Choose an inverter or UPS and the battery bank that goes with it. Enter the total watts that run together, your biggest motor load and how many hours of backup you want. You get the VA rating, system voltage and battery size.
Planning guide. Add up the watts of everything that will run together. Fridges, pumps and ACs draw a surge when they start, so enter the biggest one as the motor load.
Formulas and assumptions
- Running VA: load watts divided by power factor, plus 25 percent margin
- Surge VA: (load watts plus 2 times the largest motor watts) divided by power factor
- Inverter size: the larger of the two, rounded up to a standard size
- System voltage: 12 V up to 1000 VA, 24 V up to 2000 VA, 48 V up to 5000 VA, then 96 V
- Battery energy: watts times hours, divided by (usable fraction times inverter efficiency)
Example
An 800 W load that includes a 300 W fridge, with 4 hours of backup on lead acid batteries, needs 1,250 VA for running and 1,750 VA for the starting surge, so a 2000 VA inverter on 24 V. The battery is about 7.5 kWh, around 314 Ah at 24 V.
Frequently asked questions
How do I choose an inverter size?
Add up the watts of everything that will run together, divide by power factor to get VA, add a margin, and make sure it can handle the start-up surge of motors such as fridges and pumps.
What is the difference between VA and watts?
VA is the apparent power and watts is the real power. Watts equal VA times the power factor, usually about 0.8 for inverters.
Should I use a 12V, 24V or 48V system?
Higher voltage means lower current, thinner cables and better efficiency for large loads. Small loads under 1000 VA are fine on 12 V.
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