Formula
Usable watt-hours are amp-hours times voltage times efficiency:
hours = amp-hours × volts × efficiency / load watts
The load must also be no more than volt-amps times the power factor. A milliamp-hour gadget battery stays on the battery runtime page.
Assumptions
- The battery is fully charged.
- The load is steady.
- The power factor is the one the UPS maker states for its watt rating.
Worked example
A 100 Ah, 12 volt battery at 80% efficiency, carrying 400 watts on a 1,000 VA unit at 0.8 power factor, runs 2 h 24 min.
Input definitions
- Volt-amps
- The UPS rating. Its watt limit is that rating times the power factor.
- Power factor
- The share of the volt-amp rating that is available as watts.
Limitations
This is a planning estimate. The rate, sun hours, and losses are numbers you type. It is not a utility bill, a solar design, or a room-by-room load calculation.
Questions
Why can a 1,000 VA unit reject a 900 watt load?
At a power factor of 0.8 the watt limit is 800. The page will not invent runtime for a load the rating cannot cover.
Is this a phone power bank?
No. Power banks use milliamp-hours on the battery runtime page. This page is a UPS battery in amp-hours, with a volt-amp limit.