How do you calculate a home load for a whole house generator?

Question

How to total the house load and what size generator to handle it?

Answer 1

For a manually switched generator, you can size based on critical loads:

• Refrigerator/freezer
• Basic lighting (doesn't take much with LEDs)
• Internet equipment
• Sump pump
• HVAC - depending on environment and type of system, this can be anywhere from 0 (e.g., Empire furnace) to several hundred Watts (air handler with gas furnace) to several thousand Watts (large heat pump or air conditioner) or more (resistance heat). How critical this is depends on typical temperature range. Biggest concern usually is providing enough heat during a winter power failure to be comfortable or at least to prevent frozen pipes.

For an automatic switch, the concern is that everything running at any point in time might be running when the switch happens. There are two logical ways to size that:

Service or Panel Size

If your service is 200A and your main panel is 200A then that's 200A. If your service is 400A/Class 320 and you have two separate 200A panels then it depends on where you put the transfer switch. If you put it with the service feed then it has to switch the entire 400A (big switches!) and be able to provide power to everything. More practical, generally speaking, is to move all the critical loads to one panel and put the transfer switch there. The other panel gets the less critical loads (e.g., EVSE (car charging), hot tub, electric oven).

But that may result in a much larger generator than you actually need.

Load Calculation

An NEC Load Calculation takes into account the size of your house, required circuits (kitchen, bathroom), HVAC (largest of H or C), cooking equipment (with lots of special rules), other fixed appliances, etc. and comes up with a single number. That number is then used to determine your panel size, service size, etc. and also tells you how much extra capacity you have - e.g., if you want to add EVSE to a house that doesn't have any right now.

Logically, a proper Load Calculation is the amount of power you can reasonably ever expect to need - whether from your utility or a generator.

If you are only powering one panel then you do a load calculation for that panel, not for the whole service.

Answer 2

The best would be if the POCO reports the hourly usage to you, on they website.

Then just look at the peak usage and that is the size of the generator you need.