Starting loads require an operating scenario
Adding simultaneous running watts to only the largest extra starting load assumes staged starts: if several motors restart together, their combined starting demand can exceed that single-start allowance even when total running demand is unchanged. Starting demand is transient. Continuous and transient ratings and the load-management arrangement are separate equipment limits.
An assumed staged-start calculation
With assumed simultaneous demand of 3,000 W and a further 2,000 W for the largest staged start, the modeled requirement is 5,000 W, and applying a 20% headroom factor increases the mathematical total to 6,000 W. That factor is assumed. Neither the total nor the allowance establishes a manufacturer’s rating or a code requirement.
Current and power factor
In the single-phase model, apparent power is voltage times current, while real power also includes power factor; an assumed 1,800 W resistive load at 120 V therefore draws 15 A because its power factor is one. A motor is different. Starting demand, wiring, transfer equipment and safe installation need their own specifications.
Fuel decides how long the generator can run
A generator's runtime is its fuel supply divided by its consumption at the load you run. Consumption climbs with the load, so a generator running near its rating burns fuel much faster than one at half load. Estimate the outage length you want to cover, then check that the fuel you can store or the supply you can connect lasts that long at your real load.
A battery answers a different question
A battery stores a fixed amount of energy, measured in watt-hours, and delivers it at a rate limited by its inverter. Runtime is usable energy divided by the load. A 12 volt, 100 amp-hour battery at 80 percent usable depth holds 960 watt-hours, and a 200 watt load drains it in about 4.8 hours.
Batteries suit short outages and quiet operation, and a generator suits long ones, which is why some homes use both.
Transfer equipment matters as much as the generator
A generator should never be connected to a home's wiring without a transfer switch or interlock installed to code. Without one, power can flow back onto the utility lines and endanger line workers, and the generator can be damaged when service returns. The transfer equipment also decides which circuits the generator can power, so choose it together with the load list.
Placement, ventilation and carbon monoxide
Engines produce carbon monoxide, a gas that cannot be seen or smelled and that can be deadly in enclosed spaces. A generator must run outdoors, well away from windows, doors and vents, and a working carbon monoxide alarm inside the house is part of the plan. Nothing about sizing or cost justifies running one in a garage or a basement.
Cost over the life of the system
Buying price is only part of the cost. Add installation, the transfer equipment, fuel, regular maintenance and the cost of exercising the engine so it starts when needed. Compare a smaller generator with a careful load plan against a larger one that covers everything, and the smaller one often costs far less for the outages that happen.
When a simple sizing is not enough
A calculator gives a reasonable starting figure. Large homes, three-phase equipment, sensitive electronics and code questions call for a licensed electrician's load calculation. Use the numbers here to have a better conversation, and let the professional confirm the final size, the transfer equipment and the permit.
Assumed inputs and calculated outputs
Generator Size Calculator: input sensitivity
| Running watts of everything on together | Minimum generator size (watts) | With 20% headroom (watts) |
|---|---|---|
| 2,400 | 4,400 | 5,280 |
| 2,700 | 4,700 | 5,640 |
| 3,000 | 5,000 | 6,000 |
| 3,300 | 5,300 | 6,360 |
| 3,600 | 5,600 | 6,720 |
Open the Generator Size Calculator to use your own numbers →
What goes wrong
How to size a backup generator: common errors
| The mistake | Why it misleads | Do this instead |
|---|---|---|
| Sizing to the total of every appliance in the house | Most appliances never run at once, and the total leads to a generator far bigger and costlier than the real need. | Capacity depends on the loads running together and their starting requirements. |
| Ignoring starting surge | A motor's starting draw can be several times its running draw, and a generator sized to running watts alone trips or stalls when it starts. | Find the starting watts on the label or in the manual and add the largest one to the running total. |
| Using the wattage on a motor's label as its full current | A motor's power factor is below one, so it draws more current than watts alone imply. | Convert to amps with the power factor and check circuits and transfer equipment against the current. |
| Running a generator in a garage or near a window | Carbon monoxide builds up faster than people expect and it cannot be detected without an alarm. | Run it outdoors, well away from openings, and keep working carbon monoxide alarms inside. |
How to size a backup generator: worked calculation
Using the Generator Size Calculator worked example, moving Running watts of everything on together from 2,400 to 3,600 changes the Minimum generator size (watts) from 4,400 to 5,600.
| Input | Value | Definition |
|---|---|---|
| Running watts of everything on together | 3,000 | Entered example assumption. |
| Extra surge of the largest motor at start | 2,000 | Motors draw far more when they start. Check the label or manual for the starting wattage. |
| Minimum generator size (watts) | 5,000 |
|---|---|
| With 20% headroom (watts) | 6,000 |
| Running watts of everything on together | Minimum generator size (watts) | With 20% headroom (watts) |
|---|---|---|
| 2,400 | 4,400 | 5,280 |
| 2,700 | 4,700 | 5,640 |
| 3,000 | 5,000 | 6,000 |
| 3,300 | 5,300 | 6,360 |
| 3,600 | 5,600 | 6,720 |
Which input moves the minimum generator size (watts) most in the generator size calculator?
Ranked by how far each moves the minimum generator size (watts) across the range tested: running watts of everything on together (600, 12%) and extra surge of the largest motor at start (400, 8.0%).
If I double running watts of everything on together in the generator size calculator, does the minimum generator size (watts) double?
Doubling it from 3,000 to 6,000 takes the minimum generator size (watts) from 5,000 to 8,000, which is 1.60 times the worked-example figure. So it grows, but by less than double. Halving it to 1,500 gives 3,500.
How to size a backup generator: points to carry forward
- Size to an essential-load list, not to the whole house, and the generator gets smaller and cheaper.
- Add every running load together, then add only the largest starting surge on top.
- Convert loads to amps to check that circuits, cords and transfer equipment can carry them.
- Fuel supply and load together set runtime, and consumption rises steeply as the load approaches the rating.
- Never connect a generator to house wiring without proper transfer equipment, and never run one indoors.
How do I calculate what size generator I need?
Add the running watts of everything you want on at once, then add the largest starting surge from any single motor, and add a margin of about 20 percent. The generator size calculator does the sum from those two figures.
What is the difference between running watts and starting watts?
Running watts are what a load draws continuously. Starting watts are the brief, higher draw when a motor starts. The starting figure matters for sizing because a generator must survive it.
Will one generator run my whole house?
It can, if it is large enough and wired through a transfer switch that covers every circuit, but the cost is much higher than for an essential-load plan. Many households power only a few circuits.
How long will a battery last compared with a generator?
Battery runtime is usable watt-hours divided by the load, so it is short at high loads. A generator runs as long as it has fuel, which makes it the better choice for outages of days.
Do I need a transfer switch?
Yes for any connection to house wiring. It prevents power flowing back to the utility lines and protects the generator and the people repairing the grid. An electrician installs it to code.
Can I run a generator in my garage with the door open?
No. Carbon monoxide can still reach dangerous levels, and manufacturers and safety authorities say to run generators outdoors and away from openings.
Does altitude or temperature change how much a generator delivers?
Yes. Engines lose power at high altitude and in extreme heat, so a generator sized with no margin may fall short in those conditions. The manual lists the derating.
Should I buy a bigger generator than I need?
A margin of about 20 percent above the minimum is sensible. Much larger costs more to buy and to fuel, and it runs inefficiently at light loads.