What Size Generator Do I Need?

It’s important to have a thought out and correct load calculation to get electric generator sizing right when installing a home backup generator.

An improperly sized power generator is a waste of money if oversized, and it may damage appliances in the home and the generator itself if undersized.

When choosing a home generator, ask for a licensed electrician to perform an accurate load calculation, which should be explained in detail by the electrician while in your home, not over the phone.

For example, let’s assume a load calculation of 36,000 watts, which is typical in a 3,500-square-foot, all-electric house.

Most professional electricians who install generators will load manage the A/C or heat pump compressor, electric emergency heat strip and dryer. These three appliances will account for 16,000 watts of use. By using load management modules, the generator size is reduced to 20,000 watts.

When there isn’t enough power generated to power these heavy draw appliances, the load management systems will not power these appliances. Most of the time, when a generator is sized like this, the appliances function normally.

Where consumers and electricians get into trouble is when they decide to load manage a lot of appliances so they can install a much smaller generator.

So in addition to the loads already managed in the example above, they will add the range and the hot water heater to the load management for a reduction of an additional 10,000 watts. Then, a 10,000- or 11,000-watt electric generator will be offered to the consumer. The savings is about $1,500 to do this.

The problem is that under normal usage none of the appliances that are load managed will ever have power. The backup generator doesn't have enough reserve capacity after powering household lights, the refrigerator and small appliances to ever provide enough power for the larger ones that were load managed. Now we have a consumer with a whole house generator system with no heat or A/C, no hot water and no reserve capacity.

There are many generator sizing calculators available online and from generator manufacturers, but these are grossly inaccurate and tend to show a much smaller load than really exists in your home. They are only for approximations.

A professional electrician should do this load calculation with you, not a technician or an estimator.

The National Electrical Code (NEC) requires that a standby generator must be able to cover at 100 percent whatever loads are placed on a standby generator.

The exception to this is that the code allows for load management whereby loads may be temporarily shed so that the generator does not end up in an overload condition.