I have been using the calculation supplied in the link below for about 8
years now. With it, greenhouse size info, data from local weather stations,
BTUs per unit of fuel and heater efficiency ratings I have been able to
determine not only how large a heater I needed to get me through the coldest
possible night I was likely to face but also what it costs in fuel on any
given time period of an average year. Once these numbers were plugged into
a spreadsheet and a running log of outside temps was recorded it *did*
provide a close approximation of the fuel actually used and therefore I
think it can be used to help you predict fuel costs.

But there are a lot of variables 'assumed' in this calculation. Like wind
speed. If possible locate your greenhouse in a way that protects it from
prevailing winds in your area. Windy locations will increase heat loss
through whatever covering you use and can swing the numbers wildly. I am
fortunate that my house stands between my greenhouse and the prevailing
winter winds.

http://aggie-horticulture.tamu.edu/G...hdbk/heat.html


Ed wrote:
OK, our 125+ mature orchids are overflowing our attached sunroom, so
we are considering a 10' x 12' polycarbonate greenhouse for the
backyard. I plan to keep my paphs and phrags as well as the Vandas
and warm types in the sunroom, but would like to move my phals, blc,
lc, dendrobiums, etc into the new greenhouse. We will be growing some
veggies and other plants in there as well.

I live in Lexington, Kentucky. Our winters are "fairly" moderate,
with some days in January or February sometimes into the 50's.
However, we also get a LOT of days below freezing, into the teens on
occasion. The coldest days also have the brightest sun. Very little
snow, maybe 6-8" a year. I am probably going to use electric heat as well
as heat cables for
propagation trays I have read several greenhouse books, but have found
nothing to pedict heating costs? My target is to keep night temps in
the greenhouse at 60 degrees.

I have no experience with greenhouse culture, so I could use all the
advise the group is willing to share.

Thanks,

Ed