Heater size for a given greenhouse and geographic location depends upon the surface area of the greenhouse and the temperature difference between the inside and outside of the greenhouse. To determine the size heating system you need, calculate the total surface area of the greenhouse covering. Then determine the difference between the minimum expected outside temperature during winter in your area (see the USDA hardiness zone map, average annual minimum temperature) and the maximum night temperature you wish to maintain (generally 60 to 65 degrees F). Multiply the greenhouse surface area by the temperature difference by the heat loss conversion factor in Table 1 for the appropriate covering. The answer will be in BTUs per hour. Most heaters are rated in this unit. Many greenhouse supply or construction companies can help you determine the proper size heater for your situation.
Table 1. Heat Loss Factor for a Small Hobby Greenhouse
| Greenhouse covering | Heat loss (calm area) | Heat loss (windy area) |
| Polyethylene or fiberglass | 1.2* | 1.4 |
| Glass | 1.5 | 1.8 |
| Double-layer plastic | 0.8 | 1.0 |
| *Heat loss in BTU/hour/square feet/degree F. | ||
