When building a greenhouse to grow plants year-round, one must consider its heating cost. Active heating, unlike passive heating, has the advantage of offering a constant temperature better suited for fast growing. The formula to calculate the heating cost for your greenhouse is easy, but you will have to make sure your greenhouse is air tight because air leaks will significantly raise the heating cost.
Calculate the total surface area of the walls of the greenhouse. Take the length and the height of each side, without forgetting the roof, and multiply the length by the height. Write down the result for each side and add them all. Call this amount value A.
Check what the optimal inside temperature is for whatever you want to grow and write that temperature down.
Go to Weather.com/weather/wxclimatology/monthly/XXXXX where "XXXX" is your zip code. Look into the "Avg. Low" column and write down the temperature for each month where the temperature is lower than the temperature you want to maintain in the greenhouse. Calculate the average by adding the temperatures and dividing them by the number of months that the temperature is below the temperature you want inside the greenhouse. Subtract the inside temperature from the average outside temperature you just found. Call that number value B.
Check the glazing manufacturer's documentation to find the material's R value . Divide 1 by the R-Value and write that value down. Call that value C.
Multiply A by B by C to find the number of BTUs, or British thermal units, you need to produce per hour. Multiply that number by 360 (12 hours per day on average during 30 days) to get the BTU per month. Call that value "Total BTU."
Check the specifications of the heating system you are considering using. Write down the number of BTU's used per unit of energy (BTU per kilowatts, BTU per gallon of propane or BTU per cubic foot of natural gas). Divide Total BTU value from Step 5 by the BTU value of the heating system to know how many kilowatts, gallons of propane or cubic feet of natural gas you will be using per month.
Check your local utility company or propane provider to know the price of gas, propane or price per kilowatt hour and multiply that price by the amount you will be using to find your monthly cost during the month(s) you need to heat the greenhouse.