Generator Size Calculator

Two numbers decide the size

Two numbers decide the size of a generator, and mixing them up is the most common buying mistake. Running watts, sometimes called continuous watts, is what the machine can hold all day. Starting watts, or surge watts, is the short burst it can deliver for a second or two when a motor kicks on.

Every appliance with a motor asks for a burst at startup. A refrigerator that runs on 700 watts can pull 2,200 watts for the half second its compressor starts. A well pump that runs on 1,000 watts can pull 2,000. The generator has to survive that burst, and it has to do it while everything else is already running.

So the rule is simple to state: add up the running watts of everything you want on at the same time, then add the extra starting watts of the single biggest motor. Size to that total plus a margin, and you will not be the person whose generator trips every time the fridge cycles.

Add up everything you want to run

Start with a list. Walk the house and write down what you cannot do without for a few days: the fridge, the furnace blower, a few lights, the router, phones, and one thing that makes the wait bearable. Leave out what you can live without, because every 1,000 watts costs money and fuel.

Two cautions on the numbers. First, a nameplate rating is usually higher than what the appliance draws in normal use, so check the label on the back or the manual rather than a rule of thumb. Second, anything that heats with a resistance element, like a space heater, a coffee maker or an electric water heater, draws its full rating the whole time it is on, with no surge and no duty cycle.

Motors pull 2 to 3 times their rating at startup

Motors are the reason a generator has two ratings at all. An induction motor pulls a locked-rotor current for a fraction of a second as it comes up to speed, and that current is two to three times the running current. The bigger the motor, the bigger the multiple and the longer the burst.

This matters because the burst is not averaged out. If the fridge, the furnace and the well pump all decide to start in the same second, the loads stack. In practice they do not, which is why the sizing rule counts only the largest motor at startup rather than all of them.

Put a number on it. The table in the previous section shows the pattern: a refrigerator compressor starts at about 3.1 times its running watts, while a furnace blower, a sump pump and a garage door opener start at about 2 times. The bigger the motor and the heavier the load it turns, the larger the multiple and the longer the burst lasts.

Soft-start kits change the math. A soft starter ramps a compressor or a pump up over a second instead of slamming it on, which can cut the startup burst by half or more. On a house with a big AC or a deep well pump, a $300 soft starter can be the difference between a 7,500-watt unit and an 11,000-watt one, and it pays for itself at the fuel pump.

Worked example: a 3,200-watt load

Here is a real list for a small house in a cold climate. Everything on it runs at once during an outage, and the fridge is the biggest motor.

The fridge runs on 700 watts and starts at 2,200, so its extra starting load is 1,500 watts. Add that to the 3,200 running watts and the surge the generator has to survive is 4,700 watts.

Now the margin. Multiply both figures by 1.25: 4,000 watts of running capacity and 5,875 watts of surge. Portable units list a surge rating about 20% above their running rating, so a 5,000-watt unit with a 6,000-watt surge covers the 5,875 target with a little room. That is the answer: 5,000 running watts, not 3,200 and not 8,000.

Run the same math on a smaller setup, a fridge, a sump pump, lights and charging at 1,800 running watts with a 1,000-watt pump start. The surge is 2,300, the target after headroom is 2,875, and a 2,500-watt unit with a 3,000-watt surge handles it. On a bigger house with 8,000 running watts and a 3,000-watt pump start, the target lands at 12,500 watts of surge and the recommendation moves to 10,500 watts or the next size up.

Why the 25% headroom is not padding

The 25% margin is not padding for the salesman. It covers four things that all show up in the first hour of an outage.

• Ratings are optimistic. A 5,000-watt generator makes 5,000 watts at sea level on a 60 degree day. At 5,000 feet of elevation it makes about 15% less, and at 100 degrees it derates again.
• Loads creep. The list you wrote in July grows in January, when the space heater and the furnace blower both want to run.
• Age costs watts. A unit with 400 hours on it and a dirty air filter will not hit its sticker number.
• Starting is not clean. Voltage sags during a motor start, and electronics on the same circuit notice. Margin keeps the lights steady.

Twenty percent is the minimum sensible margin and 30% is comfortable for a house with a well pump. Below 10% you are buying a unit that will nuisance-trip on the first cold morning, and nuisance trips are how people end up running extension cords to a generator at 2 a.m.

Portable, inverter or standby

Three kinds of generator cover most homes, and the size you need usually points at one of them.

Portable open-frame. 3,000 to 8,500 watts, loud at 72 to 78 dB, burns gasoline, and needs to sit outside. Cheapest per watt by a wide margin. Fine for a fridge, lights, a furnace blower and a well pump.

Inverter portable. 2,000 to 7,000 watts, quiet at 52 to 62 dB, clean power that will not upset a furnace control board or a laptop, and parallel kits let two units run together. Costs 30% to 50% more per watt and is the right pick for electronics and for camping.

Standby. 7,000 to 22,000 watts, wired to a transfer switch and fueled by natural gas or propane, starts on its own within seconds. Installed cost runs $3,000 to $12,000 depending on size and electrical work. It is the only option that covers a central AC and an electric water heater at the same time.

Match the type to the load before you match the price. A 5,000-watt open-frame unit and a 5,000-watt inverter do the same job on paper and feel completely different in a driveway at midnight.

The transfer switch is not a DIY job

Connecting a generator to house wiring is not a cord-and-outlet job. It needs a transfer switch, and a transfer switch needs an electrician, a permit and an inspection in most jurisdictions.

The transfer switch does one thing: it disconnects the house from the utility before the generator connects. Without it, the generator backfeeds the line and puts 240 volts on wires a utility crew believes are dead. That is the failure mode behind most generator electrocutions, and it is why the illegal double-male cord, often called a suicide cord, is banned everywhere.

There are two common setups. A manual transfer switch with a handful of labeled circuits costs $300 to $800 installed and lets you choose what runs. An interlock kit on the main panel is cheaper and covers the whole panel, but every circuit is live at once, so the load calculation matters more, not less.

Carbon monoxide is the other hazard. A generator belongs outdoors, at least 20 feet from doors, windows and vents, with the exhaust pointed away. Never in a garage, a basement, a crawlspace or an attached shed, even with the door open. The Ready.gov power outage guide and the OSHA electrical safety pages both spell out the clearance and backfeed rules, and they are worth ten minutes before the storm rather than after.

Fuel, runtime and noise at each size

Fuel and runtime are where the size decision turns into a weekly chore. A bigger generator is not just more expensive to buy, it burns more every hour whether or not you use the capacity.

A 5,000-watt unit at half load gets through a 12-hour night on a little under 6 gallons, which is about $20 at $3.40 a gallon. The same house on a 10,500-watt unit burns roughly twice that to do the same work, because a large engine idling at 20% load is inefficient.

Dual-fuel units run on propane with a 10% to 20% loss of rated watts, and a 20 lb tank holds about 4.7 gallons of propane, enough for roughly 5 hours at half load on a 5,000-watt unit. Propane stores for years and does not go stale like gasoline, which matters when the generator sits in a shed for 11 months. Keep 10 gallons of treated gasoline or two full propane tanks, and test the unit under load twice a year. The Department of Energy publishes the efficiency figures behind those derates if you want the detail.