Generator fuel usage?diesel vs gas

robertglines1 · February 18, 2010, 05:34:41 AM

I have a 7.5 kohler gas now that does a fine job for my needs uses 1/2 gal per hr on a 24 hr run...Am selecting gen for next coach and need usage info to make a informed decision...have heard from some that diesel usage is 50 percent less;this post is both to learn for me and to define for others....thanks to all .

belfert · February 18, 2010, 05:48:39 AM

You're doing really good to only use 1/2 gallon an hour in a gasoline 7.5KW generator if it is fully loaded. I had an Onan 4KW in my toy hauler and it burned around 1/2 gallon an hour to run the A/C and a few other things.

My Powertech 8KW diesel is rated to burn 3/4 gallon an hour at full load. I really have no way to know how much it really takes, but I usually figure about 1/2 gallon an hour because I am almost never fully loading it.

robertglines1 · February 18, 2010, 05:58:07 AM

Brian had gen plugged into house for a week in feb ice storm last year did power management;would assume near full power output..bus ran house

luvrbus · February 18, 2010, 06:40:04 AM

Robert, that being a 45 ft coach you are going to need 4 roof airs and maybe 5 and that will put you into the 17 to 20Kw range and those will use about 1 1/2 gals per hr under load.
I had a Wirco 13 kw that used a gal per hour with 3 units running.
The Isuzu powered units will use less fuel than the rest of the small diesels but cost a little more fwiw

good luck

belfert · February 18, 2010, 07:12:49 AM

Can you explain how five roof airs require a 17 to 20KW generator? Maybe I am not understanding something.

A 15,000 BTU rooftop takes about 1,700 watts so five would take 10,560 watts if they are all running at once. An electric water heater is generally 1,750 watts or less and a fridge is perhaps 600 watts. A total of 12,910 watts which leaves a 2,000 watt margin for startup and other electricity usage on a 15KW generator.

How often would someone really need five roof airs all running at once to justify a generator larger than 15KW? With five rooftops they may be 13.5K BTU instead which lowers the power requirement a bit.

Just Dallas · February 18, 2010, 07:22:10 AM

Removed

Nusa · February 18, 2010, 01:13:49 PM

Quote from: belfert on February 18, 2010, 07:12:49 AM
Can you explain how five roof airs require a 17 to 20KW generator? Maybe I am not understanding something.
Unless otherwise stated, always assume that generator numbers are the surge rating, since that's how most are marketed and named. Actual continuous rated capacity is often about 20% less. So that 17KW label may only give you 13.5KW of continuous capacity.

A 15,000 BTU rooftop takes about 1,700 watts so five would take 10,560 watts if they are all running at once. An electric water heater is generally 1,750 watts or less and a fridge is perhaps 600 watts. A total of 12,910 watts which leaves a 2,000 watt margin for startup and other electricity usage on a 15KW generator.
Your math is funky...1700 x 5 = 8500. However, if all five A/C's try to start at the same time, that could be a problem. That can be solved manually, or with some electronics to cycle them smartly.

How often would someone really need five roof airs all running at once to justify a generator larger than 15KW? With five rooftops they may be 13.5K BTU instead which lowers the power requirement a bit.
"How often" only comes in during the spec phase. Either you have the capability to run them all at once or you don't. Once needs are determined, "how often" doesn't justify having a generator too small to accomplish the task, whatever the actual numbers work out to be.

belfert · February 18, 2010, 01:48:18 PM

I guess the question is do you buy a generator for the worst possible scenario, or the realistic scenario for a busnut? Realistically, you're probably not going to have all five roof airs starting at once. You'll stage them or something. If you are in an area were you need all five A/Cs you might turn one off if you need to use extra power for a bit.

Most busnuts would probably be willing to sacrifice a little bit to save on the generator initial cost and on fuel usage.

Certainly, if you're doing an entertainer coach you go oversize as you don't want the paying customers to have to worry about overloading the generator. The coach is making money and hopefully there is money to fix the generator if it breaks.

My math was wrong on the wattage requirements. I added in the water heater and fridge twice. So a 15KW generator should be plenty if all five rooftops don't start at once and a 12KW would work if you planned a little bit.

Ed Hackenbruch · February 18, 2010, 02:12:31 PM

Has anybody ever figured out what their cost per kw is for running a generator vs. what their cost per kw at home is?

JackConrad · February 18, 2010, 02:55:57 PM

Quote from: Ed Hackenbruch on February 18, 2010, 02:12:31 PM
Has anybody ever figured out what their cost per kw is for running a generator vs. what their cost per kw at home is?

Since our kilowatt usage varies greatly with our generator it is difficult to figure. When our generator is running, it can be anything from just charging batteries to all ACs, water heater, microwaye/convection oven, and charging batteries. On our trip last summer (5 months) we averaged 0.32 gallon per hour with our PowerTech (10 KW head on a 3 cylinder Kubota) Jack

Zeroclearance · February 18, 2010, 03:42:01 PM

I use a little over a gallon an hour with my 20KW Powertech (4 cylinder Kubota)

Nusa · February 18, 2010, 04:11:29 PM

Fuel consumption has more to do with your energy lifestyle than it has to do with the generator itself. Two people with identical generators can have vastly different fuel consumption rates.

Cost of energy from a small generator is always going to be significantly higher than the cost from the grid, unless you have a cheap fuel source besides the commercial pump. Even with nearly free fuel, you still have purchase and maintenance costs, and replacement costs when you wear them out.

belfert · February 18, 2010, 04:36:29 PM

I certainly don't know everything about generators and I'm not going to pretend I do.

I'm trying to figure out why a 17KW generator would be recommended for five roof airs? Is this just to cover the worst case scenario of all of them starting at once? The other thing I suppose that could use a lot of energy would be a clothes washer, but you probably wouldn't use one if boondocking. A TV and a stereo system will use a bit of power too.

gus · February 18, 2010, 04:44:01 PM

The sensible way to manage ACs is to block off the section of the bus you aren't using at the time, like when running down the road. We then block off the rear and at night, if necessary, we block off the front.

Not much point in AC for the whole 45' if using only the front.

Of course, this depends on the number of people in the bus and many other variables. But, for the usual two people it works well.

Zeroclearance · February 18, 2010, 04:58:41 PM

Brian, I think that you are missing "strip heat" for the winter. My coach has two 7.5KW heat strips built in.. I have a 220v cook top. The wife is now asking for a washer and dryer.. Looking at my power usage, I know that I could manage with 15kw. But then you invite some guests with you and things change. I had a situation this weekend where the women were using 2 hair dryers with the microwave running.. I heard the RPM's start to pull down when the A/C units started to kick in..