Never Trust Generator Sizing Calculators

It’s tempting.  You’re trying to size a generator for your home, and you come across an online calculator from a generator manufacturer.  You plug in your appliances, what you want to run, and it spits out a recommended generator size.  Unfortunately, these generator size calculators are almost always wrong.  Here’s why, and what you should be doing to get an accurate generator size.

Generator sizing calculators rely on a lot of assumptions.  They assume all toasters behave alike, and all air conditioners obey the same power consumption rules.  But they don’t.  There are huge power consumption differences from one air conditioner to another.

Like any calculator, they take each individual appliance and add them up to get a total amount of power needed to run the home.  Another problem with calculators is they have trouble accounting for which appliances will be on at the same time.  You don’t use your toaster all day, every day.  It’s used only for a few minutes at a time, but a sizing calculator can’t take this into account.  Big appliances like central air conditioners and heat pumps are another problem for online calculators.  The main concern with starting big appliances like these is the starting current required during the first 2 seconds of operation.  Every time your AC clicks on, it gulps down a huge surge of electricity which lasts only for a few seconds.  It then settles down to a much lower figure.  Generator sizing calculators have a hard time with this, because every AC is different depending on its age, tonnage, and other factors like temperature.  As a result, calculators tend to oversize everything they touch.

I’ve seen one sizing calculator claim that a 4-ton air conditioner uses 8,600 watts while running (no figure was given for surge at startup).  But at our company, we routinely measure 4-ton air conditioners using only 4,000 – 5,000 watts.  That’s a big difference.

In the end, an online generator sizing calculator is nothing more than a marketing gimmick.  In the end, it spits out a number that can’t be trusted and recommends a generator that is the most profitable for the company to sell.

A better way to size a generator is to have an expert come out and look at the home’s appliances and suggest a generator size.  Finding an expert who truly understands generators can be hard.  Most electricians don’t have the expertise required.  If you’re in Southwest Florida, we invite you to call Tropical Generator at 239-206-4509 or email us at  We will perform a detailed load study and recommend several options to fit your budget.