weather capacity reduction
- startup surge
- pass-through charging limits
- solar input limits
- cable losses and adapter limits
Buying decision
When comparing batteries or portable stations, do not ask “How many days will it run my house?” Ask:
- How many Wh are usable?
- What is the average watt draw of my selected loads?
- What is the continuous AC output?
- What surge can it handle?
- How will I recharge it?
This keeps the purchase grounded. For a broader system view, read Backup Power Sizing .
