Generator Size Calculator
This generator size calculator answers the question of what size generator you need. Enter the total running watts, the largest extra starting watts, the power factor, and a safety margin, and the generator size calculator returns the generator size in kW and kVA plus the peak load. Use it for portable, home backup, and standby generator sizing.
Results
The Generator Size Calculator is part of our AC & Three Phase collection. Browse more ac & three phase to size, convert, and check your work alongside this tool.
What the generator size calculator shows
A generator must cover the power of everything running at once, plus the brief surge when a motor starts. The generator size calculator adds those, applies a safety margin, and gives the size in kW and in kVA. Whether you are asking "generator, what size do I need" for a camper or planning whole house generator sizing, the method is the same.
Generator sizing formula
Generator sizing starts from the peak load:
Only the largest motor's surge is added, because motors rarely start at the same moment. Generators are rated in kW for real power and kVA for apparent power.
Estimate generator size: worked example
5,000 W of running load, a well pump that needs 2,000 W extra to start, power factor 0.8, and a 20 percent margin. The generator size calculator gives:
So choose a generator rated at least 8.4 kW running.
How to use the generator wattage calculator
List the appliances you want to run together and add their running watts. Then find the one with the highest starting surge and enter its extra starting watts. Enter the generator power factor (0.8 is typical) and a safety margin of 20 to 25 percent. The generator wattage calculator returns the size to buy.
Reading the generator load calculator result
Pick a generator whose continuous (running) rating meets the kW figure; the surge rating is a short-term number and should not be used for the running load. The margin covers future additions and keeps the engine from running flat out. A generator load calculator gives a planning figure; for a permanent standby unit, a licensed electrician should confirm the load and the transfer switch.
When to use the backup generator calculator
- Sizing a home standby unit with the backup generator calculator
- Choosing a portable generator for tools or camping
- Planning whole home generator sizing for outages
- Checking a quote with a generator selection calculator
Common generator size calculator mistakes
- Adding every starting surge. Add only the largest one.
- Sizing by surge rating. Match the running rating to the load.
- No margin. Running at 100 percent shortens engine life.
Tips for accurate results
- Read running and starting watts from appliance nameplates.
- Stagger motor starts to reduce the peak.
- Always use a transfer switch for home connections.
Whole house generator sizing
Sizing a generator for a house, whether with a whole house generator sizing calculator or a simple home generator sizing calculator, adds every circuit you want to keep live, then applies the starting surge of the biggest motor, usually central air or a well pump. Load management modules can shed large loads while others run, which lets a smaller generator cover more of the house. For three-phase sites, the three phase power calculator converts phase current and voltage into the total kW the generator must supply.
Standby generator calculator vs portable
A standby generator calculator result usually lands higher than a portable one, because standby units cover more circuits and run longer. Portable units suit a few essential loads through extension leads or an inlet box. A standby generator sizing calculator, a backup generator size calculator, and an emergency generator size calculator all answer the same question: generator, how many watts do I need. Either way, the generator must be connected through a transfer switch so it can never feed power back onto the grid.
Generator wattage estimator: typical loads
| Appliance | Running W | Extra starting W |
|---|---|---|
| Refrigerator | 700 | 1,500 |
| Sump pump (1/2 hp) | 1,000 | 2,000 |
| Window AC (10,000 BTU) | 1,200 | 1,800 |
| Lights and electronics | 500 | 0 |
| Microwave | 1,000 | 0 |
Typical values; check the nameplate.
Generator Size Calculator FAQ
What size generator do I need?
Add the running watts of everything used together, add the largest motor's extra starting watts, and add a 20 percent margin.
What is the difference between running and starting watts?
Running watts keep a device going; starting watts are a brief surge when a motor starts, often two to three times higher.
Should I size by kW or kVA?
Match the kW rating to your load. kVA equals kW divided by the power factor.
How big a generator for a whole house?
Many homes need 10 to 22 kW for essential and whole-house loads, but central air and electric heating push it higher.