In summary: Divide usable watt-hours by average load watts, then account for conversion, standby draw, temperature and reserve.
The basic formula
Divide the battery's usable watt-hours by your load's average watts to get a runtime estimate in hours -- but usable is the important qualifier, since nominal capacity and actually available energy are not the same number.
Why usable energy is lower than nominal capacity
The inverter, standby electronics and battery management system all consume some energy themselves, and manufacturers typically reserve a margin to protect battery health -- in practice, usable energy commonly runs around 80-90% of the nameplate watt-hour rating, not 100%.
Getting a realistic estimate, not just a number
Use measured or nameplate average watts for the actual load rather than a rough guess, model a range using both the 80% and 90% usable-energy assumptions rather than a single figure, and separately confirm the station's continuous output can sustain that load's watts at all -- a runtime estimate is meaningless if the output rating cannot support the load in the first place.
- Model a range rather than one percentage.
- Use measured average load, not only charger labels.
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HOW TO CALCULATE PORTABLE-POWER RUNTIME GUIDE QUESTIONS
Frequently asked questions
No. Use it to frame the decision, then verify decisive facts directly.