Inverter size calculator: VA rating, batteries and backup time
This inverter size calculator works out what a home or shop inverter needs to be: list the appliances you want to keep running in a power cut, and it adds up the load, converts watts to the VA rating that inverters are sold by, checks the starting surge of motors such as a fridge or a water pump, and then sizes the battery bank — amp-hours, number of batteries and how many hours of backup you will actually get.
It answers the questions people ask in a shop — is 900 VA enough for a fridge, how many 150 Ah batteries for four hours — and shows every step of the working. Every number — appliance watts, power factor, margin, depth of discharge and efficiency — is editable.
How to use the Inverter Size Calculator
- Edit the appliance rows or pick items from Add typical appliance; enter the running watts from each label and the quantity.
- For motors (refrigerator, pump, mixer, fixed-speed AC) keep a starting-watts figure; for lights, fans and electronics leave it equal to the running watts.
- Set the power factor (0.8 by default) and a safety margin (20–25 % is common).
- Enter the backup time you want, the battery type and the size of each 12 V battery; leave the system voltage on Auto or choose 12, 24, 36 or 48 V.
- Read the recommended VA, the battery Ah, the number of batteries and the real backup time; copy the summary to take to the dealer.
Inverter Size Calculator features
Watts to VA
VA = watts ÷ power factor, plus your safety margin, rounded up to the next common inverter rating from 600 VA to 10 kVA.
Starting surge check
The largest motor’s extra starting watts are added on top of the running load (or every motor’s, if they can start together), so the inverter does not trip when the fridge compressor kicks in.
Battery Ah and count
Battery Ah = Wh ÷ (volts × depth of discharge × efficiency), split into batteries in series for 24, 36 or 48 V and parallel strings for capacity.
Real backup time
Shows the hours the bank you would actually buy delivers at full load — often more than you asked for, because batteries come in fixed sizes.
Typical appliance list
Thirty-odd household items with typical running and starting watts, from a 30 W BLDC fan to a 2 ton AC — always editable.
Formulas used by the inverter load calculator
- Running load = Σ (watts × quantity). Defaults: 3 fans (225 W) + 6 LED lights (60 W) + TV (60 W) + router (15 W) + fridge (150 W) = 510 W.
- Continuous VA = 510 ÷ 0.8 = 637.5 VA; with a 25 % margin, 796.9 VA.
- Surge: the fridge starts at about 450 W, 300 W more than it runs, so the peak is 810 W = 1,012.5 VA. The larger figure decides: the next common rating is 1,100 VA (800 VA if you switch the surge check off).
- Battery: 510 W × 3 h = 1,530 Wh. At 12 V, 80 % depth of discharge and 85 % efficiency: 1,530 ÷ (12 × 0.8 × 0.85) = 187.5 Ah → two 150 Ah batteries in parallel, which actually give 4.8 hours.
Typical choices for Indian homes
| Load | Running watts | Inverter | Battery (3 h) |
|---|---|---|---|
| 2 fans, 4 LED lights, TV | about 250 W | 600–700 VA, 12 V | 1 × 100 Ah |
| 3 fans, 6 lights, TV, router, fridge | about 510 W | 1,100 VA, 12 V | 2 × 150 Ah |
| 4 fans, 10 lights, TV, fridge, 0.5 HP pump | about 1,000 W | 2,500 VA, 24 V | 2 × 2 × 150 Ah |
| Above plus a 1.5 ton inverter AC | about 2,500 W | 5,000 VA, 48 V | 4 × 2 × 150 Ah |
Things to check before you buy
- Surge rating: a datasheet gives continuous VA and a short surge figure; motors draw a starting current several times their running current for a fraction of a second. Fixed-speed ACs and large pumps are the usual cause of tripping.
- Depth of discharge: tubular lead-acid batteries are commonly rated for cycle life at 80 % depth of discharge; draining only 50 % makes them last longer. LiFePO4 handles 90 % routinely. The battery runtime calculator adds Peukert’s correction for lead-acid at high currents.
- Charging: a bigger bank needs more charging current and longer mains or solar hours to refill — size the charger or the solar panels to match.
- For a whole-house standby supply with heavy motors, compare with a generator size calculator; for the sanctioned load of the house itself use the electrical load calculator.
Inverter Size Calculator FAQ
How do I calculate the inverter size for my home?
Add the watts of everything you want to run at the same time, divide by the power factor (0.8) and add a 20–25 % margin; if you run a motor such as a fridge or pump, make sure the rating also covers its starting surge. The inverter size calculator does this and rounds up to a common VA rating.
What is the difference between VA and watts on an inverter?
Watts are real power; VA (volt-amperes) is apparent power, which includes the reactive current drawn by motors and electronics. With a power factor of 0.8, a 1,000 VA inverter supplies about 800 W.
How many batteries do I need for my inverter?
Battery Ah = load watts × backup hours ÷ (battery voltage × depth of discharge × inverter efficiency). For 510 W for 3 hours at 12 V, 80 % DoD and 85 % efficiency that is 187.5 Ah, so two 150 Ah batteries. A 24 V system needs batteries in pairs, 48 V in fours.
Can an inverter run a refrigerator or an AC?
Yes, if the inverter covers the starting surge. A fridge compressor briefly draws about three times its running watts; a fixed-speed 1.5 ton AC can start near 4,500 W. Inverter ACs start softly, so a 5 kVA, 48 V system with a large battery bank can run one, but backup time is short.
How long will my inverter battery last?
Backup hours = battery Ah × volts × depth of discharge × efficiency ÷ load watts. Two 150 Ah 12 V batteries (300 Ah) at 80 % DoD and 85 % efficiency run 510 W for about 4.8 hours.
Is this calculator accurate enough to buy from?
It gives the standard sizing arithmetic with typical values. Real appliance watts, battery age and temperature change the result, so confirm the datasheet ratings and let a qualified electrician install it.