(This question is about a small house in the northeastern US. Power outages are not uncommon, especially in winter, and can occasionally last for more than 24 hours. We are considering a standby generator that is capable of powering the whole house so that the water pump, electric range, freezer, and fridge can all run in the event of an outage. Our furnace is forced hot water and heating oil for fuel, so we'd also be relying on the generator to bring power to the furnace to circulate the heat.)

One electrician told me that in order to install a generator, a propane tank would be necessary. Since we would have to install a rather large propane tank, this increases the project budget significantly. Another electrician quoted the same project but did not mention the need for propane. Is the first electrician right? Is propane the only power source that can work on standby to power a small house?

  • Power outages occur mainly during the winter and you're not concerned about powering your heating source? Are you 100% on wood fireplace? Do you have natural gas?
    – MonkeyZeus
    May 24 at 12:40
  • We'd need electricity to power to the heating source, but the fuel for this is heating oil. So the electricity is just what is needed to circulate forced hot water, not to provide heat.
    – nuggethead
    May 24 at 12:47
  • So I assume that's a no for natural gas?
    – MonkeyZeus
    May 24 at 12:49
  • We don't have natural gas at the house either, if that's what you're asking.
    – nuggethead
    May 24 at 12:51
  • Water pump? Is that a well? May 24 at 13:29

Let's look at your loads.

  • Your furnace loads are fairly minimal; you can look at the nameplate data but water is easy to circulate. 600 watts wouldn't surprise me.
  • Your refrigerator requires about 120 watts when it is running. Overall it needs about 1 kilowatt-hour per day.
  • Your freezer requires about 120 watts when running. Overall it uses about 0.7 KWH per day.

So far that's only about 850 watts... won't even make a small Honda generator sweat.

  • You want a well pump to operate. Most well pumps are "hard start" meaning they presume an unlimited amount of power available from the utility. As such, they need a large generator to start. So you are probably at a 6000W generator just to be able to pick up the well pump without tripping out. (It's a shame; many wells go in with 3-wire+ground wiring because they put the start capacitor at the top of the well. Those same wires could run 3-phase delta driven from a VFD up top, so soft-start would be easy).

  • Your electric range requires as much as 6000 watts when it is running.

Holy smoke! Now we need a generator 8 times larger, +++. Do you really need to cook Thanksgiving dinner during a power outage?

Yesterday's tech

Psst: Furnace oil is close enough to diesel that an older diesel generator won't care. The newest diesel generators may have emission controls that will not work with the sulfur in furnace oil. Do not put it in your diesel car unless you like very hefty fines for evasion of road tax.

The good news for your range is diesel generators tend to be fairly hefty, so no problem with Thanksgiving dinner.

Also, they often tend to be water cooled, and that's free heat you can use!

By the way, fred_dot_u hit on an important limitation of propane. Propane is stored as liquid, and it must boil to deliver to your generator in gaseous form. Boiling requires the latent heat of vaporization to be absorbed. This is why propane bottles get colder when they are in use, and even "frost up". Look at how the air conditioning cycle works, and you will realize what's needing to happen inside the propane tank. It's a serious complication during cold snaps.

Tomorrow's tech

Given your small loads other than cooking, a battery system may suffice. The fridge and freezer need less than 2 KWH per day. Your furnace if it runs 33% of the time (8 hours out of 24) needs - well, look at its nameplate but 600W x 8 hours = 4800WH or 4.8 KWH per day.

Your well is all about the surge current; once it's running it is more reasonable. And it only runs when a spigot is on.

A big lead-acid battery is about 1 KWH ($150). A used Tesla Model S module is 6 KWH ($1100).

Size your battery charger so it can take in the full output of a little gas Honda generator such as an Eu2000. That means the battery doesn't need to make it through the entire outage: you can fire up the Honda at a time of your choosing to top up the batteries.

You want a fairly large, say 4000 watt inverter anyway, so it can handle the startup surge of the refrigerator and freezer (which will sometimes start at the same time). So it isn't a huge stretch to bump it up to a 6000W unit to handle the well also. At that point you have some headroom to run the stove under.

If you crunch the numbers, you can see that batteries, inverter, and battery charger are costly, but not as costly as either a diesel generator, or a propane generator + propane tank provisioning.

It simply becomes a matter of your choices of how much you choose to run the various utilities. If you hunker down in survival mode and do minimal cooking and water running, you won't need as much battery as if you decide to have long showers and cook Thanksgiving dinner.

  • You forgot the well pump.
    – MonkeyZeus
    May 25 at 12:23
  • Interesting, out-of-the-box thinking!
    – FreeMan
    May 25 at 17:00
  • @MonkeyZeus Oh, that was in comments. Well it makes the generator situation worse, to be sure... not that big an impact on a battery system, since it's runtime that affects battery size. May 25 at 17:50

The question about generators is always how much inconvenience are you willing to accept. If "The goal is primarily to keep the lights/heat/water running" A 7.5k will get you by with minimal management. If you want the lights to blink nothing changes but a call for more fuel when the generator turns off a 30k might work. Or not.

As you see in comments fuel storage is an issue. To get a visual a gas (gasoline) portable generator will generally have a fuel tank sized a gallon per 1000w, and will use a full tank of gas when used at 50% for 8 hours. So if you have a 5K generator, and you don't need to run all night a full generator tank and a 5 gallon can will get you through a 24hr outage. I'll let you do the imagining for a 15k generator and a 3 day storm, and how far and often you would have to travel to get fuel if you only store one gas can. Discipline to cycle unused gas through a car or truck every 6 months can be a factor too.

So then the real question is How big of a generator do I need? Everybody's behavior and definition of need is different. Really most people can get by with life safety, communication, and property preservation with a 5kw generator. Having a well a 7.5 would make things much easier. From there your installation and operating expense starts to climb pretty sharply.

All I can do without getting you all bogged down in math is tell you what works for me, and you figure out how it applies to you. I live in an only electric house, no natural gas, cable or even phone available. Since I live in the woods trees knock my power out often. I first pull out my quiet 3500w generator, turn off breakers for water heater, well, electric furnace, and freezers. Then limit my activities for a couple hours to low power consumption activities. Light the wood stove, take care of animals, check news, plug in rechargeables, and stay calm, evaluate, and plan. If still needed after a couple hours I switch to my 5k/6k generator and then I can add one major task at a time. Major tasks: filling well tank, electric water heater, cooking (stovetop only or preheat oven on broil setting), wash machine, dryer. I run the freezers and dishwasher at the same time, might not work for you. After the first couple showers the water heater will need to run for a couple hours. If I had a 7.5k I could do two of those tasks at once. If I needed to go to work and leave my wife and kids alone then I would be thinking a 10k. At 10k three tasks would mean I could pretty much turn on everything except the electric heat and ignore everything else except the noise.

If sticking with portable using a breaker interlock as a transfer switch keep scalability in mind. Up to 7.5k generator only needs a 30A breaker, receptacle, and wire. But if you set up for 7.5k max and ran 10/3 romex to your receptacle and decide you need a 10k you're starting over. Run 6/3 or a 3/4" raceway the first time.


You don't list the type of generator you are planning on using. Some will work on gasoline / propane. One point people often overlook is the amount of power the generator can handle. Generators will frequently be listed as 4,000 watts or 5,000 watts. It's important to remember that is the maximum wattage and not sustained wattage. If you calculate out all of your power needs, you should shoot for installing a generator capable of delivering at least double your desired output. If the sustained load is 2,000 watts, look for a 5,0000 watt or higher rated generator.

Gasoline vs Propane: output

The most important thing to remember is that the wattage is dependent on the fuel supplied. A gasoline generator might deliver 5,000 watts, but the same generator working with propane might deliver less than 2,000 watts. It's about the power density in the fuel. The size of the propane tank should be big enough so that you can get fuel delivered as it's convenient and a big enough tank for a sustained outage. If your generator goes through a gallon of propane an hour, a 100# (25 gallon) tank will last effectively under one day. Whereas a 400# tank will last a few days without refilling.

Fuel Stability:

One area propane beats gasoline is fuel stability. Propane is under pressure and never goes bad. So if you fill up a propane tank and don't have an outage for a year, the fuel is still good. Gasoline is not as fuel stable as it used to be with some of the additives. You will need to add stabilizing agents and rotate the fuel so you do not have an issue when the power goes out. You also need to run it dry to remove any fuel left in the system that can gum up the works as it ages.



I don't know of any generators that run on oil as a fuel source. Gasoline, propane, and natural gas are most common.

If this generator is supposed to kick on automatically then propane or natural gas is preferred since the shelf-life is so high; 30 years for propane.

If you run your current desired load full-tilt then it will consume ~3.2 gallons of propane per hour.

Your current ask:

  • Pump (8 amps)
  • Electric range (40-60 amps) Let's settle on 50 amps per leg so 100 amps actually
  • Freezer (5 amps)
  • Fridge (7 amps)
  • Hot water heat recirc pump (2 amps)

So you are looking for 122 amps (14,640 watts) of running electricity. 82% of your anticipated load will be for the electric range.

Gasoline generators max out at around 12,000 running watts so you would have to get a stationary generator instead for $4,000-$8,000. Make sure to consider a 1-3 second startup surge of 2-3x for the pump, freezer, and fridge.

With gasoline you have to make sure you have fresh gasoline always on-hand. You absolutely cannot fill up the generator today and expect to dutifully start it up 2 years later.

Diesel generators exist but those are far less common and more expensive when you compare the running watts against a gasoline/propane generator; they are however more reliable so if that's important then they might be worth their price.

This is where propane (natural gas if available) becomes attractive. Propane can stay in storage for up to 30 years.

Propane calculations:

( 14640 / 1000 ) * 2 = 29.28 horsepower

29.28 * 10000 = 292800 required btu/hour

292800 / 92000 = ~3.2 gallons of propane per hour

A 5 gallon tank of propane would last you less than 2 hours at full load.



It would be a huge cost savings to remove the electric range from the equation and settle on a 15 amp microwave. This would let you downgrade to a gasoline powered generator.

I think the electrician has to calculate maximum expected usage so pinky-promising them that that you would only use one range burner at any given time is probably not going to fly since the system will have to be inspected.

If you want to bake your cake and eat with the lights on then ditch the electric stove and get a propane stove installed because it would consume far less btus; around 60,000 on direct propane but 240,096 if the propane has to be converted to electricity. That difference is absurd.


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