Inverter / UPS Size Calculator (VA)
Inverter / UPS Size Calculator (VA)
Add up to six appliances, mark the ones with a motor, and get the running watts, the VA the inverter must supply, the starting surge and the next standard inverter size up.
Inverter size
8 × 12 W LEDs, 3 × 75 W fans, a 100 W TV and a 150 W fridge, PF 0.8, 25% margin, 3× surge
Running VA, surge VA and the size
- ΣW
- running watts of everything switched on (watts × quantity, summed)
- PF
- power factor of the load; home inverters are rated at 0.8
- S
- starting surge multiple of the one motor assumed to start
- peak
- how far the inverter can exceed its rating briefly, typically ×2
Worked example
8 × 12 W LEDs, 3 × 75 W fans, a 100 W TV and a 150 W fridge, PF 0.8, 25% margin, 3× surge
Running watts = 96 + 225 + 100 + 150 = 571 W
Running VA = 571 ÷ 0.8 = 713.75 VA; with 25% margin = 892 VA
The fridge starts while the rest runs: 571 + 150 × 2 = 871 W = 1,089 VA, which a 900 VA inverter covers on its ×2 peak
Next standard size at or above 892 VA: 900 VA
Typical household loads and whether they surge
| Appliance | Running watts | In the example | Motor? | Contribution |
|---|---|---|---|---|
| LED bulb / tube | 9–20 W | 8 | No | 96 W |
| Ceiling fan (BLDC or induction) | 28–75 W | 3 | No | 225 W |
| LED television | 60–120 W | 1 | No | 100 W |
| Refrigerator (250 L, running) | 100–200 W | 1 | Yes | 150 W |
| Water pump, 0.5 hp | ≈ 500 W | — | Yes | starts at 3× or more |
| Air conditioner, 1 ton (non-inverter) | 1,000–1,500 W | — | Yes | usually needs its own supply |
Sizing an inverter or home UPS
An inverter is rated in volt-amperes, not watts. Volt-amperes are what the inverter has to push through its transformer and transistors; watts are what the appliances turn into light, heat or movement. The two differ by the power factor: a mixed household load of fans, LED drivers and a television has a power factor near 0.8, which is exactly why home inverters are sold at 0.8 — a 900 VA unit is a 720 W unit. The example load of 571 W therefore needs 714 VA before any margin at all.
Margin. Running an inverter at its rating all evening is how inverters die in summer. A 20–25% margin covers the appliance you add next year and keeps the unit out of its hottest region: 571 W becomes 892 VA, and the next standard size is 900 VA.
The surge is the part people get wrong. A motor draws several times its running current for the fraction of a second it takes to get going — a locked rotor can be six times or more, though a refrigerator compressor or a mixer settles quickly and an inverter-driven (BLDC) fan or air conditioner barely surges at all. This page assumes one motor starts while everything else is already running, which is the realistic case in a house. The inverter does not need a continuous rating that covers the surge, because inverters can exceed their rating briefly: Victron’s Phoenix datasheet lists 2,400 W peak against 1,000 W continuous. Divide the starting VA by that peak factor and compare it with the running requirement — whichever is larger sets the size. In the example the running load wins; add a 0.5 hp pump and the surge takes over.
Then size the battery, not the inverter, for the hours you want. The VA rating decides how much you can run at once; the battery decides how long. Work that out with the inverter backup time calculator, and check the battery cable and the inverter’s input fuse for the DC current it will draw. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.
Frequently asked questions
What size inverter do I need for my home?
Add the running watts of everything you want during a cut, divide by the power factor (0.8 for a mixed load), add 20–25% margin and round up to the next standard size. The example — lights, fans, a TV and a fridge, 571 W — needs a 900 VA inverter.
What is the difference between VA and watts?
Watts are real power; VA is the product of voltage and current the inverter must deliver. Watts = VA × power factor, so a 900 VA inverter at PF 0.8 supplies 720 W.
How much inverter capacity does a fridge need?
A 150 W fridge runs on very little, but its compressor draws roughly three times that at each start. With other loads already running, allow the running total plus twice the fridge’s watts as a brief surge, and check that the inverter’s peak rating covers it.
Can I run an air conditioner on a home inverter?
Only on an inverter sized for it: a 1 ton non-inverter AC draws 1,000–1,500 W continuously and much more at start, so a 900 VA home unit cannot. Inverter-type ACs start gently but still need the continuous capacity.
Is a bigger inverter always better?
No. A much oversized inverter costs more, wastes more in standby and runs further from its best efficiency point. Size for what you actually run, plus a margin.
Related calculators
References
- Victron Energy. Phoenix Inverter 1200 VA – 5000 VA datasheet (SinusMax). Continuous output at 25 °C: 1,000 W from the 1,200 VA model and 4,000 W from the 5,000 VA model (VA × 0.8); peak 2,400 W and 10,000 W; maximum efficiency 92–95%; zero-load power 8–35 W.
- Luminous India. Home inverters product range, accessed 22 September 2026: 600, 650, 700, 800 and 900 VA, then 1.1, 1.4, 1.5, 2, 2.5, 3, 3.5, 4, 5, 5.5, 7.5 and 10 kVA.
- Vader R. Energy Unlimited. Victron Energy. Charge efficiency “will range in between 80% and 95%”; battery ageing advances “disproportionately fast” once roughly 80% depth of discharge is exceeded.