With a few simple numbers and a calculator you can easily know what the greenhouse heating requirements are for the coldest weather and the average monthly costs. The Internet is very helpful in finding out certain specifics such as the temperatures and the heat loss or R-Value of the materials which you are using.
Step 1 - Find the R-Value or Heat Loss Value
This is the amount of heat which is lost through the materials the greenhouse is made out of. Many of these numbers can be found online, or with the manufacturer of the materials. Often, if the items were purchased new, the heat loss or the R-value is printed directly on the label. Do not be confused: the heat loss value is how much heat escapes and the R-value is how much heat is retained. You want to find the Heat Loss Value. If all you have is the R-Value, just divide the R-Value by one. The number will be a decimal, but that is why you use a calculator.
Step 2 - Calculate the Exterior Surface Area
This is the entire exterior surface of the greenhouse, excluding the floor. Since the floor is not exposed directly to the outside, it is not calculated. Do not simply multiply length times width. You want to calculate the area of each wall and each ceiling panel then add them together.
Step 3 - Know Lowest Outside Temperature and the Average Outside Temperature
The Internet is another helpful tool here. If you are unable to find the exact temperatures for your area, you can check with the United States Department of Agriculture website for the Hardiness Zone or call your local agricultural extension office. The lowest outside temperature is what calculates the heating requirements and the average outside temperature will tell you the average monthly costs altogether.
Step 4 - Calculate the Inside Temperature
Whatever temperature is necessary inside the greenhouse to maintain active plant growth needs to be calculated. This information can be found on seed packets or through various gardening books. In general you want to go with a good mid-range number. Some plants will like it warmer, some will like it cooler. If the number is around 2 or 3 degrees higher than the lowest, but 2 or 3 degrees lower than the highest all the plants should do well.
Step 5 - Find the Minimum Heating Requirements
(Lowest Outside temperature - Inside temperature) X Area X Heat Loss Value
Use this formula to calculate the minimum heating requirements to maintain the desired internal temperature. This is how many BTUs of energy will be needed per hour to heat. Multiply by 24 for the daily requirement and again by 30 for the monthly.
Step 6 - Find the Average Heating Costs
(Average Outside Temp - Inside Temperature) X Area X Heat Loss Value
This formula will calculate the average number of BTU's needed per day in the month. Multiply by 24 for a daily amount and again by 30 for a monthly amount. Multiply the total BTU's per month by the cost per BTU.