Generator size calculator: running watts, starting watts, kW and kVA
This generator size calculator tells you what size of generator — in kW and kVA — will run the appliances and motors you choose. It adds the running watts, then allows for the moment a motor starts, when a fridge compressor, water pump or fixed-speed air conditioner draws two to three times its running power for a second or two. The larger of the starting peak and the running load plus a safety margin sets the size.
It then converts kW to kVA at the 0.8 power factor that generator ratings assume, rounds up to a common rating, works out the current for single-phase or three-phase supplies (230/415 V, 220/400 V or US 120/240, 208 and 480 V) and estimates daily energy in kWh and fuel per hour. Use it for a home standby set, a portable generator or a small DG set for a shop or office.
How to use the Generator Size Calculator
- List the loads: edit the rows or use Add typical appliance, with running watts, starting watts and quantity.
- Choose the supply — single-phase or three-phase — and keep the power factor at 0.8 unless your loads are known.
- Set a safety margin (25 % is common) and any derating from the maker’s altitude/temperature table.
- Tick Motors may start at the same time if you cannot stagger them; otherwise the largest motor’s surge is used.
- Enter the running hours, the average load and the fuel use to see kWh per day and litres per hour; copy the summary.
Generator Size Calculator features
Running vs starting watts
Each row has both figures; the calculator adds the largest extra starting watts (or every motor’s) to the running load.
kW to kVA
kVA = kW ÷ power factor, rounded up to the next common generator size from 1 to 2,500 kVA.
Single- and three-phase current
I = VA ÷ V or VA ÷ (√3 × V) for 230/415, 220/400 V and US 120/240, 208 and 480 V supplies, for the load and for the rated set.
kWh and fuel
Daily energy from hours and average load, and fuel from an editable litres-per-kWh figure.
Derating
Divide by (1 − derating) for hot or high sites, using the maker’s own table.
Worked example: home standby generator
Default loads: a fridge (150 W, starts at 450 W), four fans (300 W), ten LED lights (100 W), a TV (60 W), a 1 HP pump (750 W, starts 2,250 W), a fixed-speed 1.5 ton AC (1,500 W, starts 4,500 W) and a microwave (1,200 W). Running load = 4,060 W. The AC has the largest extra starting power, 3,000 W, so the peak is 7,060 W — more than the running load plus 25 % (5,075 W). 7.06 kW ÷ 0.8 = 8.83 kVA, so a 10 kVA set; on 230 V single-phase it carries 38.4 A at that peak, or 12.3 A per phase on 415 V.
Stagger the starts and the AC’s surge alone decides; if everything can start together (after a power cut, for example) the peak rises to 8,860 W and the size to 12.5 kVA. Without the AC, a 7.5 kVA set is enough.
Typical running and starting watts
The starting figures follow the common rule of about three times running watts for induction motors. Real values vary with the motor and its load, so use the nameplate’s locked-rotor current or the maker’s chart when you have it.
| Load | Running W | Starting W (typical) |
|---|---|---|
| Refrigerator | 150 | 450 |
| Water pump 1 HP | 750 | 2,250 |
| Air conditioner 1.5 ton, fixed speed | 1,500 | 4,500 |
| Air conditioner 1.5 ton, inverter | 1,500 | 1,500 (soft start) |
| Washing machine | 500 | 1,500 |
| Microwave, kettle, iron, geyser | nameplate | same as running |
Choosing between kW, kVA and phases
- kW vs kVA: generator sets are sold in kVA at 0.8 power factor, so a 10 kVA set gives 8 kW of real power. Portable generators (especially in the US) are rated in running and starting watts instead — compare with the two watt figures shown.
- Three-phase: above roughly 10–15 kVA, or with three-phase motors, a three-phase set spreads the load; balance single-phase loads across the phases.
- Loading: a diesel set that runs for hours at very light load wears faster, so do not oversize far beyond the margin. Check the share of the rating shown in the working.
- To ride through short cuts without noise, compare with an inverter and battery; for the whole house connected load, use the electrical load calculator.
Generator Size Calculator FAQ
How do I calculate what size generator I need?
Add the running watts of everything you will power, add the extra starting watts of the largest motor, and compare that with the running load plus about 25 %. Take the larger figure, divide by 1,000 for kW and by 0.8 for kVA, then pick the next size up. The generator size calculator shows each step.
What size generator do I need to run a house?
For lights, fans, a TV, fridge and a 1 HP pump, the default figures give 1,360 W running and a 2,860 W peak when the pump starts — a 5 kVA set. Adding a fixed-speed 1.5 ton AC and a microwave takes it to about 10 kVA because of the AC’s starting surge.
How do I convert kW to kVA for a generator?
kVA = kW ÷ power factor. With the standard 0.8 power factor, 8 kW is 10 kVA; 20 kW is 25 kVA.
How is generator size calculated for three-phase?
Size the kVA the same way, then the current per phase is VA ÷ (√3 × line voltage): a 10 kVA set on 415 V carries 13.9 A per phase at full rating.
How much fuel does a generator use per kWh?
Diesel sets at a reasonable load typically use about 0.25–0.35 litres per kWh; small petrol sets and light loads use more. The calculator multiplies your average load by the figure you enter — use the maker’s fuel table for the set you buy.
Can I connect a generator to my house wiring?
Only through a changeover or transfer switch installed by a licensed electrician, so it cannot back-feed the grid. Always run it outdoors, away from doors and windows.