What the Generator Size Calculator does
This calculator works out the smallest generator that can run your loads: the total running watts, the moment the biggest motor starts while everything else is already on, a running margin, and the power an engine loses at altitude and in heat. It gives the result in kW and in kVA, and estimates fuel use from your generator's own data-sheet figures.
Generators are sized by their worst moment, not their average. A fridge that runs on 700 W can ask for 2,000 W or more for the second its compressor starts, and a set that cannot deliver that stalls or trips.
How to use it
- List every load you need at the same time with its running watts and quantity. Rating plates and manuals give running watts; for motors they often also give starting (surge) watts.
- Enter starting watts for anything with a motor or compressor - pumps, fridges, air conditioners, compressors, power tools. Leave it empty for lights and electronics.
- Set a running margin (25% is common) so the generator is not run flat out for hours.
- Enter your altitude and the hottest temperature it will run in. Open the derating rules to use your manufacturer's figures, which vary by engine.
- For fuel, copy the litres per hour at 50% and 100% load from the data sheet, the hours you expect to run and, optionally, the fuel price.
Reading the results
Largest start is the running load with the single biggest motor starting on top. The calculator assumes motors start one at a time, which is normal once everything is running.
The minimum rating is the larger of running load plus margin and the largest start, divided by what is left after derating. Buy a generator whose continuous (not peak) rating meets it.
Load at that rating shows how hard the generator works on the running load. Diesel sets in particular should not idle for long periods below about 30% load.
Worked example: home essentials during a power cut
The loads are a refrigerator (700 W running, 2,200 W starting), a 1/2 hp sump pump (1,050 / 2,150 W), a 1/3 hp furnace fan (700 / 1,400 W), five 60 W lights and 50 W of router and chargers. Running total: 700 + 1,050 + 700 + 300 + 50 = 2,800 W.
The largest start extra is the fridge's 2,200 - 700 = 1,500 W, so the worst moment is 2,800 + 1,500 = 4,300 W. Running load plus 25% is only 3,500 W, so the start decides the size.
At 30 °C with derating of 1% per 5.5 °C above 25 °C, the engine loses 0.9%: 4,300 / (1 - 0.009) = 4,339 W. The generator needs a continuous rating of at least 4.34 kW, which at power factor 0.8 is 5.42 kVA.
Formulas and scoring rules
- Running load
running = sum(running W x quantity)- Starting scenario
starting = running + max(starting W - running W) over all motor loads- Derating
derate = (altitude above threshold / 300 m) x altitude % + (temperature above threshold / 5.5 °C) x temperature %Defaults of 3.5% per 300 m (about 1,000 ft) and 1% per 5.5 °C (10 °F) are common rules of thumb; manufacturers publish their own.- Minimum rating
kW = max(running x (1 + margin), starting) / (1 - derate); kVA = kW / power factor- Fuel
L/h = fuel at 50% + (fuel at 100% - fuel at 50%) x (load fraction - 0.5) / 0.5Linear between two data-sheet points; without the 50% figure, L/h = full-load fuel x load fraction (optimistic at light load).
Limitations: what the result does not prove
- Starting watts vary with the motor, its age and the voltage. Figures on rating plates or in manuals beat generic tables; a clamp meter with inrush capture beats both.
- Derating differs between engines, fuels and whether the set is turbocharged. Use the manufacturer's derating table where it exists; the defaults are generic rules of thumb.
- Some loads are sensitive to generator power quality. Electronics are safest on an inverter generator or behind a UPS.
- It does not cover wiring. Connecting a generator to a building requires a transfer switch or interlock so it cannot back-feed the grid, installed under local rules by a qualified electrician.
Privacy: where your data goes
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Standards and sources
- US Consumer Product Safety Commission - Portable generator safety
- US DOE Energy Saver - Estimating appliance and home electronic energy use
Frequently asked questions
What size generator do I need to run a house?
It depends entirely on which loads you run together. Fridge, freezer, sump pump, furnace fan and lights typically need 4-6 kW because of motor starts. Adding central air conditioning, an electric water heater or an electric range can push it past 15-20 kW. List your loads to get your figure.
What is the difference between running and starting watts?
Running watts are what an appliance draws once it is going. Starting watts are the brief surge a motor needs to get moving, often two to four times higher and lasting a second or two. A generator must supply the surge on top of everything already running.
Why is the generator bigger than my running load?
Usually because a motor's start-up surge is larger than your running load plus margin. Starting motors one at a time helps; so does a soft starter on a large pump or air conditioner, which can cut its starting demand substantially.
Why does altitude reduce a generator's output?
The engine burns fuel with the oxygen in the air it draws in. Thinner air at altitude holds less oxygen, so a naturally aspirated engine produces less power. Hot air is thinner too, which is why generators also lose power on very hot days.
What is the difference between kW and kVA on a generator?
kW is real power the loads use; kVA is the apparent power the alternator must carry. Standby generators are usually rated in kVA at a power factor of 0.8, so a 10 kVA set delivers 8 kW. Size by kW for the loads and check the kVA rating matches.
How much fuel does a generator use?
Look at the data sheet, which lists consumption at 25%, 50%, 75% and 100% load. Enter the 50% and 100% figures and the calculator interpolates for your load. Fuel use does not fall in proportion to load, so light loads burn more per kWh.
Last reviewed by the A2Z.Tools team against the sources listed above.
