Generator size calculator guide
What does this calculator help you figure out?
A generator that is too small trips on the first refrigerator start; one that is too large costs more, runs inefficiently, and can foul its exhaust with soot at light load. This calculator sizes a portable or standby set from the watts that run continuously, the extra surge the largest motor draws in the second it starts, and a safety margin, then converts kilowatts to the kVA rating printed on the nameplate using a power factor. It also estimates litres of fuel per hour from the running load and a typical specific consumption for diesel, petrol, or LPG, and turns that into a cost per hour and per day at your fuel price. Homeowners in load-shedding areas, small shops, and site managers use it to choose between a 5 kVA petrol portable and a 7.5 kVA diesel standby before asking a dealer. Derating for altitude and heat, and the tank size needed for a full day, are for you to apply on top.
How is the result calculated?
Peak demand (W) = running load + largest motor starting surge. Minimum kW = peak demand × (1 + margin ÷ 100) ÷ 1,000. Minimum kVA = kW ÷ power factor. Fuel (L/h) = running load (kW) × specific consumption, using 0.3 L/kWh diesel, 0.45 petrol, 0.5 LPG. Cost per day = L/h × fuel price × hours per day.
Worked example
Worked example: a house needs 3,000 W of lights, fans, a television, and a refrigerator running, and the largest motor starting is a 1 hp water pump with a 1,500 W surge. Peak demand is 3,000 + 1,500 = 4,500 W. A 25% margin makes the minimum rating 4,500 × 1.25 = 5,625 W, or 5.6 kW, and at a power factor of 0.8 that is 5.6 ÷ 0.8 = 7.0 kVA. Fuel is estimated on the running load only: 3 kW × 0.3 L/kWh = 0.9 litres of diesel per hour, which at 1.20 per litre costs 0.9 × 1.20 = 1.08 per hour and 1.08 × 4 = 4.32 for a 4-hour run. The calculator displays 7 kVA.
Units and conversion notes
Running and starting loads are in watts; use the wattage on the appliance label or a plug-in meter, and take the motor surge from the locked-rotor amps or LRA on the compressor or pump plate multiplied by the supply voltage. Power factor is a ratio between 0.5 and 1 with no unit. Fuel price is per litre in whatever currency you enter; if you buy petrol by the gallon, divide by 3.785 first. The result is kVA, the unit on the generator nameplate, alongside the kW figure some brands quote.
What does the result mean?
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 running load, largest motor starting surge, safety margin, power factor, fuel, fuel price, run hours per day, market / jurisdiction, effective date, data source, update owner and review date, so start there if the generator size calculator returns something you did not expect.
Common mistakes to avoid
Good to know: add only the largest surge, not every motor’s surge, because the fridge, pump, and air conditioner rarely start in the same second unless power returns to all of them at once, in which case stagger the breakers. Do not read the fuel figure as full-load consumption; it assumes the set is loafing at the running load, and a generator held below about 30% load burns more fuel per kWh and, on diesel, builds up unburnt fuel in the exhaust. Check whether a brand quotes standby or prime rating: the standby figure is for occasional use, not for running eight hours a day.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Common questions
Frequently asked questions
What size generator do I need to run a house?
Add the watts that run at once, then add the surge of the largest motor. A modest home at 3,000 W running with a 1,500 W pump surge needs 4,500 W peak, and with a 25% margin that is 5.6 kW or 7 kVA. Adding a 1.5 ton air conditioner, about 1,800 W running and 4,000 W or more starting, pushes most households to 10 to 12 kVA.
What is the difference between kW and kVA on a generator?
kVA is apparent power, kW is real power, and the ratio is the power factor. Generator sets are rated at 0.8, so a 7 kVA set delivers about 5.6 kW to a mixed household load. If your load is almost entirely heaters, kettles, and incandescent lamps, the power factor is close to 1 and the set can deliver nearly its full kVA in watts.
How much starting surge does a refrigerator or air conditioner draw?
Single-phase induction motors pull two to three times their running watts for under a second while the rotor spins up. A 150 W refrigerator compressor can demand 400 to 600 W at start; a 1.5 ton non-inverter air conditioner at 1,800 W can demand 4,000 to 5,000 W. Inverter-driven air conditioners ramp up softly and need far less.
How much fuel does a 7 kVA diesel generator use per hour?
At about half load, roughly 0.9 to 1.2 litres per hour; at full load closer to 1.6 to 1.8 litres. Small diesel sets burn about 0.3 litres per kWh produced when loaded reasonably, so multiply the kW you actually draw by 0.3. Petrol sets use about 0.45 L/kWh and LPG about 0.5 L/kWh of liquid gas.
Does altitude or heat reduce generator output?
Yes. Thinner air and hot intake air both reduce engine power. A common rule for naturally aspirated engines is about 3.5% less output for every 300 m above roughly 1,000 m, and about 1% for every 5.5 °C above 40 °C ambient; the alternator also derates in heat. Buy one size larger in a hill station or a hot, enclosed generator room.
Standby or portable generator for load shedding?
A portable petrol or diesel set of 3 to 7 kVA is cheaper and can be wheeled out, but it needs manual starting and refuelling and must sit outdoors. A standby set of 7 kVA or more sits on a pad, starts itself through an automatic transfer switch within seconds, and is built for many hours a day. Where cuts are daily, the standby set usually wins on convenience and engine life.
