Back-up generator selection questions

kitchendetectiveMarch 2, 2011

This may be the wrong place to ask, but I'm betting some of the great minds on this site can help. How does one figure out what size generator to purchase as a back-up for a whole house? Go room to room and add up all outlets and appliances and then figure some percentage of that (since I never run everything at once)? I'd prefer propane, but I would also like to consider diesel as fuel. And are there brands you specifically recommend?

Thank you for reporting this comment. Undo

On the question of how much power, there are three answers.
a) You want a smallish generator that connects to the house via a manual transfer switch and only powers a few lights, furnace, refrigerators, etc. Then add up their power requirements.

b) You want a generator that will supply the entire house no matter what you turn on or plug in and the transfer switch is automatic. Then the generator should supply up to the level of service the house has, for example, 200A at 240 Vac, or 48 kW. If you know that the house can't draw what the service level is, then a lesser generator will work, e.g., 20 kW.

c) You want a generator that will supply most of the house except those circuits that are load shed and you will use an automatic transfer switch. In that case, figure out what you can load shed without interering with your life significantly (such as air conditioning, dryer, pool pump, sauna, garage heater, etc.) and add up what is likely to be left running or potentially running.

There is a big difference between the power needed for surviving along with not having to drain plumbing or throw out food, and the power needed to continue a party centered a TKO electric kitchen where the inconvenience lasts at most for 20 seconds.

In any case, the National Electric Code requires that the possible load on the generator when the transfer switch actuates not exceed the generator capability. The generator manufacturer will likely claim some limit to how close one can operate continuously to the generator capacity, such as 80%. Load shedding is your friend if you don't have a bottomless pocketbook for an enormous generator. The transfer switch can send a signal to a relay that will allow you to depower verious high loads when running off the generator.


    Bookmark   March 2, 2011 at 6:10PM
Thank you for reporting this comment. Undo

Propane is a relatively clean-burning fuel that won't have difficulty vaporizing over a wide range of conditions. However, the pressure needed for the generator at full load requires the propane tank to have enough liquid surface area to supply the propane required at the lowest temperature the tank will see. This will make the tank rather largish in northern climates.

My Briggs & Stratton generator is rated at 20 kW, and the propane tank is 500 gallons (10 ft long). I buried it for aesthetic reasons. The tank and the burial and the underground line construction all add cost to that for the generator, transfer switch, and load shedding circuitry. Note that there are a pile of propane tank location requirements that have to be met along with generator location requirements.


    Bookmark   March 2, 2011 at 6:23PM
Thank you for reporting this comment. Undo

For a whole house back up generator you will want an automatic transfer switch. What this means is that certain circuits will be transferred to the generator when it kicks in. The number of such circuits will determine what size generator you want. The higher the KW rating of the generator, the more circuits you get. By "circuits" i mean the circuits connected to a breaker in your main power box.

We had installed a 17 KW Generac (propane) generator that has worked flawlessly. Its transfer box accommodates up to 16 circuits, where each 110/120 line counts as 1 circuit and each 220/240 counts as 2 circuits. What you need to do is decide what you want to have current when the power goes off. In our case, we're on well & sceptic so we wanted the well pump and water tank to have power. We wanted the fridges to have power. We use wood stoves so we didn't want the heat pumps on the switch but we did want lights and wall outlets in the bedrooms, family room, kitchen, etc.

When the power goes off (we've lost power 4 times so far this winter), the transfer switch automatically kicks in after about a 10 second delay. The power is well regulated so you don't notice any difference (no flickering, etc.) from the lights, computers, or TVs. When power is restored, the generator automatically turns itself off.

    Bookmark   March 2, 2011 at 7:23PM
Thank you for reporting this comment. Undo

If a 48kW LP generator were running at one half capacity, how much propane would it consume in an hour? How do I figure this? Has Briggs and Stratton stopped making a 48?
I have a 500 gallon tank that I use for the house now. I also have a 250 gallon tank that I'm not currently using. Our house is on county water, but the ranch has 2 wells for stock ponds and the house can switch over to wells in an emergency, although we've never done it. I was thinking a small solar energy system for the well pumps. As for barn and outbuildings, they won't be on the generator. Our fireplaces provide a great deal of heat in cold winters, of which there have been few. This past winter was the coldest in my 16 years here in the South. (It actually went down to 17 degrees F.) My BIG consumption of electricity happens in the months of August and September when air conditioning is mandatory. We have four AC units, two of which are heat pumps. Okay, this is long-winded, but addresses some of the issues mentioned above

    Bookmark   March 3, 2011 at 12:28AM
Thank you for reporting this comment. Undo

Some of my post did not show up. Weird.
I like the idea of automatic switching after a 10 second power delay. There's no pool pump or pool, but there are sound systems, computers, TVs, security systems, one electric convection wall oven, one electric warming drawer, a huge vent hood, one combination OTR/microwave/convection oven/external vent, another microwave oven. The other ranges are gas, as is the dryer. The septic has an electrical aeration-secondary treatment component, now that I think about it. Mandatory in my state. Off the top of my head.

    Bookmark   March 3, 2011 at 12:41AM
Thank you for reporting this comment. Undo

We have the same setup as geo91324 - the same generator in fact - plus the transfer switch. We use propane for the heating boiler anyway, plus the cooktop, so it was natural to add the line for the generator. We can pretty much run the whole house on 17kw.

    Bookmark   March 3, 2011 at 9:21AM
Thank you for reporting this comment. Undo

Kitchendetective wrote: "If a 48kW LP generator were running at one half capacity, how much propane would it consume in an hour? How do I figure this?"

Although there are some propane dealer websites with a lot of tables of information, you could scale from the 20 kW generator information at Briggs & Stratton's web site, or likely most other generator websites. (You may need to download the installation manual to get the full table set.)

The 20 kW model at full power uses 142 cu ft / hr, and the density of propane is 36 cu ft / gal, so you might need around 5 gal/hr at 24 kW.

However, your generator may need a tank larger than 500 gal to operate at 17F. There are tables for that also. You might squeak by at half load, but a 750 or 1000 gal tank would be better considering that you are also using it for heating.

On your previous question about choice of manufacturer, unless you will be the repair person, try to determine who are the factory authorized repair persons for the brands available to you. Are they well reputed? When you call them do they answer, do they still support the manufacturer, can they get parts, do they disparage the brand?

The best way to determine actual load is with an ac current meter that hooks around each feed conductor. These are not inexpensive, however, so it might be less costly to hire an electrician for an hour to make measurements as you configure the house for different load conditions.


    Bookmark   March 3, 2011 at 12:27PM
Thank you for reporting this comment. Undo

You are wonderful, folks!

    Bookmark   March 3, 2011 at 12:50PM
Thank you for reporting this comment. Undo

A 48KW generator is one large and expensive generator. Unless your backing up a commercial business or backing up absolutely everything in a home (which doesn't really make sense), even 24 KW is too much.

As Sara points out, 17 KW does a real nice job. Remember, this is a BACKUP generator--it is not your primary power source. Our house is over 7,000 sq ft and while not everything is connected to the generator, everything of importance is.

Our 17KW generator uses 0.9 gal/hr. The longest power outage this year was 22.5 hr, so the actual consumption was about 20 gallons--a small price to pay for not suffering the inconvenience of no power and, in our case, no water. (We could have manually turned off the generator when we went to bed and saved some propane but we just left it on all night).

Find a reputable electrical contractor that specializes in generators and talk to them. I personally believe that a 500 gallon tank is more than sufficient for a 20KW or smaller generator. Ask about circuits to connect and those not to connect, ask them about who does their gas hookups (usually subcontracted out), and ask them about the gas pressure regulators they use (which to me is more important then the size of the tank to ensure no problem performance).

    Bookmark   March 3, 2011 at 1:31PM
Thank you for reporting this comment. Undo

I'd agree that a 500 gal tank is sufficient. Ours is ample for the boiler, cooktop and generator (if needed). In winter we probably get refilled every 4 weeks, not at all in summer. We discussed it at length with the propane suppliers, and our GC, who has the same set up as us.
Uh, yeah on the regulator - we had to get a new regulator on the tank this winter, because it froze and the heat went off. . . . not too clever when the temps are in single digits and there's 4 feet of snow on the ground . . .

    Bookmark   March 3, 2011 at 5:00PM
Thank you for reporting this comment. Undo

geo91324, our house is close to the size of yours. We've not had power out for more than two hours at a time; however, prior to this winter, it had never gone out for that long, either. We seem to be north of the hurricanes and south of the tornadoes, but, really, not by a lot, so I'm trying to think about near-worst-case scenarios. Also, there are times when the house is full of lots of visitors for elaborate overnight fundraisers. If I have a protracted power outage during one of those events, it seems I might need a generator like one for a small hotel. I'll have to think about this.
Kas, what are the requirements for burying a propane tank? Is the equipment more likely to corrode or degrade underground, or do you encase them somehow? My current two are above ground.

    Bookmark   March 3, 2011 at 5:43PM
Thank you for reporting this comment. Undo

I just installed a backup generator with a propane supply. Some additional things I learned....

Several things are the main users of power:
--heating elements like electric heat, ranges, fireplaces
--startups of electric motors, like those used in water pumps, washing machines, etc, draw a substantial amount of power initially.
Lights, televisions, and the like really don't use much power.

Most generator company websites have calculators to help determine how big a generator you need, but your local generator installer is apt to be more helpful.

If you're planning on a service contract for regular maintenance (oil & belt changes, etc) and emergency service, you should explore this and budget for it. I discovered after the fact that contracts in my area are $500+ per year.

Your town's building and zoning codes will tell you where you can bury a propane tank. You will likely need to have an inspector sign off on the site.

According to my propane company, the longevity of a buried tank is 20-25 years. You might want to think about how this affects the resale value of your property in 15 years.

The size of the tank required is determined not only by the amount of propane your generator will require, but how quickly the liquid propane evaporates into its gaseous form. This is affected significantly by ambient temperature. Those of us in northern climates require larger tanks to facilitate that evaporation. The type of regulator is also important, as mentioned above.

    Bookmark   March 3, 2011 at 7:08PM
Thank you for reporting this comment. Undo

Ditto to what warmfridge says but it shouldn't cost you anything like $500/yr for maintenance. We were offered a $110/yr maintenance contract which also included the necessary oil change, oil filter and air filter. Since the oil and filters only cost around $30, I decided to do it myself (it is, after all, just a small 4-stroke engine).

    Bookmark   March 3, 2011 at 7:56PM
Thank you for reporting this comment. Undo

My contract is for routine maintenance and emergency calls. There aren't many companies that service generators where I live, and the cheapest was $500/year. That's why I suggested the OP check the available service in her area.

    Bookmark   March 3, 2011 at 8:07PM
Thank you for reporting this comment. Undo

The tank is buried in a hole that is at least 2 feet wider and 2 feet longer than the tank. The hole is deep enough that only the piping stack shows when filled in, probably putting the top of the tank at least a foot under. The bottom of the hole has to be deep enough that it can be backfilled with rock-free sand. The entire tank is eventually filled in with filtered sand.

In areas subject to flood or high ground water level, a concrete pad is poured into the bottom of the hole and the tank is chained to it so the tank won't rise out of the ground.

The underground tanks are epoxy painted, and electrically bonded to a bag of magnesium to act as a sacrificial corrosion element. Your propane dealer should have means to test this periodically for efficacy. Put the bag in a location near the propane tank where it can be dug up without much risk to the tank. It should be at least as deep as the bottom of the tank, and watered in before the tank space is filled. (Be sure that the bag is removed from its shipping box before placement.) I don't recall hearing a tank lifetime value, but the magnesium will have to be replaced periodically depending on soil moisture, acidity, and other factors. Your local propane dealer should provide information.

In addition to the tank hole, you also have to trench to the generator pad, and trench from the generator to the transfer switch. Don't put the generator next to the house, and ensure that it has total "air rights" above it. According to my town's fire code inspector who was once a fireman, generator fires are quite spectacular.

Don't skimp on the power feed conductors; you don't want to waste some of your power heating the ground. Large conductors are not limp. Use 2-inch PVC conduit for the power feed from the generator to make wire pulling easy. Use an expansion joint at each end above ground. The control wiring conduit can be smaller. Most of the detail you will leave to your electrician unless you are qualified yourself, and in that case you will have means to find out.


    Bookmark   March 3, 2011 at 8:21PM
Thank you for reporting this comment. Undo

Your electrical contractor/installer will be knowledgeable about all local codes and common sense recommendations in terms of installation. I don't believe I've heard of any code that forbids (or even recommends against) having your generator against your house. There must be clearance around the unit (say 3') and there are restrictions as to how close it is to any openable window.

    Bookmark   March 3, 2011 at 10:07PM
Thank you for reporting this comment. Undo

I can't take the time right now to hunt down the relevant code, but the Briggs & Stratton installation manual shows the requirement to be 2 feet from a non-combustible material with Fire Resistance Rating of 1 hr or greater, and 5 feet from a combustible material or structure with a Fire Resistance Rating of less than 1 hour. Town, state, or national code, I forget which, requires a clear vertical path above this zone of 20 feet.

What this normally means is that wood framed houses with roof overhangs require 5 foot spacing away from the roof drip line. In any case, as quiet as these larger units can be made to be relative to portable generators, putting them close to a house will increase the sound level inside, potentially to an annoying level. That may lead to additional sound inside, not emitted by the generator.

Your town's fire code inspector is the authority having jurisdiction, and should be consulted early enough to avoid costly errors. Your propane company will likely put its oar in also.


    Bookmark   March 4, 2011 at 12:09PM
Thank you for reporting this comment. Undo

I was out of town and just saw the further replies to my questions. Many thanks for your wonderfully explanatory information, everyone. I've put off this process too long. The electrician who did out home is wonderful. Nevertheless, I want to go into this with as much information as possible, so you are all helping me tremendously. (The 12V, security, and computer/TV/sound systems were done by another company and there were some problems coordinating the electrical with the former. The sound guys are no longer in business, and I'm anticipating difficulties, perhaps needlessly.)

    Bookmark   March 4, 2011 at 9:42PM
Thank you for reporting this comment. Undo

Interesting thread as this is on our to-do list as well. We will be going with NG as that is what fuels our furnace and water tank. The most our house uses - during A/C season - is about 80kWh/day. Peak hourly usage I reckon would be about triple that rate (or ~10kWh), obviously more if we run our central a/c, dryer, and range simultaneously (but, why would we during a temporary power outage?). I've spoken with our electrician and he feels that, based on our needs (well, sump, electric range, dryer, a/c, pool pump), we could get by quite comfortably with a 12kW stationary unit. We'll do a more thorough power survey before we settle on size but it's good to have a starting point.

Our neighbours both have smaller units (around 7kWh - these are smaller homes and there are no kids) - one has a propane unit and the other is NG. Both units are Briggs&Stratton and they are loud when operating. Fortunately, we live one treed estate lots and it's not overly intrusive.

FWIW, 2 years ago, CR tested back-up generators and placed the Kohler 12RES on top in the stationary category due to quietness and quality of power generated. I like Kohler's claim that this unit will restore power within 10 seconds. The more I've researched brands, the more I like the Kohler unit - although it's more expensive than Generac/Guardian or B&S, the price includes a transfer switch. OTOH, G/G and B&S appear to have a stronger support networks in my part of Ontario. I'll probably pull the trigger later in the year and have already lined up my electrician.

    Bookmark   March 5, 2011 at 9:00AM
Thank you for reporting this comment. Undo

One tidbit I learned from an owner of my electrical distributor: Don't put the unit where an automatic water sprinkler can fire a stream up into a vent. Seems a generator that he had sold wasn't performing correctly, and after the installing electrician and manufacturer's approved service people couldn't figure out the cause, he visited the residence. The time was right because as he examined the unit, the sprinkler system came on and blasted him and the internals of the generator housing.


    Bookmark   March 5, 2011 at 12:55PM
Sign Up to comment
More Discussions
Any thoughts about Bosch and GE Profile appliances?
I am building a passive house and trying to choose...
Does Wolf M-Series Double Oven come with Temperature Probe?
If yes, great. If not, does anyone know how much the...
36" induction cooktop with bridge
So far we've only found two 36" induction glass-top...
Bosch Speed Microwave Oven vs. GE Advantium?
We had a GE Advantium for 7 years and used it 2-3 times...
Dishwasher with cutlery tray and heated dry?
I seem to be coming up fairly empty handed with these...
Sponsored Products
Olde Bronze Mini Pendant 1-Light
$60.00 | PLFixtures
Hyde Park Outdoor Daybed with Cushions, Patio Furniture
Nourison Indoor/Outdoor Area Rug: Nourison Rugs Craftwork Autumn 7 ft. 6 in. x
Home Depot
Tay Tay Chrome Three-Light Square Canopy Bath Fixture with Ceylon Glass
$418.50 | Bellacor
Ribbon and Greenl Stripe Rug - 2'3" x 8'6"
Grandin Road
Poeme Blue Brown Rectangular: 5 Ft. x 8 Ft. Rug
$506.00 | Bellacor
Calhoun Sectional - Key Largo Grape Purple
Joybird Furniture
OttLite Lamps 11 in. telescoping arm blue desk lamp 964508
$35.17 | Home Depot
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™