Energy
Estimate normal load, peak load, and a practical inverter size from appliance demand.
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Inverter size calculator guide
An inverter has two ratings that matter: the continuous power it can supply and the surge it can tolerate for a few seconds while a compressor or pump motor starts. This calculator takes your normal running load, the peak starting load, and a design headroom percentage, then recommends an inverter size in kilowatts based on whichever load is larger. It is written for anyone choosing a hybrid solar inverter, an off-grid unit, or an IPS for load-shedding backup in Bangladesh, India, Pakistan, or Sri Lanka, where “VA” ratings on the box and a mix of fans, lights, refrigerators, and water pumps make the choice confusing. Add up the running watts of everything that could be on together from the essential-loads calculator; then estimate the peak by adding the start-up surge of the largest motor — typically three to five times its running watts for a refrigerator or pump, and up to three times for a fixed-speed air conditioner. Twenty percent headroom covers wiring losses, hot-weather derating, and future additions.
Recommended inverter (kW) = max(normal load, peak starting load) × (1 + headroom ÷ 100). To compare with a VA rating: VA ≈ W ÷ power factor (0.8 is the usual label assumption).
Worked example with the defaults: a normal load of 2 kW, a peak starting load of 4 kW, and 20% headroom. The larger of the two loads is 4 kW. Apply headroom: 4 × (1 + 20 ÷ 100) = 4 × 1.2 = 4.8 kW, which the calculator displays as the recommended inverter power. In practice you would buy the next standard size, a 5 kW (often labelled 6 kVA at 0.8 power factor) hybrid inverter, and check that its surge rating — commonly 1.5 to 2 times continuous for 5 to 10 seconds — covers the 4 kW start.
Both loads are in kilowatts; divide watts by 1,000, so 2,400 W is 2.4 kW. Headroom is a percentage. The recommendation is in kW of real power. Inverters in South Asia are usually sold in VA or kVA: multiply the kW figure by 1.25 (for a 0.8 power factor) to get the VA to look for, so 4.8 kW ≈ 6 kVA. Small IPS units are labelled in VA directly — 600 VA, 850 VA, 1,000 VA — and deliver about 80 percent of that in watts.
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on normal load, starting surge load, design headroom, market / jurisdiction, effective date, data source, update owner and review date, so start there if the inverter size calculator returns something you did not expect.
Good to know: sizing to the running load alone is the mistake that makes an inverter trip every time the refrigerator kicks in. The opposite error, sizing to the sum of every start-up surge at once, buys a needlessly large unit that idles inefficiently — motors almost never start in the same instant. Confusing VA with W understates capacity by about 20 percent. An inverter’s rating is only half the system: a 5 kW inverter on one 12 V 150 Ah battery will hit the battery’s discharge limit long before its own, so check the battery bank’s continuous current and the recommended DC voltage (12 V up to about 1.5 kW, 24 V to 3 kW, 48 V above that).
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
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How it works
Estimate normal load, peak load, and a practical inverter size from appliance demand. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Normal load = 2 kW · Starting surge load = 4 kW · Design headroom = 20 % → 4.8 kW (recommended inverter power). Change an input above and this example updates with your numbers.
Common questions
Add the running watts of everything you want on at once, then add the largest motor’s start-up surge, and multiply by 1.2. A 2 kW running load with a 4 kW start needs about 4.8 kW, so a 5 kW (6 kVA) inverter. Essentials only — fans, lights, router, fridge — often fit a 1 to 1.5 kW unit.
A 1.5 ton fixed-speed AC draws about 1.8 to 2 kW running and can surge to 5 to 6 kW at start, so you need a 5 kW inverter with strong surge capability or, better, an inverter-type AC which soft-starts and runs comfortably on a 3 kW unit. Add the rest of your load on top.
At the usual 0.8 power factor, 1 kW ≈ 1,250 VA. So a 4.8 kW recommendation corresponds to roughly 6 kVA, and a 1,000 VA IPS delivers about 800 W. Some lithium hybrid inverters quote unity power factor, in which case kW and kVA are equal.
Usually not reliably. A refrigerator running at 150 W may surge to 600 to 900 W for a moment at compressor start, which trips most 600 VA (about 480 W) IPS units. Choose at least 1,000 VA, or a unit with a rated surge above 1,000 W.
A 5 kW inverter draws over 100 A from a 48 V bank and more than 400 A from 12 V, so 48 V is standard at that size. Rules of thumb: 12 V for up to about 1.5 kW, 24 V for up to 3 kW, and 48 V above that.