Formula
The bank's amp-hours are the watt-hours divided by voltage and efficiency:
batteries = (watts × hours) / (volts × efficiency × amp-hours each)
The count rounds up. A kilowatt-hour pack with a depth of discharge stays on the solar battery page.
Assumptions
- The batteries are the voltage and amp-hours you enter.
- They are wired so each string matches that voltage.
- Efficiency is the inverter's, typed as a percent.
Worked example
800 watts for 8 hours from 12 volt, 100 Ah batteries at 85% efficiency needs 7 batteries.
Input definitions
- Amp-hours
- The capacity printed on one battery at the voltage you enter.
- Inverter efficiency
- The share of battery energy that becomes alternating-current 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
Does this design the wiring?
No. It counts batteries. Series and parallel connections, fuses, and cable size are outside the page.
Why is a phone battery a different page?
The battery runtime page starts from milliamp-hours and answers hours. This page starts from hours and answers a battery count.