Geek On The Hill

How Big a Whole-House Backup Generator do You Need?

A couple of years ago, I published a post about how I hooked up a portable generator to power my house during power failures. What I didn’t really go into detail about, but should have, was how I determined how powerful a portable generator I needed. I’m going to try to rectify that omission in this post because quite a few people have asked that question.

Before proceeding, I want to encourage you to consult a professional electrician to calculate your generator needs and install the generator. I am not a professional electrician. I’m a DIY guy with more then 50 years experience with wiring; but I still consult professional electricians when I’m uncertain about something, as well as to review my planned work and inspect my finished work.

Also be sure that your generator wiring includes an inlet box of the appropriate amperage rating, an approved transfer switch, or a safety interlock and backfeed circuit breaker designed for your home’s electrical distribution panel.

If you choose the safety interlock method, remember that just because running the generator through the distribution panel enables you to use any electrical device in the house, that doesn’t mean you can use them all at the same time. You’ll have to constantly consider how much wattage the devices you’re using draw, and be careful to not use more power than your generator can provide and your wiring can handle.

Understand How Backup Electric Generators are Rated

The next thing we need to understand when choosing a whole-house backup generator is how their power outputs are rated.

Almost all standby generators and portable generators actually have two ratings: the running wattage and the surge wattage. Simply stated, the running wattage is the maximum wattage the generator can provide on an ongoing basis; and the surge wattage is the maximum wattage the generator can provide for a few moments.

The reason this is important is because many electrical appliances draw more power when they’re first turned on than they do while they’re running. This is especially true of devices that have motors such as central and window air conditioners, refrigerators, well pumps, and forced-air furnaces. Some of these appliances can pull as much as 400 percent more juice when they first kick in than while they’re running.

Devices that have heating elements or that provide light, on the other hand, tend to draw a more consistent load. The burner on an electric range, for example, will draw only the wattage needed to provide the heat the user selects; so the user can avoid a surge draw by not initially turning up the burner any higher than needed. In other words, if you only need the burner on Low, then only set it to Low. Don’t set it to High and then back it down.

Add Up Your Home’s Automatic Device Power Needs

Some essential electrical devices in homes start up automatically when needed, so your backup generator must provide sufficient output wattage to power them all during both their surge and running states.

The most-common high-draw electrical devices that start automatically include well pumps, sump pumps, furnaces or boilers (even if they burn oil or gas, the fans or pumps are electrically powered), electric baseboard heaters, electric hot water heaters, refrigerators, and freezers. The generator you choose should be powerful enough to power all these devices simultaneously.

The best way to determine your devices’ surge and running power draws is to consult the manufacturer’s documentation for the devices. The following are just general guidelines and should not be taken as gospel. The first number is the typical surge draw, and the second number is the typical running draw, all expressed in Watts.

  • Electric Water Heater: 3,000 / 2,800 Watts
  • Non-Electric Furnace Fans: 2,400 / 900 Watts
  • Non-Electric Boiler Pumps: 800 / 300 Watts per heating zone
  • Sump Pump: 1,500 / 800 Watts
  • Electric Baseboard Heaters: 250 / 250 Watts per linear foot
  • Electric Space Heaters: As rated by manufacturer
  • Refrigerator: 2,500 / 800 Watts
  • Freezer: 2,500 / 800 Watts

Well pumps are a special case. Their power draw is highly variable depending on the depth of the well and the size of the pump needed. A shallow-well pump will draw about 1,500 Watts surge and about 800 Watts running; but a deep-well pump can draw as much as 30,000 Watts surge and 15,000 Watts running. Consult your well-drilling contractor or your electrician to determine your well pump’s power draw.

The generator you choose must be able to power all of these devices at both their surge and running draws.

In addition, you should also include the power needs that you personally consider essential. For example, if you work at home and need your computers in order to do that, then you’ll need to include enough juice to power everything computer-related such as modems, routers, printers, monitors, and the computers themselves.

Protecting Computers and Electronics from Power Outages

Unless they have their own batteries (for example, a laptop computer or a tablet), computers and other sensitive electronics should also be protected by high-quality uninterruptible power supplies, especially while on generator power, which tends not to be clean enough for sensitive electronic devices. The UPS will also protect your computers and data between the time the power goes out and the generator is turned on.

Add Up Your Home’s Optional Device Power Needs

Optional devices include devices that you turn on and off manually (such as an electric range, microwave oven, clothes washer or clothes dryer, garage door opener, computer, television, lighting, and so forth); or devices that you can stop from turning on and off automatically (such as your air central or window conditioners).

When calculating the size of generator you need, you should consider which optional devices you would need or like to have available during a power outage in addition to the must-have devices mentioned in the previous section. For most people, this would include the electric range and microwave oven because presumably they’d still want to eat.

The rest are pretty much a matter of what’s important to a particular person or family given their geographic location and lifestyle. For example, if you live in a place where the weather doesn’t get very hot, then maybe you’re willing to forego air conditioning during a blackout. If you live in Death Valley, on the other hand, then maybe not.

The best way to determine the wattage needed to power your optional devices is to consult the manufacturer’s documentation. The following are just rough numbers and should not by any means be considered authoritative. The first number is the typical surge draw, and the second number is the typical running draw, all expressed in watts.

  • Window Air Conditioner (8,000 BTU): 1,200 / 3,400 Watts
  • Central Air Conditioner: Highly variable depending on efficiency and size. For a high-efficiency 24,000 BTU model, 3,500 / 10,000 Watts is probably about average. Older, less-efficient models will draw much more power.
  • Electric Range (Per 8-Inch Burner): 2,200 / 2,200 Watts
  • Electric Range (Per 6-Inch Burner): 1,600 / 1,600 Watts
  • Clothes Washer: 1,200 / 2,400 Watts
  • Electric Clothes Dryer: 6,000 / 8,000 Watts
  • Non-Electric (Gas) Clothes Dryer: 800 / 2,000 Watts
  • Garage Door Opener: 900 / 2500 Watts
  • Computer, Large LCD or LED Monitor, Modem, and Router (Combined): 1,000 / 1,000
  • Laser Printer: 700 / 1,200 Watts
  • Big-Screen LCD or LED Television (without Home Theater Audio): 100 / 100 Watts
  • Lighting: As rated by the manufacturer
  • Home Theater Sound System: Ballpark it by adding 25 percent to the rated wattage

Again, the above are ballpark figures. Don’t take them as gospel.

You don’t need to buy a generator powerful enough to power all of your high-draw optional devices simultaneously. A more reasonable approach is to buy a generator big enough to power all of your essential devices plus your highest-draw optional device, and just make sure to use only one high-draw optional device at a time.

In other words, consider buying a generator powerful enough to power your essential devices plus either the electric range, microwave oven, air conditioner, or electric clothes dryer; and just make sure to use only one of those optional devices at a time. If you do, you’ll trip the circuit breaker on the panel or generator and be in the dark again.

Finally, consider replacing as many devices in your home with high-efficiency ones, such as replacing all of your incandescent bulbs with LED lamps. This will reduce the load on your generator when you use it, save you money in between, and benefit the environment.


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