Whole House Generator: How does it work
September 9, 2018 5:22 AM   Subscribe

This summer I bought a house that has a natural gas powered whole house generator. Once a week it kicks on for a test run of about 15 minutes. My husband likes motors and has been out there to check the oil and inspect everything and it looks fine. We live in the projected path of Hurricane Florence and it looks likely we're going to need it for real for the first time this week.

So, uh, how does this work? If we lose power, does it just kick on automatically, or do we have to turn it on? Are we supposed to let it run continuously the whole time the electricity is out (which, if this area's last hurricane was any indication, could be a week or more)? What is the daily cost of operating this thing going to be? Any info or advice welcome!
posted by something something to Home & Garden (16 answers total) 3 users marked this as a favorite
There should be a panel in your electrical system- look for "Generac" or a similar term. Most modern backup generators autostart. Since it autostarts when it tests itself, I'd wager it has a relay to do so when the power fails.
posted by pjern at 6:17 AM on September 9, 2018

Yes the brand and fuel would help (fuel: do you see propane tanks anywhere or do you have natural gas in the house?)

If it self tests id suspect you have a fully automatic one. It will kick on when power goes out, and automatically disconnect the street utility (using whats called an automatic transfer switch). It does this so your generator doesn't back feed the grid and electrocute a lineman working to restore service. Then when power comes back it will sample and delay for some amount of time to make sure the power is REALLY back on and not just flickering. This is a box that looks like a circuit panel but usually only has 2 giant breakers and can be either near your breaker panel or outside next to generators.

Yes, let it run 24/7. Thats what it's for. The only question is how much capacity does it have versus how much can you consume? Generally if it is the automatic type it's been sized to run your house as normal (the difference in price for capacity up to a point is VERY small).

As for costs, that depends on 1) fuel type 2) how much it CAN consume 3) how much power you use

Congrats though. It's a terrific thing to have. We got lucky with irma in tampa last year (friends lost power for WEEKS we never did) bit we are absolutely planning to put one in.

If you post model number etc i can help a little more, or memail me and we can video chat or something. Good luck with Florence.
posted by chasles at 6:35 AM on September 9, 2018 [2 favorites]

Short answer: It depends; if you can provide some pictures or info on the generator and any electrical looking panels we can help more.

Longer answer: It probably starts on its own upon loss of power. The fact that it's plumbed to natural gas and exercises weekly indicates it was either professionally installed, or the previous homeowner knew what they were doing (jankey DIY jobs can be quite dangerous to people working to restore power by back-feeding from a house into the medium voltage distribution in unexpected locations).

Depending on the size of the generator you may have a whole house automatic or manual transfer switch installed upstream of your main breaker panel that will connect then entire house of the generator, or if it is a smaller unit you may have a smaller transfer box near your main breaker panel that automatically or manually connects some of your house circuits to the generator. It will run the whole time the power is out.

To he honest, the first thing I would try if you don't have any documentation left behind is turn off all sensitive electronics and then turn off your main breaker and see what happens (ideally while holding a flashlight in case it turns out this is a manually switched generator). It may take 30 seconds for the generator to come on, and after you restore power some stay on for a few minutes to be sure the power is back for good.

As to the cost, you can lookup the fuel consumption rates for your model (ideally based on loading, since it will use more gas powering everything vs just a few lights), determine about how much it will be powering (you can turn off everything it won't be running and read power off your power company's meter on the side of your house), get your fuel cost, and pull some WAG as running costs together. But to be honest, if the power is out; you won't care in the moment.
posted by token-ring at 6:50 AM on September 9, 2018

Thanks so much for the fantastic answers so far! Good to know there is a small delay before it kicks in. We woke up one morning to flashing alarm clocks after a thunderstorm, which made me worry it wasn't working properly - but I'm guessing the power only flickered for a few seconds, not long enough for the generator to kick on.

It's a Briggs & Stratton, 15kw, model 040234. There is a large transfer box on the outside of the house near the main breaker box (which is inside). There's also an indicator light in the garage with a key to various errors that might occur. It's powered by natural gas (connected to the city system). This home was owned by an elderly woman and her grandchildren really took care of her as far as this type of thing goes; I'm pretty confident it was installed correctly and is a high quality system.
posted by something something at 7:13 AM on September 9, 2018

So, uh, how does this work? If we lose power, does it just kick on automatically, or do we have to turn it on? Are we supposed to let it run continuously the whole time the electricity is out (which, if this area's last hurricane was any indication, could be a week or more)? What is the daily cost of operating this thing going to be? Any info or advice welcome!

I have one. I think the way it works at my house is if the power is off for more than... 30 seconds maybe, the generator will kick on. As you know it's NOISY as hell and usually it's connected to either some or all of the electrical stuff in your house. So like it my house it will run all the kitchen stuff and the living room stuff but not the stuff in the remote bedrooms etc. We had to do a test to see exactly what would run and what wouldn't. I get ours serviced every year which costs about $200 and is super worthwhile, Lat year there was a storm and the generator wouldn't kick on without us starting it manually (unfun!) and this year we discovered it needed a battery so now we are all set. The serviceman said it was a good idea to turn it off and check the oil every 8 hours it's been running just to make sure it's not burning it up (I have an older model, this may be a non-issue for you). This is the sort of thing you can learn to do on YouTube. As far as cost I recall that it was expensive to run but less than staying in a hotel for the duration and it was great not to lose our food or our heating system when the power was out for four days in March.
posted by jessamyn at 7:32 AM on September 9, 2018 [2 favorites]

Twenty years ago I worked for a company that sold, installed, and serviced generators. I have no financial stake in them or any other company, but I felt like I should disclose that past connection before saying: you should call your local shop and sign up for a maintenance contract. You at least want to be on their contact list before anything hits, but the best case scenario is that they will be able to come out and inspect your unit and show you what you need to know. If the previous owners had such a maintenance contract with them they will have the service records and they can tell you when the next scheduled service would be due, and you can have a little more peace of mind that your unit is really going to work the way it's supposed to. At a minimum, even if they can't schedule a visit before the storm hits, they can give you a better walk through over the phone than strangers on the internet can. I'd look for a decal on the transfer switch or next to the generator's serial number plate and call that company first.

That self test mode it does once a week probably doesn't actually transfer the load from your house onto the generator. It's a good sign that it runs every week, but without a load test you don't really know for sure that your generator will work as well under load as it does at idle. Aside from the standard engine service (oil change, filters, spark plugs, check for leaks; all things a handy adult can do) the big thing a technician can do for you on a regular maintenance visit is run a load test. Also your first visit would be your chance to learn about what you should be doing to maintain the unit between service calls, and what to expect during an emergency.

What everybody else has said about transfer switches sounds right to me. I'd guess you have an automatic transfer switch based on the fact you have a self test happening once a week. Assuming you have an automatic transfer switch and not a manual transfer switch, it will detect a line failure, start up your generator, wait for it to stabilize, and then transfer your house's load to it. Fancier, more expensive generators start up faster and have electronic engine control and voltage stabilization; more basic generators have simpler regulators and take a few more seconds to provide stable power. After startup they're mostly comparable, although electronically controlled generators will have more consistent power output.

If it turns out you have a manual transfer switch and the self test is controlled by another device (perhaps a control panel on the generator itself) then you'd have to go start the generator, wait for it to stabilize, and then operate the transfer switch by hand. When you learn that power is restored, you'd restore the switch, wait about five or ten minutes to let the generator cool a bit at idle, and then turn it off. This is important: make sure the generator is stable before transferring your house's load to it, disconnect the load from the generator before stopping it (to avoid damaging both the generator and the devices in your house), and remember to let it idle before stopping it.

Most devices in the home won't really care if the power floats a little, but some electronics react badly to undervoltage. If you have a big home theater receiver turn it off and leave it off for the duration (I speak from personal experience on that: undervoltage killed a friend's very nice receiver, he managed to get it repaired, and then it happened again). If you have a desktop computer I'd recommend putting it on a UPS just to smooth out any problems with inconsistent power. Laptop computers have their own batteries and voltage regulators and they'll just switch to battery power in an undervoltage scenario. Your TV has its own power regulation circuitry that should be OK (and it probably draws too much current to put on a UPS). Lamps, refrigerators, and the like will all be fine. If you have an electric stove use it judiciously (and maybe don't have the TV on while using it).
posted by fedward at 7:32 AM on September 9, 2018 [2 favorites]

OK, according to the manual the fact that it's automatically doing an "exercise" once a week indicates it has an automatic transfer switch and the generator is in auto mode. But I was correct in my assumption that it's not transferring load when it does that every week. You can do a manual test to transfer load (documented in the manual), and if you can't get a service technician out before the storm is expected, I'd still do a manual test. Not only will you make sure it really works at all, as Jessamyn says this will give you a chance to figure out if the whole house is connected to it or if they just put "critical" circuits on it. 15KW seems like it should be enough for a whole house but I don't know what your air conditioning load looks like.
posted by fedward at 8:03 AM on September 9, 2018

I'm seconding fedward here about getting a maintenance contract under foot as well as talking to someone about how the system should be operated in the event of a long-term outage.

My neighbor has an automatic system and told me that the generator needs to be manually cycled off to "rest" for a short amount of time after so many days of continuous operation. That seems something that would be pretty specific to your model, so it's worth learning more.
posted by JoeZydeco at 9:41 AM on September 9, 2018

One thing that you should keep in mind is that in a disaster, the utility company could shut off the gas service for your area/town/neighborhood to isolate gas leaks.

Usually natural gas-powered appliances are safe from power outages, but if something happens that causes leaks (like, a tornado hit just a few blocks from your house) you could still be screwed. This happened to me this summer, and I was pretty glad I already had a gasoline powered generator for the week we were without natural gas AND electricity.
posted by ArgentCorvid at 1:36 PM on September 10, 2018

Hi everyone - hope it's OK to ask a few follow up questions as (understandably) I have not been able to get a tech out here this week as Florence bears down on North Carolina.

Is it really OK for me to run my air conditioning on this thing? My house is 2400 square feet and the generator is 15000 kW. Should I, in general, try not to have too many lamps on, or watch TV, etc., or can I just act like I have regular electricity?
posted by something something at 6:45 PM on September 11, 2018

Does your electric utility let you see your daily or hourly usage on their web site? If so, you should be able to see what your peak usage is. Even if you can't see hourly usage you could still look at your last electric bill to see how granular the data is there. Our bill has bar charts that show our daily usage, which isn't quite as good as hourly usage but it's better than nothing. Thumbnail guess: for a 15KW generator your house's hourly power usage at peak summer load should be around or under 8 kWh.

15KW is sort of in the middle range of home standby generators, and I would guess that it was sized to allow for air conditioning, but there are a lot of variables about your house that we can't really answer over the internet. If your generator was properly sized for your whole house, if the generator was properly installed, if its own safety systems are working correctly, and if you follow the maintenance instructions in the manual (including any rest or cooldown periods specified) then you should be OK running the air conditioner.

If my 8 kWh guess is close enough (and if the generator's rating is accurate), you shouldn't be likely to stress the generator too much, even using everything normally. The closer to the generator's maximum capacity you push it, the more likely you'll experience some sort of power fluctuation or instability. If you have a lot of stuff on and the lights flicker or pulse, or you hear the generator revving up and down, turn some stuff off until it stabilizes. Depending on a number of factors I can't predict from here, that may or may not actually happen to you, but the warning signs (fluctuating lights, weird noises from the generator) should be pretty obvious.
posted by fedward at 8:10 PM on September 11, 2018

Thank you, fedward - my bill doesn't show hourly usage but it says Average Daily Use in the month ending today (which was much much hotter on average than this week will be) was 57 kWh. It sounds like I should be OK, then?
posted by something something at 8:48 PM on September 11, 2018

it's tough to say but 57/24 is around 2.5. my house is old, out dated, and has an energy hungry pool pump, old air conditioner, 3 fridges etc, and I max out around 7.5KW
posted by chasles at 11:02 AM on September 12, 2018

and i should say, good luck with the storm!!! hopefully it's a non event for you
posted by chasles at 11:02 AM on September 12, 2018

I'd advise against doing any sort of simple division by 24 to figure out peak electric usage, if for no other reason than you're unlikely to be sustaining peak usage for all 24 hours in a day. Dividing your daily average by 8 or 10 might be a better rough estimate for peak usage, simply assuming your A/C is the biggest power consumer in your house and it runs for a total of 8 or 10 hours per day, not 24. You can also look on the information plates attached to large appliances to figure out how much power they consume, but then you have to do a little more math (volts × amps = watts).

Generator sizing accounts for steady loads (lights and appliances once they're up and running), peak loads from large appliances as they start up, and the fuel consumption and noise characteristics of the generator. Electric motors (like an air conditioner blower and compressor) consume a lot more current at startup than they do once they're running. Ever notice how all the lights in your house dim just a little bit when the A/C comes on? This is why. A generator that's been properly sized for the house will be able to handle that startup load without its output fluctuating too much, and it also won't consume too much fuel under steady load. In other words, generator sizing is a bit of a Goldilocks situation: you're aiming for not too big (inefficient), not too small (unstable), but just right.

There are a lot of variables here I can only guess at, and I don't want to give what sounds like an expert opinion for a system I didn't have any professional connection to, and then have somebody rely on my guesswork as being authoritative. Hence all the "ifs" in my last comment. Assuming the Goldilocks math was done right, it should be OK.
posted by fedward at 12:40 PM on September 12, 2018

UPDATE: After all that, we never even lost power. Now to get regular service set up before the winter's first ice storm!
posted by something something at 11:05 AM on September 17, 2018 [2 favorites]

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