Help sizing a generator for home backup?

[adettman]

Hi, folks. Does anyone have any insights regarding sizing a portable generator for home backup? My electrician is prepared to set me up with a (legitimate, up-to-code) method for backfeeding my panel in the case of a power outage. I'm barely a novice in terms of electrical work/home wiring and so please don't ask me to provide details regarding his methods or materials.. just know that he is a licensed electrician!

That being said, any recommendations regarding what'd be an adequately sized portable generator to provide backup power for my home? In terms of usage/load, my goal would be to maintain power to the pumps/computer associated with my propane boiler (home heat + hot water), well pump, freezers (one chest, one upright), refrigerator, air exchange system, and then I guess some lights and probably a laptop/phone charger. I don't own air conditioners nor would I plan on running power tools during a power outage. Additionally, I think I could handle the responsibility/"burden" of unplugging my freezers once they got to temp (just meaning that some stuff wouldn't have to stay plugged in/operational constantly). My primary concern would be to keep my boiler functioning (as this will provide my home heat in addition to my woodstove) and my well pump operational.. as well as my fridge and some lights I suppose.

In light of all this, would a 4,000W generator suffice? Or is that cutting it close? Is something like 6,500W more appropriate? [LAST MINUTE EDIT BEFORE BED: Upon some more investigating, sure seems like 6500W is more appropriate in my case. I have yet to dig out the literature on my well pump but was unaware that those were such a heavy draw. Sounds like a 4000W generator would likely handle a well pump just fine but then maybe not much more..? Any thoughts?]

I should also mention (perhaps very important) that my power doesn't go out very regularly. Furthermore, if and when it does go out it rarely lasts more than a few hours. And again, this is perhaps twice a year. I just happen to live in a very cold part of the country (Northern Minnesota) and a modestly priced generator seems like reasonable "insurance" against frozen pipes!

Thanks for your time..

 

[CobyRupert]

You’ve made a pretty good list of the loads you expect to run on the generator. Next step is to add up their wattages.

 

[BrokenTrack]

Usually with a generator, you run out of amps before you run out of wattage.

For instance my generator is a PTO Generator (runs off my tractor) and so it is a 83 amp/20,000 watt generator.

Since a home can generally be run with little worry about shutting this off, or that off at 10,000 watts, I could almost run (2) houses watt wise, but the amps is the limiting factor. To reword that, can you imagine what people would say if I said, "I only have an 80 amp service to my home" when most homes today have 200 amp service? But that is what I have when I am operating by my generator; 80 amps.

If you are on here, I assume you have a tractor. I really like PTO generators because they have clean power unlike portable generators, and since you are not buying an engine, for the money you can get a lot of back-up power for very little money. And there are a lot of rebuilt pto generators (used generators) that would net you three times the power, for the same money as a new portable generator.

The downside is, you are using your tractor so you are putting hours on it just making juice for the family, and sometimes you want to use the tractor at the same time the family wants to watch TV. But I do not find it to be too bad. The family can go without power for the hour it takes to plow my driveway of snow for instance, and I did buy my tractor to use, so what is 18 extra hours on it when my family is happy with power in the house?

And with PTO generators you get a lot better power as it is clean power (safe for electronics) and the tractor can regulate loading better since it has more turning mass, and a better generator like when the water pump snaps on. And a person tends to have plenty of fuel on hand for the tractor, and the fact that it always starts because portable generators can varnish the gasoline if not run now and then. Since most people use their tractors a lot, they always start.

I like PTO generators. When lit, my family cannot tell the difference between grid power and back-up power; which really says a lot.

 

[RandyT]

Need to look up all the running watts of everything that you plan on running at the same time. then add the starting wattage of the largest thing on the list which is most likely the well pump.and then add about 20% to that number and then get the generator with at least that but would recommend more watts than your list showed.

 

[seville009]

Lots of generator sizing websites to assist you - here’s one random site

Norwall Power Expert—Sizing Calculators and Selection Guides

 

[defranks]

Usually with a generator, you run out of amps before you run out of wattage.

For instance my generator is a PTO Generator (runs off my tractor) and so it is a 83 amp/20,000 watt generator.

Since a home can generally be run with little worry about shutting this off, or that off at 10,000 watts, I could almost run (2) houses watt wise, but the amps is the limiting factor. To reword that, can you imagine what people would say if I said, "I only have an 80 amp service to my home" when most homes today have 200 amp service? But that is what I have when I am operating by my generator; 80 amps.

If you are on here, I assume you have a tractor. I really like PTO generators because they have clean power unlike portable generators, and since you are not buying an engine, for the money you can get a lot of back-up power for very little money. And there are a lot of rebuilt pto generators (used generators) that would net you three times the power, for the same money as a new portable generator.

The downside is, you are using your tractor so you are putting hours on it just making juice for the family, and sometimes you want to use the tractor at the same time the family wants to watch TV. But I do not find it to be too bad. The family can go without power for the hour it takes to plow my driveway of snow for instance, and I did buy my tractor to use, so what is 18 extra hours on it when my family is happy with power in the house?

And with PTO generators you get a lot better power as it is clean power (safe for electronics) and the tractor can regulate loading better since it has more turning mass, and a better generator like when the water pump snaps on. And a person tends to have plenty of fuel on hand for the tractor, and the fact that it always starts because portable generators can varnish the gasoline if not run now and then. Since most people use their tractors a lot, they always start.

I like PTO generators. When lit, my family cannot tell the difference between grid power and back-up power; which really says a lot.

Broken Track is missing that current (amps) and power (watts) are co-dependent. The basic formula is power = voltage times current, P=IxE. Your voltage won't change at 230V (two 115v phases) so current and watts will track one another. So consider your load, as others have mentioned, and calculate wattage. Bear in mind that things with motors, your well for example, will require a higher start-up current than it will use while running.

 

[TnAndy]

Something else you might factor in.....the type of fuel. Given you only plan to use this occasionally, you might consider a propane or natural gas fueled unit. Gasoline units tend to be way less forgiving about storage for long periods without use.

 

[seville009]

Also - as with almost any equipment, it’s 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.

I had a “portable” 12,000 watt gas generator before I switched to a whole house 18,000 watt propane stand by generator. The 12,000 watt ran great with everything running; no real hiccups when the furnace or water pump kicked on.

 

[Carl_NH]

.

In light of all this, would a 4,000W generator suffice? Or is that cutting it close? Is something like 6,500W more appropriate? [LAST MINUTE EDIT BEFORE BED: Upon some more investigating, sure seems like 6500W is more appropriate in my case. I have yet to dig out the literature on my well pump but was unaware that those were such a heavy draw. Sounds like a 4000W generator would likely handle a well pump just fine but then maybe not much more..? Any thoughts?]
I
Thanks for your time..

We have the same setup, an oil boiler for DHW and heat and a fridge and lights/tv, but the biggest draw is the 1.5 HP well pump 20 amps starting amps 13 running so we have a 7500W generator with 32 amps @ 240 volts and 9000W starting. It runs everything fine, but we limit the water usage and have a 26 gallon drawdown (two tanks) so it runs maybe 2-3 times in a typical outage of a day, as we limit water use to minimize pump starts.

So depending on your pump size then the rest of the load, I would consider a 8000 or 9000 W running generator.

 

[YanmarFever]

We have the same setup, an oil boiler for DHW and heat and a fridge and lights/tv, but the biggest draw is the 1.5 HP well pump 20 amps starting amps 13 running so we have a 7500W generator with 32 amps @ 240 volts and 9000W starting. It runs everything fine, but we limit the water usage and have a 26 gallon drawdown (two tanks) so it runs maybe 2-3 times in a typical outage of a day, as we limit water use to minimize pump starts.

So depending on your pump size then the rest of the load, I would consider a 8000 or 9000 W running generator.

This apply's to tractors and generators.
Find the one that you want, then buy the next size up.

 

[ericm979]

A well pump can be pretty hefty- ours is 1.5hp @ 220v and is fed from a 30a breaker. Our water system is gravity feed from a 10k gallon tank so we don't need to run the pump during outages even if they're a week.

There's a lot of generator size calculators on the internet like this one: Home Standby Generator Calculator - Easily Size Your Home Standby Generator

Another option is to use meters to check the usage with everything you want to run during an outage on. Then factor in some overload for the extra load when motors start. Your electrician should be able to help with that.

 

[PILOON]

Going from 3/4 to 6/8 KW is not that much of a $ jump to really be a deterrent IMHO.
Later when you need it you will be happy.
Also the 6/8 range usually have electric starting.

 

[EddieWalker]

Where I live in East Texas, we lose power all the time. In the winter, it's from branches freezing with ice, breaking off and damaging the lines. Then we have severe thunder storms in the Spring and Summer, which can take out the power too. Worse case for me has been five days without power in the middle of winter when temps never got into the 30's.

I have a wood stove, so heat and warm water are taken care of.

Since I consider a loss of power an emergency situation, I hunker down and switch to survival mode and only use what I have to use. One light, my phone charger, the computer or the TV and that's really all I have to use. I will not open the refrigerator or the freezer when the power is out so everything remains cold inside them. When the power was out for five days, I did plug in the fridge for a couple of hours every day, and then the freezer to kind of give them a charge at keeping everything cold. I didn't know how long the power would be out, so I made it part of my routine to do that until the power came on. That's the only time it's been out more then a few hours.

My generator is the little Honda 2000 inverter type. It's super portable, starts easy, uses very little fuel and it's super reliable. It only puts out 13 amps, which is plenty for getting by. I don't plan on powering my entire house or anything more then I have to, so this has worked out perfectly for me. I also use it for projects on my land, so the small size is a huge bonus.

My parents had a Onin Generator powered by a Kubota diesel engine for their house back in CA that was pretty expensive. It worked great until the water pump went out, then it was worthless until they got it fixed. Fortunately the Kubota tractor dealer was able to get the pump faster and cheaper then the place they bought the generator from!!! Unfortunately, they didn't have the generator working when they needed it and they had to live in their RV for a few days. They could have ran an extension cord to the house, but the RV was fully functional and easier for them to live in for the short term.

Back in my Marine Corps days we had massive generators at the embassies I was stationed at. Once a month we had to run them for several hours to make sure they where working properly and the fuel was fresh. I only had to do it a few times, and all I did was start it up, and then turn it off a few hours later, and record it in the log book. If you get a full house generator, be sure it's not something that you have to run all the time to make sure it's working when you need it. If it's diesel, how long will the fuel sit? If it's gasoline, the longer the fuel sits, the worse it gets. I've thought about propane, but I always run the Honda generator until it's empty, so that's solved my fuel issues for now.

 

[shooterdon]

The issue I had with capacity was the cost to run a bigger unit. It can add up over a long outage.

My well pump is 220 on a 20 amp breaker and I am guessing it draws about 15 amps when running but will peak higher for a few seconds on start up.

FYI, I get a lot of data from my power company. My average draw is less than 1kW/hour. You really do not use much power hour to hour but you need to be able to start motors and have enough when more than one appliance is running. But how often will you well pump, sump pump, fridge and freezer all start within a couple of minutes of each other and you are running the microwave, toaster and coffee maker as well.

I can run on 5kW if I am careful. 7.5 kW is easy. My whole house is 13kW only because of my silly code requirements.

Gas is OK but can a PITA due to fuel issues...at least use non-ethanol gas with stabilizer. Get a dual fuel and hook it up to LP for ease of use and no fuel issues. It is easy to switch and run on gas if you ever need to. Get electric start and put a good trickle charger on it.

 

[John_Mc]

We are set up to backfeed our panel (with a code-compliant installation). The nice thing about this is that we can manage loads by simply shutting off circuit breakers for the circuit we don't need

I ran the essentials at our home for years during power outages with a good-quality portable 4000 watt generator with a 6600 watt surge capacity. (The surge is used to start up some loads, such as refrigerators and especially our 500' deep well pump.) Our needs were similar to what you listed: Well pump, fridge, freezer, the control system and pumps on our boiler heating system, a few lights and misc., occasional use of our microwave, and the small pump on our mound-type septic system (you don't want to forget that in an extended outage). It too some load management: Since the well pump was a significant start-up load, I'd leave it turned off and minimize water usage. We could get a bit of water use without the pump running, due to the pressure tank. When we needed more water to flush toilets or take a shower, I'd flip off the circuit breakers for the fridge and freezer, then kick the pump breaker on. I might have been fine with one or both of the fridge/freezer on, depending on where they were in their cycle, but didn't want to chance it, since starting running an electric motor under low voltage conditions is hard on it. We'd do similar load management if we wanted to use the microwave. My load management efforts were probably overkill, but we never once tripped the breaker on our generator, even during 4 or 5 day outages.

When that 4000 watt generator died (lightning strike while sitting hooked up to some tools for tree house construction), it was replaced with a Honda generator rated for 5500 watts continuous and 6500 surge. This has reduced the need for some of the load management I used with the smaller generator. We do have to be careful, but we are comfortable during power outages.

I've occasionally thought about a larger generator, or a whole-house permanently installed backup generator, but we really have no need for it. What we have does require us to put some thought into our needs during an extended outage, but we can live with that. We don't need to live as if nothing unusual is going on every moment of our lives. (If we lived further south, AC might not be viewed as such a luxury and would require a larger generator.) I didn't go with the permanently installed generator because I need a portable one anyway, and don't want to maintain two engines.

 

[John_Mc]

Two things to consider:

If you want to run sensitive electronics, get a generator that puts out clean electric power. Note that "sensitive electronics" does not mean just your TV or computer. The electronic controls on some home heating systems will not function if the power is too "dirty" - something you want to be sure to test out before you have a power outage in the middle of winter. The gold standard for quality power are inverter generators, The power quality is generally better than grid power, but they are expensive. You can find non-inverter generators that have good power quality if you dig a bit. You want good voltage and frequency regulation, and Total Harmonic Distortion (THD) of about 5 or 6% or less.

Think about the fuel you run it on. How much gas do you want to store and keep fresh? If you already have fuel stored for some other purpose, consider getting a generator that will run on that also. In my case, I have a 500 gallon buried propane tank for our heat and hot water. Since we mostly heat with wood, that tank doesn't get drained very quickly. I added a conversion kit to our gasoline-fueled generator so that it can run on either gas or propane. (Simple to install, just about anyone can do it with a few simple tools.) For short outages, I'll run the gas we have around for lawnmowers and such, but if things are going to be out for a while, I plug into the quick-disconnect to run on propane. It's a bit of peace of mind, since getting gasoline can be difficult during an extended outage.

 

[CobyRupert]

Note that not every motor is drawing its full load rated amps. It depends on the motor’s actual load and how motor was sized compared to that load. But use that number anyways.

As a rule of thumb, starting currents on motors can be 6 times or more of the motors rated full load amps. Sometimes more. Starting currents are only momentary. Whether these starting currents push generator over its limit depends on a lot of factors:
-Other loads on generator. One can see how the starting sequence of loads can make a difference. That is, if motor loads are started last, their starting current may push genny over limit, rather than if other loads are added after the motor loads are started first. Obviously for motors that automatically cycle (well pump, frig, etc...) this has to be manage manually if genny is not “oversized”.
- Torque response and governor response speed of the genny’s motor.
- Rotational momentum of engine and genny...and other motor loads on system.

 

[jima1]

Two things to consider:

If you want to run sensitive electronics, get a generator that puts out clean electric power. Note that "sensitive electronics" does not mean just your TV or computer. The electronic controls on some home heating systems will not function if the power is too "dirty" - something you want to be sure to test out before you have a power outage in the middle of winter. The gold standard for quality power are inverter generators, The power quality is generally better than grid power, but they are expensive. You can find non-inverter generators that have good power quality if you dig a bit. You want good voltage and frequency regulation, and Total Harmonic Distortion (THD) of about 5 or 6% or less.

This is an excellent point -- generated power can be a lot "dirtier" than what is coming in on the power line. Another help for addressing it is to ask your electrician to install a "whole house" surge suppressor on your electrical panel when the work to install the transfer switch is done. Should not add much cost if the electrician is there already, and you'll never have to worry about buying the surge suppressor power strips for electronics inside your house.

 

[YanmarFever]

We have a Yanmar 12kw diesel gen set we purchased in Japan. Its very quiet because its enclosed, It will run all day on a gallon of diesel at 1800 rpm. Plus it puts out clean power.
It has bailed us out quite a few times. We had a licenced electrician wire it to the house with a transfer switch. It runs the air cond, the well, security system and 90% of the house.
Love diesel, it stores for a long time and its cheap!

 

[BrokenTrack]

I do not have this luxury because I got what I got, but if I was to buy a back-up generator, I would actually buy two smaller units instead of one. That way if I was only using a little bit of power, I would run one, but if I had bigger electrical loads, then I could fire up the secondary unit and synchronize them together to get plenty of power for the load.

It really is only between 5-9 AM and 4-8 PM that a lot of electricity is consumed.

I looked into synchronizers, and they were not as expensive as I thought, so the money saved in fuel consumption would pay for the synchronizers in quick order.

(I am pulling the heat out of the coolant system to heat my radiant floor heated home too...something called cogeneration or combined power and heat (cph), so this really appeals to me as I would not need to operate a big engine if I just wanted the heat from it).