How to Calculate Greenhouse Heating

How do I calculate energy requirements to decide what size heater I need for my greenhouse?

First of all, brace yourself. Heating a greenhouse will likely be expensive. However, there is nothing like a tomato freshly picked from one's own greenhouse in midwinter. Follow the steps below to calculate your energy requirements. Only you can decide what price you are willing to pay for that tomato or to see those orchid blossoms.
These calculations assume that you have first done everything you can to seal any cracks and secure any openings in your greenhouse.
It couldn't hurt to have a few barrels of water or a big pile of rocks in the greenhouse and to have your plants in larger pots as large masses hold their heat longer. There are many ways to save energy costs. You may have a light on in there part of the time. You may be using heating cables or mats. None of this is factored in. I am also not addressing what type of heating to use. This first attempt is only about the basic calculation. On to simple math...




H=height to eaves
L=length
W=width
R=height to ridge
S=length of roof slope

The basic calculation is based on a single layer glass greenhouse with glazing to ground level. This is a rough calculation, but certainly close enough for your needs.
If you have lower walls that are solid (such as brick), measure them separately and use 1/2 the result in your calculations (because heat loss is less).
If your greenhouse is double-glazed (glass or polycarbonate), the heat requirement figure is about 30% less. (Other materials will differ slightly. I will try to help you with that another time. Meanwhile, this should be good enough for a rough guess.)

My husband says that you should picture placing a heater outside and turning it on. That is what it is like to attempt to heat a greenhouse.

(I don't think it's that bad!)

Calculating your energy requirements for greenhouse heating:

1. Figure out the total inside surface area measurement of your greenhouse:

Surface area of walls and roof slope = 2 x (H + S) x L
Surface area of end walls = (R + H) x W
Add these together for "total inside surface area" (S. A.)
[2 x (H + S) x L] + [(R+H) x W] = S. A.
Using the example of my greenhouse above,
Surface area of walls and roof slope = 2 x (5 + 6.5) x 12 = 276 sq ft
Surface area of end walls = (10 + 5) x 8 = 120 sq ft
276 + 120 = 396 square feet of surface area (We'll round it to 400 sq ft)

2. Calculate the "temperature difference." By this I mean the difference between the desired minimum greenhouse temperature and the average lowest winter temperature in your area.

Let's say that I want to grow tomatoes.
The ideal minimum greenhouse temperature for tomatoes in winter is 60 degrees Fahrenheit.
My average coldest temperature in winter is 33.5 degrees Fahrenheit. (I know this because the National Weather Service has kindly put up on the web the average temperature data for the past 50+ years for my area. See next paragraph for link.)
The difference between the two temperatures in my example is 26.5 degrees F. (T. D.)
Try to be as accurate as possible with your "average coldest winter temperature." For those in the USA, there are some great sites on the web. The National Weather Service now has online Regional Centers web sites full of just the information you as a gardener need! (For those of you elsewhere, I'll keep looking, and put in links as I find them.)

3. Multiply the "total inside surface area" by the "temperature difference."
(S. A.) x (T. D.) = BTU'S (British Thermal Units) required for heating the greenhouse to please the tomatoes.
To continue the example:
400 x 26.5 = 10,600 BTU's required for Sherry's Greenhouse.
But wait! Sherry's Greenhouse is glazed with twin-wall polycarbonate which should reduce the heating requirement by 30%. To further continue the example:
10,600 x 0.7 = 7,420 BTU's required to heat Sherry's Greenhouse in winter to please those tomatoes. That's better.

4. How about kilowatt hours?

Just divide the BTU figure by 3.413
7,420 / 3.413 = 2174 (That's 2.1 kWh)
In other words, if your 1500 watt (or 1.5 kW) heater needs to run 6 hours, energy consumption is (6 x 1.5 )
or 9 kilowatt hours. Check your monthly statement to see what you are being charged per kilowatt hour. So, if you are being charged $0.06 per kilowatt hour, cost for running the example heater would be $0.5

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