How to calculate greenhouse costs

Some people have asked how to calculate the cost of heating and cooling a greehouse. I hope that this thread is helpful in this regard. It is fairly technical, but I hope to explain it fully enough to allow anyone to plug in their own numbers.

First some formulas:
Watts=amps*volts
1 KWH = 3,413 BTU
1500W electric heater produces 5120BTU
Propane is 91,600BTU/gallon
Q=deltaT*A/R
Q=BTU heat requirement per hour
delta T is the difference between outside temp and desired inside temp
A= the surface are of the greenhouse is square feet, not including the floor; ie, the roof, side walls, and end walls
R= the R value of the glazing

Two important points about this formula...some heating calculators use "heat loss value" (HLV) instead of R-value. Don't get confused, the heat loss value is just 1/R. The formula using heat loss value is then Q=deltaT*A*HLV. The other important point is that you use a different temperature when calculating your average energy costs over a month than you do when you you calculate what size heater you need. Heater calculations are used to determine how much heat you will need on the coldest night of the year. For this, you use your coldest winter temps, for example the colder end of your zone. For calculating heating costs you need the average temperature for the month.

Here are some typical R-values for common glazings or coverings
4 mil polyethylene 0.83
4 mm (5/32") twinwall polycarbonate 1.43
6 mil polyethylene 0.87
6 mm (1/4") twinwall polycarbonate 1.54
6 mil poly double layer (inflated) 1.43
11 mil woven polyethylene 0.95
3 mm (1/8") glass (single layer) 0.88
16 mm (5/8") triplewall polycarbonate 2.5
Polycarbonate / fiberglass (single layer) 0.83

OK, let's say you are contemplating an ACF 10 x 12 Cottage Greenhouse. We first need to calculate the surface area of the greenhouse. Using the diminsions found on the website and some simple geometry, I calculate the surface area of each sidewall to be 55 sq ft, each roof side to be 64.6 sq ft, and each end wall to be 75 sq ft. Therefore the total area (A) is sq ft is (55*2)+(64.6*2)+(75*2)=389.2 sq ft.

The website tells me that this GH comes in 6mm twinwall polycarbonate, so the R-value is 1.54. A quick Google search shows that in this particular zone 7a, the average January temperature to be 28.4 degrees. I want to keep my greenhouse at 50 degrees. So the "delta T" is 50-28.4=21.6 degrees. Now we can calculate the BTU per hour necessary to heat the greenhouse in January. Q=389.2*21.6/1.54=5459 BTU. To figure daily use, multiply by 24 hours to get 131016 BTU/day. To figure monthly use, multiply again by 30 (days in the month) to get 3,930,480 BTU/month.

Now, lets say you heat with propane. Knowing the BTU in a gallon of propane, we can take our BTU/month and divide by the number of BTU in a gallon of propane. 3,930,480/91,600BTU/gal propane to get 42.9 gallons of propane we would expect to burn in January. We just bought propane at $1.69/gal, so this would mean my January cost would be 42.6*$1.69=$72.52/month.

Let's say you heat with electric. Take the same BTU value and instead of dividing by the BTU/gallon of propane, divide by BTU/KWH. 3,930,480/3413BTU per KWH=1151KWH. If my energy cost is 7.169cents/KWH, then heating electric should cost about $82.52 for the month of January.

If anyone finds a math error, please bring it to my attention and I will edit the post. I will return to work through a summer cooling example later.

SB

A few comments to consider in your post...

Have you considered the efficiency in the heater? Depending on the type of furnace, propane or natural gas heaters have efficiencies as bad as 60%. (Compare this to home condensing natural gas furnaces at close to 95%) An unvented open flame heater would have close to 100% efficiency, but other factors such as CO, safety, etc. often warrant a vented heater. I use an updraft NG forced air furnace (just like the ones in many homes) and I doubt it's better than 60% (maybe when new) and it's delivering heat over the plants instead of under them.

It would also be good to add natural gas (typically 1,020 btu/cubic foot) and #2 oil (138,000 btu/gal). Gas is often priced per therm or dekatherm (dth=1,000,000 btu)[CORRECTED]. It's hard to predict fuel prices with the current volatility (especially #2 oil and propane) - winter season prices (futures) for natural gas are currently in the $11.00/dth range. Propane winter futures are currently about $1.20/gal. Of course the delivered price of NG or propane will be higher to cover the transportation, other overheads, and profit. Of course propane and oil can be stored to try to miss the peak Winter prices. Propane unfortunately seems to be a bit more prone to price spiking.

With electricity the efficiency is 100% for resistance heating, although using a heat pump (especially ground source) can produce effective efficiencies actually over 100%.

I don't really miss being in the energy and energy delivery business. Years ago things were so much simpler (and dare I say more reliable).

Bravo for starting this important thread. Heating costs will unfortunately hurt the greenhouse grower and force many to not even try.
Dave

p.s. Dave's energy rant - Here we are in the US with an incredible wealth of coal, admittedly not the cleanest of fuels. At current fuel prices coal gasification is clearly where this country should be headed. I'm not a hydrogen economy proponent (primarily due to complexities), but coal to gas IS the way to go. Our current national energy policy belongs with the dinosaurs it depends on.

This message was edited Aug 27, 2006 1:39 PM

OK, well, our hypothetical DW says that the winter costs are too high and she cannot be convinced to go ahead with the greenhouse. Let's look at what we can do to lower the winter costs.

Let's insulate the north wall...no sun comes through this wall, and if we insulate it with foil-backed polyisocyanurate 1" board, which has an R-value of 7.04, we raise the R-value of the north wall to 1.54+7.04=8.58.

We can recalculate Q using the same formula, but adding together the BTU lost through the north wall and the BTU lost through the rest of the greenhouse. The formula would be Q=(deltaT*A/R)+(deltaT*A/R). Our greenhouse is oriented north-south, so the north wall is an end wall with a square footage of 55 sq ft. Our BTU formula changes:

Q = (334.2*21.6/1.54) + (55*21.6/8.58) = 4826 BTU/hour

This equals a monthly use of 3,474,720 BTU, equal to about $72.99/month heating electric.

What else could we do? Well, we could add some thermal mass. Let's put 8 black 55 gallon drums in the greenhouse. One BTU is defined as the amount of heat required to raise one pound of water one degree F. The water will warm during the day and release this heat at night. One chap I know has measured the temperatures of his water at dawn and at dusk. The temp is 74F at dusk and 66F in the morning, a drop of 8F.

Eight 55 gallon drums hold 440 gallons of water which is 3520 pounds of water. A 1F drop in the water temp releases 3520 BTU. An 8F drop releases 28120 BTU overnight. Over the course of a month, this is 844,800 BTU. Therefore, the total monthly BTU could hypothetically drop further to 3,474,720-844,800=2,629,920 BTU. This is 770.55KWH heating electric. At $0.07169/KWH, this is $55.24 for the month of January.

Your mileage may vary!

Edited for clarification of formulas

This message was edited Aug 27, 2006 10:35 AM

Just sort of skimmed over the numbers myself.....in part because it doesn't affect me in that I'm committed to propane..for the near future. But I'm with Dave here, you have to include other means of heating besides electricity. By the way, do they even make electric 200k btu heaters for grhouses, and if so, given the prices of propane now-would it be cheaper?

Dave, I totally agree that propane prices spike in the winter-no matter the cost of oil, and you just reminded me to get the tanks filled-about 2000 gallons worth, and even now thats going to be costly!!

In regards to insulating the north wall-if you have grhouse plastic, as I do-the light reflects off the plastic. Wouldn't making the north side dark affect the ability to light up that section of the grhouse?

Last, out of curiousity, SB, what kind of heating do you use?

What is everyone paying per gallon for propane last time you bought?

And off topic....please let it RAIN here!!! I am so sick of watering...
sorry...just having some crybaby soup

I want to thank my fellow Missourian and tigerlily for pointing out the problem with heater (in)efficiency. Our calculations assume 100% efficiency. As you say, heaters, particularly vented heaters, are not 100% efficient. If you know the efficiency of your heater, you can adjust the calculations accordingly!

TL, we prebought propane for the winter at $1.69/gal. I use two direct vented Empire DV-35 heaters with propane.

In my example, we used foil-backed polyisocyanurate board. The foil backing is there to reflect light (and presumably heat) back into the greenhouse.

Now on to summer costs. The greenhouse in the example will need an exhaust fan and HAF fans. First the exhaust fan. The greenhouse is 900 cubic feet in volume. Therefore, we need an exhaust fan that is rated at least 900 cfm. I found a 16" exhaust fan at Charley's rated at 1095 cfm. The website tells me it draws 1.8 amps. Remember that watts=amps*volts. So when it is on, the fan uses 1.8*120=216 watts. If this fan runs 16 hours per day in the summer, it will use 3.456KWH/day. This is 103.68KWH/month, which at the same price of energy we have been using above, amounts to $7.43/month for the exhaust fan.

Let's say you use two 8" long life HAF fans. This may be overkill for a greenhouse of this size, but you will get the idea. Each fan draws 0.6 amps, and at 120V, this is 72 watts. Running 24 hours a day, each fan uses 1.728KWH/day, or 51.84KWH/month. Total use with the two fans is again 103.68KWH/month or $7.43/month.

Anyone spotting flaws in the calculations, please point them out and I will edit the post. I hope that this is helpful to people. Thanks again TL and digital_dave!

SB

Great job!
I'm a guy who doesn't like surprises but if someone's getting into a greenhouse they need to know what they're up against. My greenhouse is a lean-to with the north wall insulated with similar reflective material. You really can get more light than the sun directly delivers! The picture shows some of the details when we recovered the plastic a few years ago.

I have a problem at the top of the GH with sealing it, and also in trying to keep the inside layer reasonably "tight." This is really important since as we know, heat rises. I also keep about 5-10 5 gallon plastic buckets to try and maintain some heat at night. (Many of the buckets are directly under the misting nozzles which seem to leak no matter what.) I'd love to figure a way to delivr the heated air under the plants. I guess maybe I should have bought a downdraft furnace instead. But that's a disadvantage of forced air heat. It's simple to install but "delivery" is more difficult than hot water or steam.

I agree with SB about fans. "Overkill" is far far better than underkill. And having two fans is far more reliable than one.

I'm no expert, but as the person who asked the question "how much will my hobby greenhouse between 8"-10" x 12" in zone 7a cost to run?", I want to thank the actual experts who responded. Stressbaby, Tigerlily123, and Digital_Dave, you have helped me immesurably to figure out the actual costs. I can't thank you enough. I now feel I have enough information to go forward. You guys are the best!

Capecod...are you zone7a??? Because you are on the ocean? I had no idea there was a zone 7a that far north!! I remember as a kid sliding down those huge sand dunes.....

Hi Tigerlily:
Yes, according to the Better Homes & Gardens chart, everyone on Cape Cod is zone 7a. I guess because of the tempering effects of the ocean on three sides. I just believe what the experts tell me, because I just moved to this area a year ago, from central coast California (talk about a different climate!) I know nothing. Actually there are a few similarities with our former home and Cape Cod: a maritime climate, lots of sand dunes, GREAT seafood--and tourists--but a FEW notable differences: what is this white stuff in the wintertime? Thus the need for a Greenhouse, hence my questions. Again, thanks so much for your patience with a newbie.
--Emily

Does anyone know if natural gas can be used to heat a greenhouse? We have natural gas for our inside heater and also for our water heater and stove. So since we already have it for those things, it would be easy to run a line out to the greenhouse. We have a guy that we hire at times to run the lines for us. He did it for our stove. So getting someone to do it is not a problem. I was just wondering if they make natural gas heaters for greenhouses and, if so, do they have to be vented?

Oh, one other question. What do you all keep your temperature set at in your greenhouses? We have this huge attic fan that we sort of took out of the attic because that was all we had at the time. We had just installed both of them in the attic, one to pull air in, and the other to blow it out. So we took one of the fans and put it in the greenhouse. It has a thermostat on it that ranges from 60° to 140°. What should I have it set on? I think it is actually too big for the greenhouse but that is all I have at this time. It says it cools attics up to 2400 sq ft and our greenhouse is only 144 sq ft. Should I continue to use this or should I get a smaller one? Here is a photo of the front and side of the box it came in. I hope it is ok but it if isn't, please tell me so I can get the proper thing. I know one thing for sure, it is never hot in my greenhouse.

Thanks,
Jesse

Editted to say that I did not mean to hijack this thread. I meant to put it as a topic of its own. I'm sorry if I got everyone off on a rabbit trail.

Jesse

This message was edited Aug 31, 2006 9:14 PM

Jesse, yes, you can heat your g-house with natural gas. Most gas heaters have the option of burning natural gas or propane simply by changing the orifices.

As for your fan, I used one like that in my first home-made greenhouse years ago and it worked great! (That g-house was only 12 X 16 or so.) Just make sure you have an inlet of some kind so it can pull air in.

Shoe.

What about using solar to power the greenhouse? Is that feasible?

Um, how about I give you my stats and somebody can figure it out for me instead? LOL just kidding! Wow SB that is so cool of you to explain that formula but whew is it ever over my head. (Oops I forgot I'm not supposed to be posting yet. I'm just here to lurk and learn.)

I heat with propane and looked at the figures but you may as well be asking me to build a rocket as figure this out!!!!

I'm just wondering why everyone but me is paying less than $2 per G for propane!!! I have been paying anywhere from (never lower than->) $2.59 -$3.00 for almost 4 years now.

I had to regroup and get rid of a lot of plants because this is just a hobby for me and I couldn't afford to heat 2 gh's. last year I used 400 g's of gas and it cost me almost $1300 just to heat them for about 14 weeks. I am now only heating 1 gh!!

dmichael619 ,

LOL you forgot to check the dates those messages were written! This thread begins in 2006 when prices were lower. Right now I'm paying $2./gallon for propane.

MollyD