Help sizing a generator for home backup?

[s219]

As mentioned, look at all your loads to get a total, but also consider what loads are critical and what loads may dominate. Decide if you can get by without the big ones, or if they are necessary then plan those into your peak demand. And consider running and startup loads.

I forget the numbers now, but my max load was our geothermal system (no surprise). I was able to reduce the startup load quite a bit by installing a soft start kit. That helped knock down the max generator wattage I'd need to start that system, which in turn lowered my generator requirements down into a more sensible range. If I don't have to run the geothermal system I won't, but it's nice to know I have the capability.

 

[grsthegreat]

I agree that something in the area of 6500 watts (continuous output rating) is a good size range to be in for a typical home in the Northern US, if you are willing to do some load management, you can get by without a large air conditioning system, and don't have a whole lot of heavy electrical loads (electric heat or electric hot water, for example). I get by with 5500 watts continuous with an acceptable (to me) amount of ongoing load management. Were it not for the starting current of the deep-well pump, I could get by without much ongoing load management at all. I just shut down non-essential circuits at the beginning of an outage. I'll generally leave the well pump off until needed, just to be on the safe side. I calculated that I'm probably OK as long as both fridge and freezer are not running (or especially, starting up) as the well pump starts, but I prefer to stay on the safe side. When I was running off a 4000 watt generator, I had to be much more active in my ongoing load management. Even then, were it not for the generator's unusualy large surge capability I may have struggled

I see a 240V outlet on just about every generator over about 3000 watts continuous or so. This is probably because there is not a commonly used outlet rated for more than 30 Amps (there is a 50A 120V NEMA standard outlet, but it's not very common). The common household outlets are rated for either 15 or 20 amps (generally 15 amp outlets on a 20 amp circuit breaker in most modern homes). So the generator would require multiple separate 120v outlets and cords to deliver that power. You just can't easily deliver more than that through one circuit.

A 4,000 watt gen is only providing 16 amps at 240. Really hard on a well pump motor. Also the 4,000 watt units generally have a 120v 30 Amp rv outlet, not 240....at least the ones I’ve seen at Home Depot.

 

[CobyRupert]

Can get average kw loads from you electric bills. From those, I could tell that the Isuzu would be running lightly loaded most of the time, about 2.5 kw vs. 12.5 kw capacity.

....but what does the average KW load tell you about sizing the generator? Peak load is what you need.

Example:
I drive my car to work 5 times a week. Say: 30 miles & 1 hour round trip.
So the average speed is: 30mph? No. Not if like a KW meter it’s (5days x 30miles)/ (7days x 24 hours)= .9 mph average car speed for the week.
I often hit 65 mph on my way to work.
Buying tires that are only rated for 1 mph (or 30 mph) top speed would be bad.

 

[CADplans]

My generator is a welder , so it gets run when I need to do a welding project, which is at least monthly,,,
the unit seldom sets idle for 3 months,, even then, it easily starts,,
I keep no-ethanol gas for the generator/welder.

As far as watts, it is rated at 10,000 watts,, I have it on an incoming breaker that is 240 volt, 30 amps.
That is just over half the rating of the unit, but, I have only tripped that breaker a half dozen times in ten years.

I feel WAY better running the generator below maximum load, there is plenty of reserve to "start" motors like the well pump,
without doing damage,,,

 

[BrokenTrack]

Come on over and I will give you a demo. This gen set was purchased IN JAPAN with a load of yanmar tractors.
You can stand next to it and talk while its running.
Its both 3ph and single
3 cylinder engine
yanmar ag12s-2 - Google Search

Mathematically that does not really sound correct.

If a gallon of diesel fuel has 131,000 btus per gallon, and 10% is known to go up the stack (10%), or in the water jacket (75%), then that only leaves 85,000 btu's for mechanical power production.

But an engine has to run at least 1/3 throttle for clean power, so a 12,000 watt generator would be producing 4000 watts at that throttle position. But we know from conversion charts that 4000 watts requires 13,600 btus. At that rate, you would consume a gallon of diesel fuel every 6 hours and 15 minutes. Or, close to 4 gallons per day.

That is still really good fuel consumption, but as stated by another member, that long-term throttle position would lead to diesel engine "wet stacking', and is hardly great power production (4000 watts). That is why I say, if only 4000 watts was needed, obtaining (2) 6000 watt generators would be a better plan, and then have the second come online, and synced together at times when more power that 6000 watts was needed.

The make, model, of the engine, nor how it is configures matters at all. A person cannot get more energy out of something than what it originally contains.

 

[John_Mc]

A 4,000 watt gen is only providing 16 amps at 240. Really hard on a well pump motor. Also the 4,000 watt units generally have a 120v 30 Amp rv outlet, not 240....at least the ones I’ve seen at Home Depot.

Yes, 4000 watts is small for a well pump, especially if you have a deep well with a larger pump motor. Whether it will work or not depends on the well pump motor size and the generator's surge capacity. I had a 4000 watt continuous rated generator with a 6600 watt surge. The pump for my 500' deep well is on a 20 Amp 240V circuit breaker. Since that surge rating amounts to 27 Amps, it handled the pump without any issues. I did make sure I was not running any other significant loads during the pump start-up.

6600 watt surge is a little unusual for a 4000 watt continuous rated generator. This one was nice because it also had relatively clean power (5% THD max, if I recall correctly) so I could run sensitive electronics on it as well. I would not try it with something that only had a 4500 watt surge for example, unless I knew my well pump was small enough (not likely in our area. We need to go deep to get decent water flow.)

As far as 240V outlet on a small generator, I looked at pages of generators offered by Northern tool to back up my recollection that most generators over about 3000 CONTINUOUS watts have a 240V outlet. After seeing your post, I looked at Home Depot online, where a 240 V outlet on a small generator appears to be the exception, rather than the norm. It's interesting that there is such a difference in their offerings.

It appears my statement "just about every generator over about 3000 watts continuous" was an overstatement. There are certainly generators out there of that size and larger which have only a 120V outlet. However, it's not at all uncommon for at 3500 watt generator to have a 240 outlet. Personally, I have no use for a 3000 watt generator with a 240 outlet. If I had enough load to need 240, I'd go bigger anyway. (I'd go back and edit my post to add a note about your correction, but the system will not let me. Apparently, too much time has passed.)

Smaller generators aimed at the RV market generally have a 120V 30A outlet on them, since that if the typical campground power for smaller RVs (such as pop-ups). If there is no need for 240, it's certainly less expensive to make than it would be to also add the 240 outlet.

 

[TheMan419]

We live in Northern Indiana. It gets cold here like it does for OP. We have a boiler and of course well water. Our boiler is old so no computer running that thing.

We have a 6900/8500 watt portable. I am not sure I would want the fridge, boiler, and well pump to all kick on at the same time. So we do some load management. When we want to run the water to shower I turn the heat down so the boiler won't kick on. Once the pump kicks off after the shower I turn that breaker off and kick the heat back to where I want it.

I could probably get buy with a bit smaller unit if I wanted to watch the watts all the time. In 5 years we have been in our current home we have used it twice to power the house. We used to live more suburban and in 5 years used it once. If I were buying again I would get an inverter style. I would also get one that could run on propane as well as gasoline.

Electric start is great. I can pull start the unit unless my back is out. Mrs. The Man cannot pull start it. So with electric I can go into work and she can fire up the generator as needed around the farm.

It is portable only in the worst sense of the word. It is on wheels and weighs a couple hundred pounds. When I want to use it as aux power around the farm I need to move it with the tractor.

 

[RalphVa]

One thing the electrician ran into when he was planning what to include in that 200 amp generator box was the backup heat coil for the heat pump upstairs. It's only 2 ton, but the backup electric coil was a bunch of amps. He rigged something with a couple solenoids that would run it off when the generator kicked on.

Have since changed out that whole unit, and the control allows you to lock out the coil: heat pump only all the time. Luckily, it runs this way fine down to around 10 F that we've gotten down to since then.

The ductless heat pump system for the basement has no backup coil. It's on the generator, too.

Ralph

 

[grsthegreat]

One thing the electrician ran into when he was planning what to include in that 200 amp generator box was the backup heat coil for the heat pump upstairs. It's only 2 ton, but the backup electric coil was a bunch of amps. He rigged something with a couple solenoids that would run it off when the generator kicked on.

Have since changed out that whole unit, and the control allows you to lock out the coil: heat pump only all the time. Luckily, it runs this way fine down to around 10 F that we've gotten down to since then.

The ductless heat pump system for the basement has no backup coil. It's on the generator, too.

Ralph

Those pesky electric heat strips are my nemesis. Hate them with a passion. i use Generac load shed modules for them unless client wants to spend big bucks for a liquid cooled generator.

luckily i have gas forced heat in my house...no issues with the 22kw. i demoed my heat pump with electric backup heat years ago and went with high eff gas heat and a/c unit.

 

[not2old]

My well pump is on a 15A 240V breaker and produces more water than all my neighbours. I haven't tested it on my generator yet, but expected no problem with 5000W and 6500 surge. This thread has prompted me to try it.