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I see 20kW backup generators advertised for homes, but this seems like total overkill, or maybe my math is wrong?

According to my energy bill I use about 400 kW hours of electricity per month. So, that would be about 13.3 kW hours per day or 0.5 kW average load. Now, if a generator can generate 20 kW, then it would be total overkill, 40 times what is needed. Am I calculating this wrong?

  • Will you have a battery bank? How big? – Harper - Reinstate Monica Jun 20 '18 at 20:35
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    You can't take your average consumption as an indication of your peak consumption. You may well draw 5 or 10 kW at certain times. – isherwood Jun 20 '18 at 20:42
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    And "20 kW per hour" doesn't really make sense. Watts are an instantaneous measurement of energy consumption. The term "killowatt-hour" refers to 1000 watts being consumed for an hour, not in an hour. – isherwood Jun 20 '18 at 20:43
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You can't average the monthly usage over a number of days because the loads aren't constant. Consider the following two scenarios:

  • A 100W light bulb that's turned on constantly for the whole month (30 days). This uses 100W * 24hr / day * 30 day / month = 72 kWh for the month.
  • A 2400W heater that runs one a day for an hour. This uses 2400W * 1hr / day * 30 day / month = 72kWh.

Doing an average usage as you did, you would incorrectly assume that both scenarios need the same size generator. But, the second needs a bigger generator since the load is bigger (2400W vs 100W). They use the same amount of electricity because the smaller load is on for much longer.

Your house is similar, in that there are some big loads that run for short periods of time (AC being the biggest usually), and then there are some small loads that run for much longer (cable boxes, lights, etc). Your generator needs to be sized to provide enough power for all the loads at once.

To determine the size of generator to get, you'll need to consider what you want to run off of it. A lot of people only run a subset of their circuits from a generator to get the essentials - HVAC, a fridge, maybe a couple general receptacles. As a simplified estimate you can take the size of the breakers you want to run (in amps) and multiply it by the voltage (120V for single breakers, 240V for double) and add them up. AC units can very quickly use up most of a generator's capacity!

  • I would agree but hvac and other motors require a generator that has a higher peak value to allow the motors to start, for example if your AC is one of your backed up devices it can take 3-5x the power to get it up to speed (normally 10-20 seconds) the same is true with well pumps and refrigerators, freezers. So by the time critical motor loads are added a larger generator is needed than the raw KW of the total loads, I use a 1.6x as a minimum size unless the loads are mostly motor then I will use up to 3x. This would be the reason for a 15-20kw whole system generator size. – Ed Beal Jun 21 '18 at 0:15
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AC power doesn't store. It's not like a 0.5kw generator could chug along all day slowly filling a huge... I don't know... tank of water maybe, that you can draw from on demand. The generator needs to provide peak demand for when the coffeemaker and hairdryer and every other darned thing are in use at once.

The rest of the time, the generator is its own "spinning reserve" for in case you turn on one of those loads. It's at full speed but doing next to nothing, powering only your internet router and cell phone charger. That is inefficient, and thus, expensive.


Now your house is a fairly small average load, and may be a candidate for a different way of doing things that allows solar as backup. After all, your 0.5kw is only two 270W solar panels, and with the usual 6x design factor for nights and rainy days, that is still only twelve panels.

The problem is storage, and that requires DC power, preferably low voltage DC, and then you can use batteries for storage. This ideally involves making wide use of low voltage DC for loads which lend themselves such as lighting, some air handling, TV, router, certain PCs, etc.

It's possible to use an inverter to make AC from DC, but the inverter is inefficient all the time, particularly under light load. So you have the same "spinning reserve" problem you have with a generator, where you're paying a king's ransom all the time to have AC power on tap the moment you want it. Better to architect for DC and only spin up an inverter when really needed for a short term use.

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Many other factors involved here.

1) Amperage of all the appliances you are hooking up. ie size of Ac Air Handler(with heat Strips installed) and AC Comperssor number of other major appliances you want to run at the same time off the generator 2) Do you want to install the generator covering all circuits or just a few. ie. are you wanting to run the washer and dryer and oven when running off the generator? 3) Bigger is better in this case, especially if you want to cover entire house power when running on generator power.

Now a word on transfer Switches(gotta have one) Get on that covers 200 Amps and either replace the existing Circuit Breaker panel or Make it First Sub Panel.

From My experience installing a 16KW 65 Amps Whole House Generator last year I had to make some hard choices.....living in FL I choose AC over Oven and Dryer!

Edit most 22Kw backup generators are about 91 Amp Capacity. My 16Kw Is 65 Amps.

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