What size generator works for you?

[Wild Turkey]

We just moved out in the country and are considering getting a generator.

I've done some preliminary calculations but what the calculations don't tell you is what do you really need to live for a few days "off the grid"


For example -- I assume we will want to run the fridge full time plus some lights (LED) in main room, one big TV and my computer during the day.  That's not a huge load, but if we add the water heater the total jumps up a bunch.

Could we get one just big enough to run water heater and nothing else every few days, then shut it off and pug fridge, etc back in?

Planning on an LP-fueled backup heat source so we don't want to get anything big enough to run HVAC.

So trying to figure out how big is big enough but don't want to overspend.

What's the brain trust's experience?

[Snipe Hunter]

Are you looking for portable or a stand-by?
Do you have a heat pump?
Do you have an outside spot to put a gen close to your main service panel?
Do you intend to install a transfer switch?

Being without power or being without enough power for a couple 3 days really sucks. Our 1st generator was too small (6000w) so it was still like camping in the house but we had hot water and a well. The replacement was 15000w (22500w start up) and it runs everything, well, water heater, heat pump and everything else in the house. It was a Generac Portable. Feeding it gas evary 5 hours was a pain, we turned it off at night and just used more blankets. If I wasn't so cheap, I wold have put in a stand-by with a propane tank. Much easier to deal with but we loose power in the winter, usually from ice storms. 24 hour running would have been worth the extra loot. We moved and have more reliable power now but still have the big gen.

If you can swing it, get something big enough to do your whole house.

[blackhat]

Fuel source. Storage and life of fuel. Convenience / hassle of refueling. Convenience / hassle of making electric connections to whatever you want to run. Coffee pot, toaster, well pump, sewage pump, power for that LPG backup heat, freezer,….? 
Portability for set up versus security from theft. 

Lots of things to consider besides size. Most folks consider a 10ish kw standby as a minimum. Run it from the fuel tank for the fireplace and up size that a bit to get 50 or 60 hours of runtime.

[fredhargis]

We had to put a standby in about 4 years ago. I went through some of what you are considering. Being on LP I was leaning toward a smaller unit (17KW, in my case) thinking the 500 gal. dedicated tank would last longer. At the time I looked at some specs that said the fuel consumption difference wasn't all that much, and the 22KW I bought wasn't any higher in price (this was piggybacked with a new heating system). The one thing I wish I had looked into a little more was the required maintenance over the years. We bought a Honeywell (Generac) and their engine design needs the valves adjusted after 25 hours, and then periodically (maybe every 200 hours or so?) afterwards. This is a service call for me, I don't want to fool with that part. I can do oil changes, battery swaps and such...but not the valve adjustment. What I should have done was check to see of if the Kohler generators also needed this....I might have bought one of theirs if they didn't have this requirement. Now, for the sizing....we have a ground source heat pump for primary heat, with a gas furnace back up. The heat pump is 60K BTU, and the generator does support it, but it's a big load and I doubt the tank of LP would last more than 5-6 days or so. My plan is if we have a long power outage, we switch the system over to the gas furnace. Meantime, everything else in the house is working.

[R Clark]

When we moved in, there was a manually switched 7KW stationary generator installed.  Since the house essential systems were all electric, that small generator was only capable of feeding a few circuits and keeping the water well going.  The heat source was a heat pump with electric aux (60Amp!).  We have two LP fireplaces, but they're no real emergency heat solution for 3200 sq ft of house.  Don't even think about using the electric cooktop with that small generator.

The worst thing about that old generator was that the Briggs & Stratton engine would shut down from carburetor icing in certain conditions:  high relative humidity with temps between 29 degrees and 34 degrees.  That meant the generator was not reliable in freezing rain situations when we were at highest risk from power outages.

Long story, but I gradually configured the house with long term goal of upgrading the generator to a unit that would keep us really up and running.

Installed a 22kw Generac with auto switching in February 2019.  Once I was into the 15kw-plus range, it made sense to go all the way to 22kw; the cost difference just wasn't that great.  When I bought mine, the largest air-cooled residential unit was 22kw.  I think the largest air-cooled units are larger now 26kw?

I can power the heat pump and aux furnace (this unit was swapped out in January 2018 and the aux burner is now LP-fueled.)  The backup generator will run typical loads in the house, the shop building (LP shop heater), and even supply basic electricity to the machine shed.  Of course, I don't go crazy and run everything all at once.  I do not have Generac's "smart" power management systems installed.  My main power panel in the house is split between two boxes and handles 400A service.  The smart switching setup necessary for my situation was going to cost $2K, so I opted to forego that.  I can add it later if necessary.  I haven't had the need, yet.

Fuel burn:  My unit will consume about 3 1/2 gallons per hour of propane under heavy load.  I have a max of 800 gallons of propane available at any time (a propane tank is "full" at 80% of stated capacity).  So, under full load, the unit will consume 75 gallons of fuel per day.  There's a lot of variables, but if my outage catches me at a bad time (propane tank <50% full and cold winter weather <20 degrees), I'll need to call my propane guy for a quick delivery if I think my outage is going to last more than 24-36 hours.

If you're switching heating methods right now, there's a part of my experience and the current cost curve of propane and natural gas that may come into play.  I will avoid "going political" on energy costs.  When I installed the new heat pump and backup burner in January 2018, I got in a rush and didn't really consider the cost-benefit of switching to geo-thermal at that time.  I wish I had done that.  Switching to geo-thermal would have saved electrical loading over conventional heat pump and would have reduced power demand on the generator both in cold winter and hot summer conditions, reducing propane usage during outages.  Don't know if that applies to your situation or not.  But if you have the ability to go geo-thermal, give it a strong consideration.

[R Clark]

We just moved out in the country and are considering getting a generator.

...

Could we get one just big enough to run water heater and nothing else every few days, then shut it off and pug fridge, etc back in?

Planning on an LP-fueled backup heat source so we don't want to get anything big enough to run HVAC.

...

Please allow me to add a couple things into consideration for you.

Are you on well water?  You live in the country, so chances are you do.  My well is on a 30A 240V circuit.  The power draw is significant, and it runs anytime we flush a toilet or open a water faucet.

As I considered backup power upgrades, I didn't want my wife to have to do manual switching of anything to keep normal life going, particularly if I was away from home.  From other posts, I think you've mentioned health issues with your wife.  Do you really want to closely mind the backup power generator during an outage?  I think saving $1-2K on a backup solution and taking on switching duties and the like might be a false economy.

HVAC:  It's easy to say that you don't need HVAC now and that you'll run an alternate heat source.  Fair enough in cold weather.  But if there are health issues in the household and you're running your generator after a severe storm in hot weather, then the decision to go at reduced capacity might be regretted since you can't run the AC.  There was a tornado here locally several years ago.  I wasn't affected, but I worked with many people who were.  Some of them were without power for two weeks or more.  Their discomfort wasn't limited to losing a fridge full of food or uncomfortable sleeping conditions.  The humid and hot weather gave many of them mold problems; their modern houses weren't built with good ventilation in the absence of electrical power.

All the above is to suggest that you really consider all of the long term impacts of near-term cost considerations and compromises.

[TDKPE]

Could we get one just big enough to run water heater and nothing else every few days, then shut it off and pug fridge, etc back in?

Planning on an LP-fueled backup heat source so we don't want to get anything big enough to run HVAC.

So trying to figure out how big is big enough but don't want to overspend.

What's the brain trust's experience?

Computers and TV sets are puny loads, as are refrigerators as long as they're not defrosting the evaporator coil.  The blower in a fan-forced gas furnace can be a big load, though, but it's not clear what your propane back-up system uses.

The water heater is the 800 lb gorilla in this scenario, since most are 4500W, and some are 5500W.  Either wattage will use a 30A 240V circuit, so you can't tell from the breaker panel.  You'll have to check the data plate on the water heater to know for sure. 

That's what you should size the generator for, as a minimum.  A 5500W (with a higher surge wattage) unit should be fine, and will still allow you to run a computer and some lights at the same time.  Just make sure the output rating is based on propane fuel, which will usually be lower than the gasoline value if it's multifuel.  Like this one, which has two different ratings.

7750-Watt peak 6250-Watt rated (gas)
6250-Watt peak 5950-Watt rated (LPG)

https://www.homedepot.com/p/Pulsar-7-750.../302907261

I'm not recommending that unit, and don't know anything about it, but I wanted to point out the two different output ratings.

I run my house on a 4000W cont., 6000W surge Generac that's decades old.  NG fired furnace with 3/4 hp blower, power vent water heater, some lights, TV, two large fridge/freezer units, and a medium size freezer, all running at the same time.  I just have to start the furnace first and wait for the blower to spool up before closing the breakers on the other loads, staggered a few seconds apart to allow the refrigerant compressors to start. 

I've also balanced the loads between the two hot legs from its 30A 4-wire shore power cord.  A 240V load like the water heater, on the other hand, is by definition balanced across the two legs as it doesn't use a neutral.  So the only balancing would be between the 120V loads, which are additive to each leg if running concurrently.

[brianwelch]

I've also balanced the loads between the two hot legs from its 30A 4-wire shore power cord.  A 240V load like the water heater, on the other hand, is by definition balanced across the two legs as it doesn't use a neutral.  So the only balancing would be between the 120V loads, which are additive to each leg if running concurrently.

Balancing the loads is a great point. Thanks for the response...

[WoodworkerTom]

I ended up getting a Generac 24kw nat gas generator. 3000 sq ft house 2, central ACs and furnaces. The bump in price fron 22kw to 24kw was negligible.

We lose power frequently since moving and I finally bit the bullet and bought the generator. Basement is fully finished so that helps ease my mind on wayer issues.

I also expect with the huge push to electrify everything to save the planet, the grid is going to be strained especially when its hot outside in the summer. Seems in the Chicagoland area, all the coal plants nearby have been decommissioned and that's all I hear about on the news is more coal going offline.

Perfect storms brewing, it's just a matter of when.

[crokett™]

My generator is a 10kw propane. If you expect to lose power for more than a day or so, get propane. First it doesn't go bad, so there's no worries about it sitting. Second, a big enough tank gives you days of run time, not the hours that the gas tank on the generator does. Third, generally less competition for fuel - you aren't running out to fill gas tanks and your car along with everybody else.