Buy an inverter too small and it trips; too big and you overpay. Sizing is simple arithmetic once you know two things: how much load you want to run, and for how long. Here are the formulas.
Step 1 — add up your load (watts)
List the appliances you want on backup and their wattage. A rough guide:
- LED bulb: ~9 W • Ceiling fan: ~70 W • LED TV: ~90 W • Laptop: ~65 W • Fridge: ~150–200 W (with surge on start) • Water pump: ~750 W+
Add them up for your total running watts. Be realistic about what runs at the same time.
Step 2 — inverter size (VA)
Inverters are rated in VA (volt-amperes), not watts. Convert using the power factor (typically ~0.8):
VA = Total Watts ÷ 0.8
So an 800 W load needs about 1000 VA. Add headroom for surge (motors, fridges, pumps draw several times their running watts at startup) and for future additions — sizing ~20–30% above your steady load is sensible.
Step 3 — battery capacity (Ah)
Battery capacity in amp-hours (Ah) determines how long you get backup:
Battery Ah = (Load in Watts × Backup hours) ÷ (Battery Voltage × efficiency)
Using a 12 V battery and ~0.8 efficiency, to run 400 W for 3 hours: Ah = (400 × 3) ÷ (12 × 0.8) ≈ 125 Ah. So a 150 Ah battery comfortably covers it.
For longer backup you either raise the Ah or use multiple batteries (which usually means a higher-voltage inverter, e.g. 24 V/48 V).
Matching battery voltage to the inverter
- Small inverters run on one 12 V battery.
- Larger inverters need 24 V or 48 V — i.e. two or four batteries in series. The inverter's rated system voltage tells you how many.
Sine wave matters
- Pure sine wave output is clean and safe for sensitive electronics (computers, medical devices, modern appliances).
- Modified/square wave is cheaper but can cause hum, heat or issues with some devices. For a home with electronics, pure sine wave is the safer choice.
Round up, don't shave. An inverter running near its limit runs hot and dies young; a battery drained too deep every day ages fast.
Quick recap
- Total the watts you'll run together.
- VA = Watts ÷ 0.8, plus surge/headroom.
- Ah = (Watts × hours) ÷ (Volts × 0.8).
- Match battery count to the inverter's system voltage.
- Prefer pure sine wave for electronics.
Get these four numbers right and the rest is just choosing quality components. Our companion guide compares lithium vs tubular lead-acid batteries — the choice that most affects lifespan and cost.