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I am (finally) looking seriously at a backup generator for my house, plus panel replacement.

Current panel is 2 fuse boxes + an 8-space breaker panel (installed with kitchen upgrade in 2000). Rule of 6, just barely enough capacity for everything, no AFCI, various other "because it is all old" issues, but no real problems. Been thinking about a full replacement for years, now is the time.

Not sure what size the current service is (probably 100A), though my electrician told me that current county rules are that with any change like this an upgrade to 150A will be required. Should not require a new line to the house as all the outside service lines in the neighborhood were upgraded several years ago after numerous outages.

Thanks to the smart meter, I know my usage.

Top usage for the last year (hourly readings, not sure why they can't give homeowners industry-standard 15-minute readings, but then again, the average homeowner has no idea what any of this means) is 9.98 kWh, so just under 10 kW. Dropping down to 99% of hourly readings gets to just under 7 kW.

Low usage is 0.41 kWh - in fact, 46% of usage hours < 1 kWh and 71% < 2 kWh.

With current usage, and not planning on major new equipment (no Bitcoin mining, no grow lamps), an 8,000W - 10,000W (more or less) generator will do just fine. So not planning on a true "whole house" permanently installed generator but rather a "portable" to be used when needed. That also means that except for making sure I don't run air conditioner, electric oven and electric dryer at the same time, I really can run everything without too much worry. FYI, water heater, furnace and cooktop are all natural gas.

Key questions:

  • Since I really can power everything from a 50A (electrician said probably even 30A) inlet from a generator, is there anything wrong with simply having 50A generator breaker on one side, 150A utility breaker on the other side with an interlock and always switching everything? Having to install two panels and then be limited in what I can power (power out on a hot day and I'll want to run the air conditioning at least for part of the day) doesn't make sense to me.
  • Is there anything else I need to consider in sizing a generator, or is the peak usage enough?
  • Is the low usage going to be a problem or do typical generators run OK (if not quite as efficiently) at low load?

I will likely have more questions after electrician does a site visit. He has worked on my house before but hasn't been here in a few years. Work will be coordinated/permitted/etc. with utility and county as needed.

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  • Per my extensive research before ending up on-grid, diesel scales well down to about 1/4 power (fuel use proportional to power use) while gasoline/propane basically suck nearly the same fuel for no load as for full load. 1800 RPM generators (4 pole) are a lot less annoying than 3600 RPM generators (2-pole). The throughly modern low-load and short-term solution these days is your lithium-battery inverter, but they do get a bit spendy.
    – Ecnerwal
    Nov 15, 2021 at 0:21
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    Batteries get expensive for more than a few hours. Outages are relatively rare the last few years but when they happen can easily be multi-day. Solar plus batteries would be ideal but for a bunch of reasons I don't think so practical for me. Nov 15, 2021 at 0:26
  • IF even a few hours of battery (at low load) is an option, you can recharge those when the generator runs and then shut the generator off without having everything go dead - but it may not be economically viable. Incidentally, my utility shows hourly on the website, but the downloadable data is 15 minutes.
    – Ecnerwal
    Nov 15, 2021 at 0:30
  • @Ecnerwal Problem is that would still need charger/inverter/batteries/etc. If, as hoped, the outages are few and far between it just isn't worth it, since I'd still need the generator for the big stuff. My utility (Pepco) only has hourly, as far as I can tell, for residential. Commercial (on a specific account basis, not automatic) has 15-minute data. Nov 15, 2021 at 0:33
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    Not sure how long you're planning on living in the house, but being able to reasonably run the whole house off the generator could be a selling point worth a few extra bucks when the time comes. Just something to consider.
    – FreeMan
    Nov 15, 2021 at 13:47

1 Answer 1

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To answer your questions:

  1. Yes a 50 amp breaker is fine as long as you use conductors with sufficient ampacity. Backfeeding is fine as long as you install an approved interlock mechanism listed for your panel.

  2. You should size the generator for the loads it will be expected to serve. Keep in mind any motor loads that require startup current. Most motor loads in houses are delayed by timers, such as a furnace blower motor.

  3. Low usage is not a problem other than wasted fuel, but that's the nature of generators. They are only to be used in emergencies, not to serve as a self contained generating station. POCOs have very complex software for load management they bring alternators off and online to maintain peak efficiency.

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  • Generator inlet will be on the other side of the wall - 2 feet from the panel as the crow flies (if crows could fly through walls) - so the wires for that won't cost much even if copper. First tank of gas for the generator will cost more (a lot more at the rate things are going). Nov 15, 2021 at 2:22

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