Help sizing a generator for home backup?

[truckdiagnostics]

We put soft start on the geo compressor. Helped with starting amps. Maybe time to consider new well pump with a soft start. Then you can use smaller generator. Lot there would depend on age of your well pump.

 

[RalphVa]

I had a bit of experience with what ran on our 4.5 kw generator when planning for the whole house one. By my calcs, could do it with 12.5 kw one we bought from central Maine in last 2011; my electrician agreed. That, combined with some fancy circuits on 4 to 6 of the big breakers on the new 200 amp generator switch box from Gentron, got us whole house with even one electric hot water heater and our clothes dryer on it. Basically, the 4 ton heat pump, electric stove top burners, swimming pool pump and carriage house got left on the other 200 amp panel off the generator. Has worked great.

Had a couple of "whole house" outfits come out. They wanted to install about a 30 kw diesel generator to get the job done.

The generator is a little gem: Isuzu powered. Will use 2.5 gallons of diesel for the year doing its 15 minute runs every other week. Will last 2 weeks on the 60 gallon diesel supply during an outage.

Ralph

 

[PEJ5]

The best way to properly size a generator is to measure your energy consumption. You could have your electrician measure each circuit while operating the load and add them up. A better way is to install something like this. TED Pro Home is latest technology in home energy management
It will help you understand your daily routine of consuming energy. You may find ways to save as you discover how much each load consumes - maybe replace an old fridge. It will help you size your generator. It will also help you understand how much may be turned on while you are running on generator to ensure you do not overload it.
A generator should not be loaded above 80% ever, and motors typically draw 150% when starting. So factor that in. Install UPSs on sensitive equipment to protect it from poor quality power.

 

[grsthegreat]

With a well, I’d go with a minimum of 6500-7500 running. Besides, I doubt a 4000 watt unit will have a 4 wire 240 outlet to power the house. Ive never have Seen a diesel generator that uses only 1 gal of fuel a day, and I’ve been around generators for 20 years. I could sell a million of those. Heck I’d cut the utility feed and power my house 24/7 at a cost of $2.50 a day

 

[BrokenTrack]

Ive never have Seen a diesel generator that uses only 1 gal of fuel a day, and I致e been around generators for 20 years. I could sell a million of those. Heck I壇 cut the utility feed and power my house 24/7 at a cost of $2.50 a day

I think that was a misprint, and meant to say 1/2 gallon per hour or something.

I do not work for, nor endorse Central Maine Diesel, but they have one heck of a website. On it they have a ton of generators, and it lists everything, including fuel consumption at partial, half and full loads. They have a 12,000 watt Perkins diesel turning 1800 rpm. It had a consumption rate of .4 gallons per hour partial load, and full load of .9 gallons per hour at full load.

That is about what I would expect.

Fuel consumption is just the Laws of Thermal Dynamics at work since energy can neither be created, nor destroyed, just converted. So it does not matter if a person is talking btu's, boiler horsepower, mechanical horsepower, kilowatts, or fuel consumption, there is 131,000 btu's in a gallon of diesel fuel, and a person can only do so much with it. Pick whatever you want to convert too, and run the numbers. Just make sure the waste of conversion is factored in to get some proper numbers.

 

[BrokenTrack]

Here is an example of those conversion numbers.

My genset has a Perkins 4.108 diesel engine, so I know it is 55 mechanical horsepower. But I want to use the coolant from the engine to help heat my home as it is making electricity, so how big of a heater is that engine?

Well I know an 75% of an engine's heat goes into the water jacket, and 10% up the exhaust stack, so combined that means it is 65% as a heating appliance. Since a gallon of diesel fuel has 131,000 btu's per gallon, I can multiply 131,000 times 65% and get 85,000 btu's. So it is the equivalent of a 85,000 btu stove.

Since my house requires 700,000 btu's on the coldest day of the year, I can deduce I need to operate that genset at least 8 hours per day to get 100% of my heat needed for my home.

But what would this engine be in terms of boiler horsepower instead of mechanical horsepower. Well a boiler horsepower is 33,000 btu's, so it is a 2.5 boiler horsepower engine.

But what is the largest generating head I can put on it? Well (1) mechanical horsepower is 750 watts, so I can power a 41 kw generating head with this engine.

The point here is, as long as you know the conversion rates, ANYTHING can be converted to anything using math because energy can neither be created nor destroyed, just converted.

 

[YanmarFever]

With a well, I壇 go with a minimum of 6500-7500 running. Besides, I doubt a 4000 watt unit will have a 4 wire 240 outlet to power the house. Ive never have Seen a diesel generator that uses only 1 gal of fuel a day, and I致e been around generators for 20 years. I could sell a million of those. Heck I壇 cut the utility feed and power my house 24/7 at a cost of $2.50 a day

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

 

[John_Mc]

Also - as with almost any equipment, it痴 good to have too much capability as opposed to too little, so if it were me, get a generator that is larger than the max you think you'll need.

There is a downside to this, however: A 12,000 watt generator that is only covering a 1 or 2,000 watt load will burn more gas than a 4,000 watt generator handling the same load. Best not to go too far in extreme in either direction.

Inverter generators also tend to draw a significantly less fuel under partial loads than similarly sized non-inverter generators. This is due to the fact that a non-inverter generator always turns at 3600 RPMs (or at 1800 RPMs, depending on the generator design) in order to give the required 60 Hz frequency (at least in the US). An inverter generator's frequency is set by the inverter, so they can run at half speed (roughly speaking) if under half load, and at just about idle if all you are running is a light bulb or phone charger.

Speaking of idling, it's best not to use the "idle save" feature found on many non-inverter portable generators when running it as backup power to your house. This is supposed to save gas by idling the engine when there is no load. I've found it rare that there is actually zero load when powering a house. It takes a couple seconds for a generator to spin up to speed when a load is sensed. Until it's up to speed, you have low voltage and low frequency power being fed to those loads. Some loads can't tolerate that very well. Electric motors in particular don't like starting up under load with low power, and it can shorten their life. You don't need to have your well pump or heating pumps die in the middle of a power outage. It may be that your generator never idles down anyway, due to the loads provided by a cordless phone's base station or a clock, etc., but I prefer not to take the chance.

 

[RalphVa]

We put soft start on the geo compressor. Helped with starting amps. Maybe time to consider new well pump with a soft start. Then you can use smaller generator. Lot there would depend on age of your well pump.

Yes. On the 4.5 kw generator, had to take everything off it and start the well pump. Then turned off the well pump and turned everything back on. Could run for quite a while from the pump pressure tank.

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. Estimated the fuel usage would be something like 0.2 gph. Has turned out to be a tad more than that. Little gen it is.

Oh, someone mentioned about not going too high installed load. With a diesel generator, you can get into wet stacking if it sits there running a tiny load relative to its size.

Ralph

 

[John_Mc]

With a well, I’d go with a minimum of 6500-7500 running. Besides, I doubt a 4000 watt unit will have a 4 wire 240 outlet to power the house.

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.

 

[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.