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Generator Fuel cost comparison

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Posted by James Baber on December 30, 2004, 7:00 am
I am considering purchase of a standby 10 - 15 kW generator.
  I have seen several units that look good. The Onan and
Generac units appear to use the same Honda motor that Honda
uses on their similar models, and it will run on either
Propane or NG (with kits I believe).  I would prefer using
NG (Natural Gas) since I do not have Propane service at home
and the existing NG meter is 9 feet from the point I intend
to install the generator.

A major question for me is the fuel cost.  My natural gas is
delivered at a cost for each Therm.

These NG fueled generators are rated as using so many Cu.
Ft. of NG burned at various percentages of rated output.

My question is how many Cu. Ft. of natural Gas are in a
Therm, so that I can estimate the cost of operation of the

I do know that a Therm is 100,000 BTUs of energy, so if any
of you know how many BTUs are in a Cu. Ft. of Natural Gas,
that would also solve my problem.
Jim Baber
Email jim@NOJUNKbaber.org
1350 W Mesa Ave.
Fresno CA, 93711
(559) 435-9068
(559) 905-2204 (A no charge Verizon IN cellphone to other
Verizon IN accounts)
See our 10kW grid tied solar system at "www.baber.org"

Posted by Vaughn on December 30, 2004, 11:18 am

     A really good question!  Also, how does one compare LP gas (measured in
pounds or gallons) to Natural Gas (measured in Cu. Ft. or Therms).

     I CAN tell you that when I found myself offgrid after the Florida
hurricanes, I was appalled at the cost of running my LP gas generator.  The grid
is likely the cheapest energy you will ever buy.


Posted by Ecnerwal on December 30, 2004, 1:29 pm

It varies, which is one reason why it's sold that way - some of the gas
you get has more BTUs/cubic foot, some has less, the meter typically
records cubic feet, and the gas bill is adjusted depending on what the
energy content of the gas they have been pumping this month is. Your
bill might contain the information you need, if you look into it
carefully. The consumption rate on the generators will also vary with
energy content, to roughly the same extent.

For price comparison, LPG has about 91,500 BTUs/gallon, and the number
of BTUs you need to buy will be similar, regardless of fuel type - but
the presence of the gas connection probably trumps any fuel price
difference, in the long run. It is all more expensive than grid
electricity; as a backup, convenient fuel source trumps a small price
difference (now) which could well be reversed at later times.

Generally speaking, diesel without road taxes is the lowest cost fuel
per KWh generated (diesel engines are somewhat more efficient than gas
type engines). It still costs more than grid electricity, if you have
grid electricity available.

Cats, Coffee, Chocolate...vices to live by

Posted by daestrom on December 30, 2004, 4:02 pm

If you already have NG service, the answer is on your bill.  NG is often
metered in 100's of cubic feet. And then the gas company puts an
'adjustment' of about 1.045 to convert it to therms.  This is because
methane (the primary constituent of NG) has 1045 BTU in one Cu. Ft at
standard conditions.  So 100 ft^3 of methane, * 1045 BTU/ft^3 => 104500 BTU
=> 1.045 therm.

Actual NG isn't pure methane, so the 'adjustment' varies somewhat from month
to month.  The gas company takes samples and analyses them to come up with
the monthly adjustment based on actual heat content of the NG.  But it runs
pretty close to methane's 1.045 Therms/100ft^3


Posted by James Baber on December 31, 2004, 1:31 am
 daestrom wrote:


I've spent a lot of time referencing my PG&E bill, but my
interest has always been on the electric part of the bill,
since for 9 months of the year the gas is $.00.

Before I installed my solar panels the electric was between
$00.00 and $50.00 so an occasional $0.00 or so for gas in
the winter when my electric use was way down just didn't get
my attention.

Now that I am generating as much as I do, I only pay the
standby electric charges each month (less than $.00) plus
the gas used for heating.  The gas runs less than $0.00 and
only in the winter it seemed so insignificant that I had set
my bill up to be an auto paid account without an monthly

I do still do get a monthly statement reconciling my
electrical usage / production, and I do have to pay for
power used that is not offset by production.

But, since after 199 days in this years annual billing
cycle, I still have a $67.00 CR. that has been applied to
my account as of today, so I have not paid much attention to
the bill. (other than to gloat a little) This winter has
really cut into my solar production, but I hope the spring
will be kinder.

Jim Baber
Email jim@NOJUNKbaber.org
1350 W Mesa Ave.
Fresno CA, 93711
(559) 435-9068
(559) 905-2204 (A no charge Verizon IN cellphone to other
Verizon IN accounts)
See our 10kW grid tied solar system at "www.baber.org"

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