How to use this generator size calculator
- Total running watts, add up the wattage of every appliance you want to power at the same time and enter the total.
- Extra starting / surge watts, add the extra burst that motors in fridges, pumps and air conditioners pull when they switch on, or leave it at 0 for resistive loads only.
- Power factor, keep the default 0.8 for a normal mixed home load, or use the figure on your equipment if you have it.
- Headroom margin, leave it at 25% so the generator is not run flat out, or raise it for heavy motor loads.
- Read the required running power, peak power and recommended generator size, they update instantly as you type.
Nothing is submitted or stored: the numbers never leave your device, so you can plan a backup, off-grid or site power setup privately.
How to size a generator
Sizing a generator comes down to two numbers: running watts and starting watts. Running watts are the steady power your appliances use while they operate, so you sum the wattage of everything you would run at the same time. Starting, or surge, watts are the much larger short burst some appliances pull at the instant they switch on. Motors in fridges, freezers, pumps, washing machines and air conditioners can spike to several times their running watts for a fraction of a second, so you add that surge on top before sizing.
Generators are rated in kVA (kilovolt-amps), not just kW, because they supply apparent power, not only the real power that does work. The two are linked by the power factor, which is about 0.8 for a typical mixed household load, so the conversion is kVA = kW ÷ power factor. On top of that you add about 20-25% headroom so the generator is not running flat out, and you should never run a generator at 100% of its rating. This tool does each step for you: it adds your surge to the running watts, divides by the power factor, applies your margin, then rounds up to the next common generator size such as 5, 6.5, 8, 10, 12.5 or 15 kVA.
Common appliance wattages
- LED bulb: 5-12 W
- Laptop: 50-90 W
- Ceiling fan: 60-80 W
- TV (LED): 80-150 W
- Refrigerator (running): 100-250 W (surge 600-1200 W)
- Microwave: 800-1200 W
- Water pump (0.5 HP): 370 W (surge 1100-2000 W)
- Window air conditioner: 900-1500 W (high surge)
- Electric kettle: 1500-2000 W
Add together only the items you would realistically run at the same time. The running watts in this list are for guidance, your appliance labels are the most accurate source, and remember that anything with a motor draws a much higher surge than its running figure suggests, so feed that surge into the calculator.
Note: This calculator gives an estimate and is a planning guide for general use. It does not replace a qualified electrician or the manufacturer's specifications. For permanent installations, transfer switches and anything safety critical, have your design checked by a licensed professional.
Frequently asked questions
What size generator do I need?
Add up the running watts of everything you want to power at once, add the starting surge of any motors, then divide by the power factor (usually 0.8) and add about 25% headroom. For example 3000 running watts becomes roughly 4.7 kVA after a 0.8 power factor and 25% margin, so a 5 kVA generator is the smallest sensible choice.
How many kVA do I need for my house?
It depends on what you run at the same time. A few lights, a fridge and a TV may only need 2 to 3 kVA, while adding a water pump, microwave and air conditioner can push you to 8 kVA or more. Total only the loads you would run together, then size up with headroom.
What is the difference between kW and kVA?
kW is real power that does work, while kVA is apparent power, the total the generator must supply. They are linked by the power factor: kVA = kW divided by power factor. Generators are rated in kVA, so you size in kVA, not just kW.
Why add headroom and starting watts?
Motors in fridges, pumps and air conditioners draw two to six times their running watts for a moment when they start, and a generator run flat out overheats and ages fast. Adding the surge and about 25% headroom keeps the generator stable, cooler and ready for the next motor start.
Can I run a generator at 100% of its rating?
No. Running a generator continuously at its full nameplate rating overheats it, cuts its life and leaves no room for motor surges. Aim to load it to about 70 to 80% of its kVA rating, which is why this tool adds headroom before recommending a size.