Generator Size Calculator — Running and Starting Watts

Generators are sized by the worst moment, not the average one

When to use this: buying a portable generator for outages, specifying a standby unit, sizing for an RV or a jobsite, or working out whether the generator you already own will carry what you need.

A generator has to survive the instant a motor starts. Everything else about sizing follows from that, and it means two numbers matter rather than one:

• Running watts — continuous draw once everything is up to speed. This sets fuel burn and engine loading.
• Starting watts — the momentary draw as a motor overcomes inertia, three to six times running, lasting a second or two. This sets the size you buy.

The rule that decides the answer

Peak demand = every running watt + the surge of ONE motor.

Not the sum of every surge. Motors do not start together — you start them one at a time, and everything else is already running when they do. This one rule separates a sensible answer from an expensive one.

A house with four motor loads, worked through:

The correct method asks for a 14,000 W generator. Summing the surges asks for 22,000 W — thousands more, heavier, thirstier, and no more capable.

Note also that the surge counted is the increment above running, not the whole starting figure. When the compressor starts it is about to contribute its running watts anyway; adding the full starting number on top of the running total double-counts it.

Running and starting watts by load

Two things stand out. The refrigerator has the highest ratio on the list at 6× — but it is 6× a very small number, so it only matters when it is the only motor you have. The water heater draws 4,500 W continuously and surges not at all, because a resistance element has no inertia to overcome.

The ECM furnace blower is worth noticing too. A conventional blower jumps to three times running; an ECM variable-speed motor ramps, so its surge is negligible. Where you have the choice, that difference can decide the generator.

Our appliance wattage chart carries running, surge and standby figures for around 58 household appliances if your list is longer than the worksheet above.

Soft start: the cheapest way to buy a smaller generator

On almost any house with central air conditioning, the compressor surge is what sets the generator size — it is frequently larger than every other load added together. A soft start module ramps the compressor over a second or two instead of connecting it across the line, cutting inrush by roughly 65%.

On the four-load example above, that takes peak demand from 13,500 W to 10,500 W, and the generator from 14,000 W to 11,000 W. The module costs a small fraction of the difference between those two machines, which makes it one of the few genuinely free lunches in this area.

Derating: the label is a sea-level figure

Engines are rated at sea level in mild conditions. Thin air means less oxygen per cylinder charge and less power; heat does the same. Neither appears on the box.

At 8,000 ft on a 95 °F day a generator delivers 77% of its rating — a 7,500 W machine gives about 5,775 W. If you live somewhere high, size the load first and then divide by the derating factor, which is what this calculator does.

Load factor: the smallest generator that fits is not the right one

Sizing to exactly your peak leaves nothing in reserve. Aim to run continuously at 50–75% of the rating.

• Above 80% there is no headroom for the next motor start, and the engine runs hot with no margin for a load you forgot.
• Below 25% a diesel engine wet-stacks — unburnt fuel accumulates and glazes the cylinder bores, shortening engine life. Petrol engines tolerate light loading better, but you have still paid for capacity you never use.

The summer example — fridge, blower, 3-ton AC and a 3/4 HP well pump at 95 °F — comes out at a 14,000 W generator running at 48% continuous load, which is squarely in the band.

Fuel and runtime

Consumption is close to linear in load above about a quarter output, with a fixed idle component underneath. Running at half load does not halve the burn.

Propane needs roughly 40% more volume for the same energy, but it stores indefinitely without degrading — which is why standby installations favour it and why a jerrycan of petrol in the garage is a worse plan than it looks. Diesel is the most efficient per gallon and the least tolerant of light loading.

Connecting it safely

A generator feeding house circuits must go through a transfer switch or an interlock kit, installed by a qualified electrician. Back-feeding through a dryer outlet — the so-called suicide cord — energises the utility lines outside your house and can kill a lineworker restoring power. It is also illegal essentially everywhere.

A transfer switch is not only a safety device. It is what lets you sequence loads, which is precisely the assumption the surge rule rests on: start the largest motor first, or bring circuits up one at a time, and your peak stays where this calculator put it.

The generator's output circuit needs sizing like any other. Our wire size calculator handles the run from generator to transfer switch, and the load calculator works out the whole-house demand if you are considering a standby unit that carries everything.