How Much Power Backup Do You Need? A Simple VA & Battery Sizing Guide
Turn your appliance list into the exact inverter VA and battery Ah you need, with a backup-hours table.
The most common buying mistake is sizing a backup system by gut feel — ending up short on power or paying for capacity you'll never use. A few minutes of arithmetic gives you the right answer. Here's the method we use with customers, mapped to the real Microtek range.
Watts vs VA — the difference that trips people up
Appliances are rated in watts (real power consumed). Inverters are rated in VA (apparent power delivered). They're linked by the power factor — for a mix of home loads, about 0.7 to 0.8. So an inverter's usable wattage is roughly 70–80% of its VA rating, which is why you can't buy a '500 VA' inverter for a 500 W load.
A simple 4-step sizing method
- 1. Add up the wattage of everything you'll run at the same time during a cut.
- 2. Divide that total by 0.7 to convert watts into the VA the inverter must supply.
- 3. Add a 20–25% margin for startup surges and future additions.
- 4. Round up to the nearest model — e.g. LUXE 1400 (1100 VA) or LUXE 1900 (1650 VA).
Worked example: a typical 2BHK home
Say you want to run 6 LED lights (60 W), 4 fans (280 W), a TV (100 W) and Wi-Fi (30 W) — about 470 W together. Divide by 0.7 to get roughly 670 VA, then add ~25% margin to reach about 840 VA. A 1000–1100 VA pure sine wave inverter like the Microtek LUXE 1400 fits this home comfortably, with a little headroom.
Then size the battery for backup hours
Once the inverter is sized, the battery decides how long the backup lasts. A handy estimate is: backup hours ≈ (battery Ah × 12 × 0.8) ÷ load in watts. Here's how the two common sizes compare:
| Battery | At ~300 W load | At ~500 W load |
|---|---|---|
| 150Ah | about 4 hours | about 2.5 hours |
| 200Ah | about 5.5 hours | about 3 hours |
Approximate backup time (single 12V tubular battery)
Want longer backup? Step up the Ah (200Ah), reduce the running load during a cut, or move to a high-capacity system with a multi-battery bank.
When to oversize (a little)
Leave some headroom if you expect to add appliances, run a fridge occasionally, or face very long outages. But don't dramatically oversize — an inverter run far below capacity wastes money, and a battery bank too large for the inverter won't charge properly. Aim for a comfortable margin, not the maximum.
Where to next
Frequently Asked Questions
Why can't I just match the inverter VA to my total watts?
Because of the power factor, an inverter delivers only about 70–80% of its VA rating as usable watts. Sizing by watts alone leaves you short, so divide your watt total by roughly 0.7 to find the VA you need.
Does running fewer appliances extend backup time?
Yes — backup time is inversely proportional to load. Halving the running wattage roughly doubles how long the same battery lasts, so switching off non-essentials during a long cut helps a lot.
Should I size for startup surge?
Yes. Motors in fridges, pumps and coolers draw a brief surge several times their running wattage. The 20–25% margin in the method covers typical home surges; heavy motor loads may need more.
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