Generator Size Calculator
Find the generator size in kW and kVA for home or business backup. List your appliances, and Advanced adds margin, power factor, three phase, fuel, runtime and inverter batteries.
Typical figures, check the label on your appliance. Running watts are what it uses once going; starting watts are the total it draws for a second or two as a motor or compressor starts. Leave starting the same as running for lights, heaters and electronics.
Homes: 230 V in India, the UK, Europe and the Gulf, 120 V (or 120/240 V) in the US. 400 V and 415 V are three-phase supplies for shops and businesses.
The usual method adds only the biggest starting surge, because motors rarely start in the same second. Pick all at once if everything comes on together when the power returns, for example with no staggered start.
A margin of 20–25% is typical guidance: it leaves room for appliances you add later and keeps the set from running flat out. A power factor of 0.8 is the common rating basis for generators (kW = kVA × 0.8).
Your generator's manual or spec sheet gives its fuel use at different loads. With the spec sheet option, the use at your load is read off the straight line through the 50% and 100% figures (a guess below half load). The load is your running watts on the suggested size.
Depth of discharge (DoD) is how much of the battery you use: about 50% for lead-acid and 80–90% for lithium (LiFePO4) is typical guidance. Inverters are often 85–95% efficient. Uses your running watts.
Watts by appliance
| Appliance | How many | Running | Starting each | Extra to start |
|---|
How your generator size is worked out
Add the running watts, add the biggest starting surge, add a safety margin, then divide by the power factor for kVA.
- 1
Running watts
5 appliances onwatts × how many= 2,275 W
- 2
Starting peak
2,275 W running+ 3,200 W to start(Split AC)= 5,475 W peak
- 3
Safety margin
5,475 W × 1.25= 6.84 kW
- 4
Size in kVA
6.84 kW ÷ 0.8 PF= 8.55 kVA→ 10 kVA set
Read more: the formula, a worked example, typical appliance watts, amps, fuel and batteries
A generator has to cover two things: the running watts of everything that is on together, and the short surge when a motor or compressor starts. Size it for the running load plus the biggest starting surge, add a safety margin, then turn kW into kVA with the power factor, because generators are sold by kVA.
The formula
running watts = sum of (running watts × how many) for everything on
extra to start = starting watts − running watts (for each appliance)
peak = running watts + the biggest extra to start
(or + every extra, if all motors start at once)
needed kW = peak × (1 + margin %) ÷ 1000
needed kVA = needed kW ÷ power factor
size to buy = the next common size at or above that kVA
single phase amps = watts ÷ (volts × power factor)
three phase amps = watts ÷ (1.732 × volts × power factor)
Worked example
A home at 230 V single phase with a fridge (250 W running, 1,250 W starting), 10 LED bulbs (10 W each), 3 ceiling fans (75 W, 150 W starting), a TV (100 W) and a 1.5 ton split AC (1,600 W running, 4,800 W starting). The water pump is switched off.
- Running watts: 250 + 10 × 10 + 3 × 75 + 100 + 1,600 = 250 + 100 + 225 + 100 + 1,600 = 2,275 W.
- Extra to start: fridge 1,250 − 250 = 1,000 W, fan 150 − 75 = 75 W, AC 4,800 − 1,600 = 3,200 W. The biggest is the AC, so the peak is 2,275 + 3,200 = 5,475 W.
- With a 25% margin: 5,475 × 1.25 = 6,843.75 W = 6.84 kW.
- At a power factor of 0.8: 6.84375 ÷ 0.8 = 8.5546875, about 8.55 kVA. The next common size is a 10 kVA set, which gives 10 × 0.8 = 8 kW.
- Running current: 2,275 ÷ (230 × 0.8) = 2,275 ÷ 184 = 12.36 A. The 10 kVA set's full-load current is 10,000 ÷ 230 = 43.48 A.
- Load on the set: 2.275 kW ÷ 8 kW = 28.4% while everything runs.
If all three motors started in the same second, the peak would be 2,275 + 1,000 + 3 × 75 + 3,200 = 6,700 W instead.
kW, kVA and power factor
kW is the real power your appliances turn into light, heat and motion. kVA is the apparent power the generator's alternator has to carry, including current that motors and electronics push back and forth without using. kW = kVA × power factor. Generators are commonly rated at 0.8, so 10 kVA = 8 kW, 62.5 kVA = 50 kW and 125 kVA = 100 kW. Common set sizes in kVA are 1, 2, 3, 5, 6, 7.5, 10, 12.5, 15, 20, 25, 30, 40, 50, 62.5, 75, 100, 125, 160, 200 and up.
Amps on single and three phase
Single phase: amps = watts ÷ (volts × power factor). Three phase: amps = watts ÷ (√3 × volts × power factor), where √3 ≈ 1.732, and that is the current in each phase. A 10 kVA set gives 10,000 ÷ 230 = 43.48 A at 230 V single phase, 10,000 ÷ 120 = 83.33 A at 120 V, and 10,000 ÷ (1.732 × 400) = 14.43 A per phase at 400 V three phase. Pick cables, breakers and a changeover (transfer) switch rated for the current, with an electrician.
Fuel and runtime
Fuel use rises with the load, so use your generator's own figures. Spec sheets usually give litres or gallons an hour at 50%, 75% and 100% load. Example: 1.8 L/h at half load and 3.0 L/h at full load, running at 28.4375% load: 1.8 + (3.0 − 1.8) × (0.284375 − 0.5) ÷ 0.5 = 1.8 − 0.5175 = 1.2825 L/h. A 20 litre tank lasts 20 ÷ 1.2825 = 15.59 hours. Run 6 hours a day, that is 1.2825 × 6 = 7.695 litres a day; at 1.50 a litre it costs 1.92 an hour and 11.54 a day. One US gallon is 3.785 litres.
Diesel sets that run for long spells below about 30% load can build up unburnt fuel (“wet stacking”); many makers advise at least 30–40% load. This is general guidance; follow your generator's manual.
Inverter and UPS batteries
Battery energy needed (Wh) = load watts × backup hours ÷ (depth of discharge × inverter efficiency). Amp-hours = Wh ÷ battery bank volts. For the example's 2,275 W for 4 hours on lithium (80% usable) through a 90% efficient inverter: 2,275 × 4 = 9,100 Wh ÷ (0.8 × 0.9 = 0.72) = 12,638.9 Wh, about 12.64 kWh, or 12,638.9 ÷ 24 = 526.6 Ah at 24 V. The inverter should be about 2,275 × 1.25 ÷ 0.8 = 3,554.7 VA (so a 3.6 kVA or bigger unit), and it must also handle the motors' starting surge. Leaving a big AC off the battery list cuts the size a lot.
Typical running and starting watts
| Appliance | Running | Starting |
|---|---|---|
| Fridge (fridge-freezer) | 250 W | 1,250 W |
| Chest or upright freezer | 200 W | 1,000 W |
| Split AC, 1 ton (fixed speed) | 1,100 W | 3,300 W |
| Split AC, 1.5 ton (fixed speed) | 1,600 W | 4,800 W |
| Split AC, 2 ton (fixed speed) | 2,100 W | 6,300 W |
| Inverter split AC, 1.5 ton | 1,400 W | 1,800 W |
| Window AC, 1 ton (12,000 BTU) | 1,300 W | 3,900 W |
| Air cooler (desert cooler) | 200 W | 400 W |
| Ceiling fan | 75 W | 150 W |
| LED light bulb | 10 W | 10 W |
| LED tube light | 20 W | 20 W |
| TV (LED, 50 inch) | 100 W | 100 W |
| Laptop | 60 W | 60 W |
| Desktop computer + screen | 250 W | 250 W |
| Wi-Fi router | 15 W | 15 W |
| CCTV recorder + cameras | 50 W | 50 W |
| Water pump, 0.5 HP | 750 W | 2,250 W |
| Water pump, 1 HP | 1,100 W | 3,300 W |
| Well pump, 1 HP (submersible) | 1,500 W | 4,500 W |
| Sump pump, 0.5 HP | 800 W | 2,200 W |
| Washing machine | 800 W | 2,000 W |
| Clothes dryer (electric) | 5,400 W | 6,750 W |
| Dishwasher | 1,200 W | 1,500 W |
| Microwave | 1,200 W | 1,200 W |
| Electric kettle | 2,000 W | 2,000 W |
| Clothes iron | 1,200 W | 1,200 W |
| Water heater (geyser, 2 kW) | 2,000 W | 2,000 W |
| Space heater | 1,500 W | 1,500 W |
| Electric oven | 2,400 W | 2,400 W |
| Induction hob (one zone) | 2,000 W | 2,000 W |
| Furnace fan, 0.5 HP | 800 W | 2,350 W |
| Garage door opener, 0.5 HP | 875 W | 2,350 W |
| Air compressor, 1 HP | 1,600 W | 4,500 W |
| CPAP machine | 60 W | 60 W |
| Vacuum cleaner | 1,000 W | 1,500 W |
These are rough typical figures, not measurements: check the label or rating plate on your appliance, or its manual. Starting watts for motors are often two to three times the running watts. Inverter ACs and fridges, and soft starters on pumps, cut the surge a lot.
Safety
- Never run a petrol, diesel or gas generator indoors, in a garage or near windows and vents: the exhaust has carbon monoxide, which can kill.
- Connect a generator to a building's wiring only through a proper changeover (transfer) switch fitted by a qualified electrician, so it can't feed back into the grid.
- Local rules on generator size, noise, fuel storage and earthing differ by country and city. Check them before you buy.
Common questions
What size generator do I need?
Add up the running watts of everything you want on at the same time, then add the extra watts the biggest motor needs for a second or two to start. Add a safety margin of about 20–25%. A fridge, 10 LED bulbs, 3 ceiling fans, a TV and a 1.5 ton AC run on 2,275 W, peak at 5,475 W when the AC starts, and need about 6.84 kW, which is 8.55 kVA at 0.8 power factor, so a 10 kVA set.
What is the difference between kW and kVA?
kW is the real power your appliances use. kVA is the apparent power the generator has to supply, which includes the part that motors and electronics push back and forth. kW = kVA × power factor. Generators are commonly rated at a power factor of 0.8, so a 10 kVA set gives 8 kW.
What are starting watts?
Motors and compressors (fridges, ACs, pumps, compressors) draw two to three times their running power for a second or two as they start. A generator must cover that surge on top of everything already running, or it will stall or trip. Lights, heaters, kettles and TVs don't have a surge.
Can I run an AC on a generator?
Yes, if the generator covers the AC's starting surge on top of the rest of the load. A fixed-speed 1.5 ton split AC typically runs on about 1,600 W but may need about 4,800 W to start. Inverter ACs start much more gently, so they suit smaller generators better.
How many amps does a generator give?
Single phase: amps = watts ÷ (volts × power factor). Three phase: amps = watts ÷ (1.732 × volts × power factor). A 10 kVA single-phase set at 230 V gives 10,000 ÷ 230 = 43.5 A at full load; the same set on three phase 400 V gives 10,000 ÷ (1.732 × 400) = 14.4 A per phase.
How long will a generator run on a tank of fuel?
Divide the tank size by the fuel the generator uses an hour at your load. A set burning 1.28 litres an hour runs about 15.6 hours on a 20 litre tank. Fuel use depends on the load, so check the spec sheet: it usually gives litres or gallons an hour at 50%, 75% and 100% load.
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