Help sizing a generator for home backup?

[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).