Standby generator

[nhraracer90]

Hello! I am thinking of getting a standby generator. Either. 12 or 16000 watt. Roughly what size propane tank should I get? I was told by one company at least a 250 gallon tank as anything else would be to small because the smaller tanks wouldn’t be able to produce enough vapors to keep up with the demand from the generator depending on the load. I am not sure how true that is. Any help would be appreciated.

Thanks


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[sixty4]

Hello! I am thinking of getting a standby generator. Either. 12 or 16000 watt. Roughly what size propane tank should I get? I was told by one company at least a 250 gallon tank as anything else would be to small because the smaller tanks wouldn’t be able to produce enough vapors to keep up with the demand from the generator depending on the load. I am not sure how true that is. Any help would be appreciated.

Thanks


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I would agree with the 250 size. I have a tank butler on mine that alerts the lp company if tank gets to low. I run house outdoor garage cooking hw heater and 20 kw however my tank is larger.

 

[ezover]

the question is, how long do you want it to run for.. I would put in a 500 gal tank.

my house runs on lpg and I have 1 500 gal tank. I have been meaning to get a second one just for the generator.

 

[Stuart in MN]

The generator spec sheet should have a figure for gas consumption per hour. Get that number, multiply it by how many hours you want it to run on a tank, and you'll have your answer.

 

[ant.foste]

How long would you expect the generator to run, and at what time of the year?

 

[wyliesdiesels]

Lets back up

What are your loads?

You may be able to get a smaller gen and run longer on the same size tank.

Do you really need that much power?

Thats 50-67a @ 240v or 100-134a @ 120v

You have that much running at once?

 

[Bretny]

The generator spec sheet should have a figure for gas consumption per hour. Get that number, multiply it by how many hours you want it to run on a tank, and you'll have your answer.

Stuart is right. And so was the propane company but it can depend on if the tank is underground or above.

You really need to figure out two things, burn rate and how long you want to run the thing for.

Maybe someone in FL or some other high generator demand area could say how long from when they call the propane to time of delivery? I'm in NY and have seen fuel shortages, not sure on propane.

 

[Fasthotrod]

I was told by one company at least a 250 gallon tank as anything else would be to small because the smaller tanks wouldn’t be able to produce enough vapors to keep up with the demand from the generator depending on the load. I am not sure how true that is.

Absolutely 100% true. The thing about LP powered systems is that you typically have a vapor withdraw system. The propane in the tank wants to be a gas at typical ambient temperatures, and it's pressure is directly related to the ambient temperature of the tank.

Have you ever used a can of compressed air to clean off your keyboard? Notice how the tank gets really cold when you hold down the trigger for a while? That's the Ideal Gas Law at work: PV=nRT where:

P = Pressure
V = Volume
n = Amount of Substance
R = Ideal Gas Constant
T = Temperature

So as you release Pressure from a tank with a fixed Volume, the Temperature drops. (That's how an air conditioning system works.) So guess what happens when the small tank gets cold? You lose vapor pressure and you starve the engine for fuel... and it's game over.

So if the tank is too small, the propane release will cause a cooling effect, and your fuel pressure will drop. The bigger the tank (or if it's buried in the ground) allows for more heat transfer into the tank which allows the tank pressure to stay at an operating level.

I've been in the power industry for about 30 years, and this is one area that many people get wrong... we had a mechanical engineer basically make this his career, going out and fixing people's mistakes after a new installation didn't work the way it was "supposed to work." One big tank, multiple small tanks... whatever it takes to increase the overall surface area that can act like a heat sink to draw heat into the tank to prevent the pressure drop is what we're looking for here.

Not to mention overall run time and tank capacity... don't forget that you typically don't fill a tank to 100%. There's some head room to consider there for the vapor in the tank... so most are filled to about 80% total capacity.

I don't know where you live, but if you are in an area that has lower ambient temperatures in the winter, this problem just gets worse.

Hope this helps.

Mark

 

[yeldogt]

Keep in mind that you can only fill them at 80% .. and you can not allow them to go under 10% ... some places it 20%. If they fall under this level many companies will not fill -- unless you are on "auto" and they are tracking usage. Have to have a leak test done to get refilled if it's an "on call" tank

A bigger tank gives you some flexibility ---- but -- how much flex do you need? If that's the only thing on the tank and it's only typically needed for 1hr twice a year ... how big do you need? If your power can go off for days -- you need to always have that on site.

What I discovered: I ended up with the 20/22 Cummins unit that was redesigned a couple years ago. The 20/22 is typically the largest air cooled unit w/ all the manufacturers. When I looked at the specs the unit had a burn rate not much different vs the 12/14 ,, especially when you factor the load --- the smaller unit will be running at full. The larger units running at 1/2 load did not burn much more.

The other factor for me was the cost -- the cost difference between the different units is small. The savings going smaller is not much ... I bought online and ordered everything including the pad ... extra control for inside the house ... load relays. Around 5k

Depending on how you will be cutting in to your house -- the bigger units can again be less expensive. You can just cut between the meter and the panel with a unit big enough to do the whole house. Smaller units often need separate dedicated panel and rewiring.

Today code mandates the unit must be able to take care of the house if has an auto start -- I did need to add relays to load shed my electric range and a couple other items. But the unit came with this capacity and they only cost was the relay pac -- $200 for 2 loads.

So, it's not as straightforward and logical as one would think. I started out thinking small and dedicated to most needed items -- ended up with larger and the whole house. If you have an AC ..make sure whatever unit you purchase can start the unit. I was not really thinking about AC initially ..my main concern in PA was longer winter outage. We only lose power in the summer for 15-30 min ... but it's common .... in that time the house is always fine with AC off. In WV that may be needed for you -- don't know.

I also went with a 1k tank -- but -- I'm using the propane for other things as well (heat/stove/dryer/outbuilding).

 

[Jagmandave]

Good info yeldogt, the bride has been talking about a whole house genset for a while now and we've been looking into it lately. I also thought I could just get an either/or switch and tie it into the main panel thru that......mine would run on nat gas as I'm in the city. Being able to start the whole house A/C is a major factor, that and her being able to start it with minimal action on her part.

I'm probably going to wind up with a fully automatic system......

 

[CudaChick1968]

We bought one last April and, after much research and YouTube videos, went with a natural gas option versus propane.


Imagine the power going out long term. Any sized tank is going to get used up pretty quickly, and the propane version NEVER empties the tank completely before it stops pumping. Where are you going to get propane if, as now, many proprietors are closed and only emergency services are allowed on the roads?

 

[Jagmandave]

I'm looking at a Generac 16K with the "whole house" switchover, and wondering if that can suffice as an outside disconnect too? Doing more research for now....

 

[Fasthotrod]

I'm looking at a Generac 16K with the "whole house" switchover, and wondering if that can suffice as an outside disconnect too? Doing more research for now....

Yes, it can... provided you get one that is rated as a Service Entrance switch. You would install it between the meter can and the main panel, and convert the main panel to a sub-panel by installing ground bars, moving the grounds over, and isolating the neutrals by removing the neutral/ground bond in the panel. (The neutral/ground bond now takes place inside the ATS.)

I installed a Kohler 20kW with a 100A service entrance rated Automatic Transfer Switch (ATS) at my in-laws. They love it.

I've considered one at our place, but thankfully our power doesn't go out very often.

http://www.kohlerpower.com/home/home-generators/transfer-switches

Hope this helps.

Mark

 

[yeldogt]

Good info yeldogt, the bride has been talking about a whole house genset for a while now and we've been looking into it lately. I also thought I could just get an either/or switch and tie it into the main panel thru that......mine would run on nat gas as I'm in the city. Being able to start the whole house A/C is a major factor, that and her being able to start it with minimal action on her part.

I'm probably going to wind up with a fully automatic system......

I really liked the Cummins unit ... take a look ... the 20 that comes with the transfer switch. The Kohler is nice as well.

The transfer switches are very large ....

It'e SE rated .... if you have a spot outside the transfer can go outside and it just gets cut into the line after the meter -- transfer to existing panel.

The generator goes right into the transfer switch.

With NG get a unit that will do the whole house.

In my case the house has my studio with AC -- a very large stove with two electric ovens. Two HP's in the main house ... well pump ... all the normal electric items. It's a big house -- so even the 20 was a tad short.

Don't get the 16 if the 20 does it all without issue .. and you need to load shed w/ 16

It has to be idiot proof .. Power goes out and someone starts turning the house on ..

 

[nhraracer90]

I have a small 1200 sq ft house. I don't plan on running a bunch of stuff. I would like to run the central AC/heat (2.5 ton), fridge, freezer, well & sump pump and some lights. I don't have natural gas where I live so propane is my only option. I think a 12k generator would work for me since I don't need everything running at once. I would like it to be able to run for a few days. Depending on the time of year, I wouldn't really need it running all the time.

 

[yeldogt]

I have a small 1200 sq ft house. I don't plan on running a bunch of stuff. I would like to run the central AC/heat (2.5 ton), fridge, freezer, well & sump pump and some lights. I don't have natural gas where I live so propane is my only option. I think a 12k generator would work for me since I don't need everything running at once. I would like it to be able to run for a few days. Depending on the time of year, I wouldn't really need it running all the time.

Read and understand my post above ..

Code does not care what you "want" ... you have three choices. Dedicated panel / unit with load shedding / unit able to run whole house.

You can not have an auto transfer switch w/o the unit being able to deal with whatever is thrown at it.

Figure out what the whole house needs -- see if an air cooled unit will do it. Cut in the transfer switch. With a dedicated panel you need to pull those items from the main panel and rewire .. In my case the cost to do the dedicated panel was higher than doing the full cut in. So the bigger generator was more money -- minus the labor cost. The difference was not worth it ..

I can run anything I want .. with little fuel difference.

Years ago there was limited sizes .. one had little choice except the dedicated panel. That's what I have at my beach house ... there was no practical way of doing the whole house. I think it was around $800 difference ... no way the limitations are worth going that route

 

[moab11]

Something to keep in mind is that most manufacturers list full load fuel consumption only, and real fuel usage is usually quite a bit lower.
We have a Generac 30kw unit running off of a 500gal tank that also runs the garage heat. Everyone was freaking out when we had it installed that the generator wouldn't run for a day on that tank, and going by the full load fuel consumption, that may be true. Pretty hard to run this big of a generator at full load though. In reality with normal household usage, the generator will run for days easily, and has run for 48+ hours with no issues.

 

[theoldwizard1]

Code does not care what you "want" ... you have three choices. Dedicated panel / unit with load shedding / unit able to run whole house.

You can not have an auto transfer switch w/o the unit being able to deal with whatever is thrown at it.

There is another "special case". If you have special service for an air conditioner or water heater, where the PoCo can shut it off during high loads it will be wired to a separate meter.

When your generator kicks in, your A/C (water heater) will not get power ! However, the installer can add a separate "bypass" switch wired to your main panel and then you can "manually" engage power to your A/C.

Automatic load shedding transfer switches can not do this.

 

[theoldwizard1]

Lets back up

What are your loads?

You may be able to get a smaller gen and run longer on the same size tank.

There you go, trying to be LOGICAL ! This is GJ, where OVERKILL is the rule !

(With careful management of your loads, and no 240V loads, the "typical" 2000 sq ft house can run on a 2000W generator. Maybe even one small window A/C !)

 

[yeldogt]

There is another "special case". If you have special service for an air conditioner or hat water heater, where the PoCo can shut it off during high loads it will be wired to a separate meter.

When your generator kicks in, your A/C (water heater) will not get power ! However, the installer can add a separate "bypass" switch wired to your main panel and then you can "manually" engage power to your A/C.

Automatic load shedding transfer switches can not do this.

The way they do it around me is a relay on the units -- no special meter. Still have to factor them.

There are always going to be odd situations ...

I'm only trying to point out that if you are doing a auto set up -- the rules can make the final decision different than what you think after everything is factored and priced

 

[ericm]

Generac makes load-shed devices that do not require extra wiring to be run. They sense the AC cycles and have adjustments so that multiple loads don't try to turn on at once. This is probably not quite as good as wired load shedding but it will cost less.

Load calcs and load shedding to meet the generator's capacity are requirements for permits here. An electrician friend just went through this on a generator install.

All the whole home generator specs I have looked at have fuel usage at 100% and 50% capacity. Some list other fuel usages too. It's not a direct correlation. The generator uses more fuel per watt at lower loads than at higher loads. It varies between generators due to the engine size and other design factors. These generators all run at a fixed rpm with a governor controlling the throttle to add power and maintain rpms when more output is requested. There are parasitic power losses with an engine running at 3600 rpm (or whatever the fixed rpm is, that's a common one to get 60 cycles). Oil pump, pushing air around inside the crankcases, bearing and piston friction, cooling fan, etc. all eat a fixed amount of power.

So the most efficient generator will be one that's running close to full out most of the time. Of course you want some head room in case you add something to the house in the future.

AC, well and sump pumps all take a fair amount of power.

Keep in mind that when you're running off a propane tank, it may be a while for the propane company to fill you up. If it's the same tank that you use for household use, your outage might start when the tank is low and is due for a fill. If you're in a place like I am where power outages often coincide with a road closure, size the tank appropriately.

You'll want a way to turn the automated system off. Folks up here have gone on vacation, had one of our common days-long power outages while they're gone and come back to a cold dark house and an empty propane tank.

 

[theoldwizard1]

The way they do it around me is a relay on the units -- no special meter.

The service is metered because it is billed at a different rate.

 

[theoldwizard1]

Generac makes load-shed devices that do not require extra wiring to be run.

Load shedding automatic transfer switches are common. They don't work if the A/C is on a separate meter.

 

[Stuart in MN]

The original poster didn't say anything about load shedding. I don't know why you guys are talking about it, you'll just confuse the issue.

 

[nhraracer90]

The original poster didn't say anything about load shedding. I don't know why you guys are talking about it, you'll just confuse the issue.

Thank you lol. I know nothing about that.


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[nhraracer90]

Also, what size wire should be ran from the generator to the house?


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[yeldogt]

The original poster didn't say anything about load shedding. I don't know why you guys are talking about it, you'll just confuse the issue.

You have to have it based on the load of the house and the generator ... asking about it does not = not eventually needing to know about it.

 

[yeldogt]

Thank you lol. I know nothing about that.


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That's why some of us have mentioned it and tried to give you some information so you can search a bit more.

Then you ask about wire size ?

If this is inspected -- you will need to do a load calculation. It has to be done correctly with an auto switch (even with dedicated panel) and the unit must be able to manage the load. That can be done different ways.

Even if not inspected (not sure what utility that would be) ... you want it correct and you want the most bang for your buck.

 

[yeldogt]

The service is metered because it is billed at a different rate.

They would give a discount ... previously

....now I'm told they can do it with an amp meter.

The old system required a larger tank and the unit would not heat during peak times.

 

[Junkman]

I know that you have said that you want to go with propane, however, have you considered diesel? Fuel is easily stored, readily available, and a diesel generator is extremely efficient. I run mine on home heating oil. I installed an ASCO automatic transfer switch. Since we have frequent short outages, I have installed a switch between the automatic switch and the generator switch that I can turn off in the home, so the generator will not come on for short periods. If we were out for 8 or 12 hours, and the power failed, the generator will not come one, until we come home and flip the switch. If we are going to be gone for a longer time, we can either have a neighbor come over and turn on the generator by flipping the switch, or flip it before we leave. Something like this will save you a lot of fuel and can easily be installed on most units, regardless of the fuel type.
Before we got the generator, we would have 7 - 10 day power outages. After installing the generator, the longest one that we have had is about 12 hours. Seems that once you buy the generator, you won't need it, but it gives you peace of mind knowing that you have it.

 

[archtimb]

How about a different tack?
We already have an installed system. No load calcs, wire size or fuel decisions to make.
Propane. 300 gallon tank. 12k Kohler.

What about multiple 100 gallon tanks that can be personally transported to somewhere you CAN get propane in times of need, or a couple extra tanks in the shed?

Can this get around the vapor requirements, vis a vis, surface area?

Feedback?

 

[theoldwizard1]

The original poster didn't say anything about load shedding. I don't know why you guys are talking about it, you'll just confuse the issue.

Because he will likely GET IT EVEN IF HE DOES NOT WANT IT !

 

[archtimb]

How about a different tack?
We already have an installed system. No load calcs, wire size or fuel decisions to make.
Propane. 300 gallon tank. 12k Kohler.

What about multiple 100 gallon tanks that can be personally transported to somewhere you CAN get propane in times of need, or a couple extra tanks in the shed?

Can this get around the vapor requirements, vis a vis, surface area?

Feedback?

So nobody has tried this?
I guess I am the Thread Killer

 

[Farmall450]

We bought one last April and, after much research and YouTube videos, went with a natural gas option versus propane.


Imagine the power going out long term. Any sized tank is going to get used up pretty quickly, and the propane version NEVER empties the tank completely before it stops pumping. Where are you going to get propane if, as now, many proprietors are closed and only emergency services are allowed on the roads?

Many people don't have the luxury of NG

 

[yeldogt]

So nobody has tried this?
I guess I am the Thread Killer

Have you ever tried to move a 100g tank -- it's way way too heavy.

100g does not = 100lb


There is also a limit to the size tank you are allowed to move w/o special permit.

 

[wyliesdiesels]

How about a different tack?
We already have an installed system. No load calcs, wire size or fuel decisions to make.
Propane. 300 gallon tank. 12k Kohler.

What about multiple 100 gallon tanks that can be personally transported to somewhere you CAN get propane in times of need, or a couple extra tanks in the shed?

Can this get around the vapor requirements, vis a vis, surface area?

Feedback?

Good luck with that.

Propane weighs 4.24Lbs per gallon

Thats 424Lbs plus the weight of the tank.

 

[archtimb]

Mmmm. Yeah I meant 100#, not 100 gallon.

 

[dfiler2]

Around here the problem with a 100# tank is the will freeze up if you try to pull too much off of them which is what I think th OP's supplier was concerned with. You probably don't have to worry about -44 but you can run into an issue by having too small of a tank.

//www.powerblanket.com/blog/how-can-i-prevent-my-propane-tank-from-freezing/

 

[mike93lx]

Even a 100lb tank weighs a lot. Add in moving them over ice and snow...

You really need a horizontal tank to get surface area.

My parents have two sets of 3 200lb tanks for their pool heater as they couldn't do a large horizontal due to location. Even in 100 degree heat and sitting in the sun, they frost up under heavy load in the summer. In the winter, they would be completely frozen

 

[archtimb]

Thank you for the replies. I do understand the freezing up concerns. What I was really wondering was if the load was split between 2-3 tanks tied together, the draw on each tank might be less and not lead to freezing?

After using construction heaters and 100# tanks over the last 40 years, I am quite familiar with moving these around in all types of conditions.

Also since moving from northern PA to western OR, I mostly thumb my nose at winter! I don't miss my plow truck, snow shovels or ice spreaders one bit. Not to mention mud season!