INVERTER
Inverter & Battery Size Calculator for Home and Shop
List your fans, lights and appliances and the backup hours you need: get the inverter VA and the battery Ah, with the number of 12 V batteries.
What will run on the inverter
Columns: appliance, watts (on its label), how many.
Assumptions used EDIT ▾
0.8 for home inverters.
Spare capacity for new loads.
About 80% tubular lead-acid, 90% lithium.
Inverter VA = load ÷ power factor, plus headroom, rounded up to a standard rating. Battery Ah = load × hours ÷ (battery voltage × inverter efficiency × usable depth of discharge). Tubular lead-acid batteries last longest when not drained below about 20% left; lithium batteries can use more of their capacity. Real backup falls as batteries age. Do not run ACs, geysers, irons or heaters on a home inverter.
INVERTER SIZE
600VA
For 319 W of load · 12 V system
- Battery12 V, one battery · needs 117 Ah
- 1 × 120 Ah
- Energy for the backup319 W × 3 h
- 957 Wh
How it’s calculated
VA = load W ÷ 0.8 × (1 + headroom); battery Ah = load W × hours ÷ (battery V × 0.85 × 0.8)
Example: 319 W for 3 hours: 319 ÷ 0.8 × 1.2 = 479 VA → 600 VA inverter; 319 × 3 ÷ (12 × 0.85 × 0.8) = 117 Ah → one 120 Ah battery.
Questions
Why is the inverter size in VA and not watts?+
Inverters are rated in VA (volt-amperes). With the usual power factor of 0.8, a 1000 VA inverter can run about 800 W of load.
Can I run an AC or a geyser on a home inverter?+
No. ACs, geysers, irons and heaters draw far more than a home inverter and battery can supply. Keep them off the inverter circuit.
Why does my backup get shorter every year?+
Batteries lose capacity with age and deep discharges. Topping up distilled water on time (for tubular batteries) and not draining them flat helps them last.
Should I choose a bigger battery than the calculator says?+
One size up is sensible if power cuts are getting longer or you will add loads. A bigger battery also works less hard on each cut.
Estimates for planning, based on the assumptions shown. They are not a guarantee of quantities or professional engineering advice; check critical work with a qualified professional.