Free tool
Genset load calculator
- 1Add what you'll runPick a preset, then adjust each item
- 2We add the motor surgeThe biggest motor starting, on top of the rest
- 3Get your size and modelsRounded up to a rating dealers stock
What are you powering?
Start from a preset, then adjust — every wattage is editable if you know your equipment's actual nameplate rating.
The method
How genset sizing actually works
A genset has to cover two different numbers, not one: the steady load once everything is running, and the brief surge when a motor-driven appliance — an AC compressor, a water pump, a lift — starts up. Most oversizing or undersizing mistakes come from calculating only one of these.
Running load is straightforward: add up the wattage of everything that will be on at the same time, then convert to kVA by dividing by the power factor (0.8 is standard for a diesel genset).
Starting load is where it gets technical. A motor started direct-on-line draws several times its running current for a second or two. The genset needs enough headroom to supply that surge on top of whatever else is already running — usually just the single largest motor in your setup, since it's rare for every motor to start in the same instant.
This calculator runs both numbers and recommends whichever is higher, rounded up to a genset size that's actually manufactured — see the standard sizes table below.
Typical appliance wattages
Starting points used in the calculator above — every value is editable if you know your equipment's actual nameplate rating, which is always more accurate than a typical figure.
| Appliance | Typical watts | Motor load |
|---|---|---|
| LED bulb / tube light | 15 W | — |
| Ceiling fan | 75 W | — |
| Pedestal / table fan | 60 W | — |
| Exhaust fan | 50 W | — |
| Split AC — 1 Ton | 1200 W | Starting surge |
| Split AC — 1.5 Ton | 1650 W | Starting surge |
| Split AC — 2 Ton | 2200 W | Starting surge |
| Window AC — 1.5 Ton | 1700 W | Starting surge |
| Refrigerator — single door | 150 W | Starting surge |
| Refrigerator — double door / frost-free | 250 W | Starting surge |
| Deep freezer | 300 W | Starting surge |
| Water cooler | 200 W | Starting surge |
| Microwave oven | 1200 W | — |
| Mixer grinder | 500 W | Starting surge |
| Electric kettle | 1500 W | — |
| Induction cooktop | 2000 W | — |
| Washing machine — top load | 500 W | Starting surge |
| Washing machine — front load | 800 W | Starting surge |
| Iron / press | 1000 W | — |
| Geyser / water heater | 2000 W | — |
| Desktop computer | 200 W | — |
| Laptop | 65 W | — |
| Printer | 300 W | — |
| Photocopier | 1200 W | — |
| LED TV (42") | 120 W | — |
| Water pump — 0.5 HP | 500 W | Starting surge |
| Water pump — 1 HP | 1000 W | Starting surge |
| Borewell submersible pump — 3 HP | 2500 W | Starting surge |
| Borewell submersible pump — 5 HP | 4000 W | Starting surge |
| Irrigation pump — 7.5 HP | 6000 W | Starting surge |
| Welding machine (single phase) | 4000 W | — |
| Air compressor — 2 HP | 1800 W | Starting surge |
| Lathe machine — 2 HP | 1800 W | Starting surge |
| Small goods lift / hoist | 3000 W | Starting surge |
Frequently asked questions
How do I calculate the genset size I need in kVA?+
Add up the running wattage of everything you'll power at once, convert to kVA by dividing by the power factor (0.8 for a diesel genset), then check whether your single largest motor's starting surge (roughly 3x its running watts) pushes the requirement higher than the running total. Add a 20-25% safety margin, then round up to the nearest standard genset size. This tool does all of that automatically.
What's the difference between kW and kVA for a generator?+
kW is real (usable) power; kVA is apparent power, which includes the reactive power that inductive loads like motors and compressors draw. Generators are rated in kVA because that's what the alternator has to supply regardless of the load's power factor. For a typical diesel genset, kW ≈ kVA × 0.8.
Why does motor starting current matter for genset sizing?+
A motor started direct-on-line (the common wiring method for pumps, compressors, and AC units) draws roughly 3x its running current for a second or two at startup. If your genset can't supply that surge, the voltage sags, the motor fails to start, and in some cases the genset trips out. Sizing only for steady running load is the most common reason a 'correctly sized' genset can't actually start the equipment it was bought for.
What safety margin should I add when sizing a genset?+
20-25% above your calculated requirement is standard for stable loads with no near-term expansion plans. For critical loads (hospitals, data centers, cold storage) or if you expect to add equipment within a couple of years, 30-40% is safer — it's cheaper to buy the right size once than to replace an undersized set later.
Can I use a smaller genset if I don't run everything at once?+
Yes — size for your actual simultaneous load, not the sum of every appliance you own. This tool's preset lists are a starting point; remove anything that won't realistically run at the same time (e.g. a borewell pump and a washing machine that are never used together) to avoid oversizing.
What happens if I undersize my diesel genset?+
Best case, it trips on overload and shuts down when a motor starts. Worse case, it runs chronically overloaded, which shortens engine life, increases fuel consumption per unit of useful output, and can damage connected equipment through voltage instability. Undersizing is a false economy — the fuel and maintenance cost of running an overloaded set typically exceeds the price difference to the next standard size up.