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I'd like to size my generator for the whole house. Can I use the usage stats from my electric bill or do I need to check the individual appliances. For example, the house uses 34 kwh per day on a heavy use day. Average is closer to 25 kwh. So first off, can I use a daily rate to size or do I need more granular (hourly, etc) Or do I really need to check all the appliances and add in the safety margin. Also, I'm planning on having an electrician come out to install the transfer switch etc. Thanks.

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    You'd probably want to do a full NEC load calculation to determine what your expected load is. If you're not sure (even after some web searching) how to do this, I'm sure the electrician would be happy to do it for you.
    – FreeMan
    Commented Apr 18 at 17:05
  • Apart from anything else, you almost certainly use much less while you're asleep. A daytime average is probably a better starting point than a 24 hour average
    – Chris H
    Commented Apr 18 at 19:06

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The ideal is an NEC Load Calculation. This incorporates large fixed loads (e.g., EV charging, water heater), a general factor for lighting and other loads based on the size of the house, cooking equipment (special rules), HVAC and a bunch of other details to come up with a single number. That number is used to determine the required service size (utility feed) and can also be used for a whole-house generator.

There are two problems with utility bill information: granularity and "what if". As far as granularity, the industry standard is 15-minute readings for both usage and peak demand. The value you need here is the peak demand. Typical utility bill information is monthly usage, possibly daily usage, and possibly even hourly usage as a downloadable file. Most that I have seen do not offer 15-minute readings for residential accounts. If you can get 15-minute usage and demand then you can use the highest demand value. If you can only get 15-minute usage then that is close to peak demand and might be good enough. But if you can only get hourly data, that really isn't good enough as there can be large loads that run for well under an hour at a time.

In addition, there is "what if" factor. Peak heating or cooling days. Peak cooking days - what if you hardly cook and this year you decide to host Thanksgiving dinner for the extended family...and it is a super-cold day and the power goes out. And a number of other possibilities.

There are online Load Calculators, some better than others. An electrician or a generator company should be able to do a proper calculation.

One way to cut down on the generator size is to exclude certain loads. That requires some extra wiring (critical loads panel) but that extra cost may be worth it to have an automatic whole-house generator that is much smaller than your entire service.

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  • My solar install gives 15-minute consumption numbers, which were super handy in electrifying the house. So even if a utility can't give you 15 minute numbers, if you have solar you may be able to get the numbers out of that system. Also 100% agree on it being useful to shed some loads. You don't usually need items like hot tubs or the clothes dryer on the generator.
    – KMJ
    Commented Apr 18 at 17:28
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    I'm asking the utility if they have the 15 minute loads I can use. Based on the recommendations though, I'm going to do an NEC calc as well. Thanks. Commented Apr 18 at 22:14
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    The load calculation is by far better. Many loads don't run 15 minutes, (or up to 30, depending when the load started .vs. the 15 minute granularity timing) so there's still a danger of being undersized from that data (my utility does offer it, but only if I download the data - their web-display tool granulates by hour at best.) And I, at least, use power very differently during an outage, as well (fridge cold, pipes not frozen, toilet flushed and the piddling power for some LED lighting.)
    – Ecnerwal
    Commented Apr 19 at 21:00
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Yeah, the problem is that power company data is either "the kWH you used in that hour" and showing the highest hour in the month, or "the kWH you used in that 30 minute period x 2", or "the kWH you used in that 15 minute period x 4".

So it's not peak data at all. It's averages at pretty long intervals, I mean a water heater on maintenance mode will throw a 4500W load for a minute or two. You cook on the range, but not for a whole 30 minutes. Dryer loads cycle on/off thermostatically, with 5500W load coming on-off-on-off, and if they're running a 20% duty cycle the power company peak data sees that as a 1100W load.

Code requires that with an automatic transfer switch, the generator must be large enough to pick up "the load to be served" per the NEC Load Calculation, minus any appliances which are on load shed devices (which monitor for the signs of a generator bogging, and drop out for awhile).

220.83 allows use of a load study for load calculation, but I really think that is intended for industrial application where you can measure a high-production day. I don't think it's a good fit for dwellings because (think March 2020) habits can abruptly change when you get laid off or a new family moves in.

NEC 220.82 is the normal dwelling load calc.

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A permit will probably be required for this so the NEC load calculations will be needed for the transfer switch (at least in Florida). You do have a little more flexability in sizing the generator so the utility information, especially the hourly and 15 minute comsumption, could come in handy. If your usage is fairly level hour to hour, you might be able to get a smaller generator that you would if the loads are much higher during certain hours and you're sizing for that.

Keep in mind that your habits might change during an outage.

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