Sizing a greenhouse heater comes down to one core idea: however fast heat escapes through the covering is exactly how fast a heater must replace it to hold steady temperature. That escape rate depends on only three things — how much covering surface area you have, how good an insulator that covering is, and how big a temperature gap you're trying to maintain — and multiplying them together gives a heating requirement in BTU/hr that maps directly onto heater capacity ratings.
Why Surface Area, Not Floor Area, Drives Heat Loss
It's tempting to size a heater off a greenhouse's floor square footage, but heat doesn't escape through the floor — it escapes through the covering: the roof and wall glazing (and any solid end walls) that separate the heated interior from outside air. A tall, narrow greenhouse can have significantly more covering surface area than a low, wide one with the same floor space, and therefore need more heating capacity even though it "looks" smaller on a site plan. Always measure the actual glazing and wall surface area the heat has to pass through, not the footprint.
U-value: The Covering's Insulation Rating
U-value measures how many BTUs pass through one square foot of covering per hour for each degree Fahrenheit of temperature difference — the inverse of R-value used in home insulation. A single layer of poly film (U ≈ 1.2) is a poor insulator by house standards, which is exactly why greenhouses are relatively expensive to heat compared to a similarly sized, well-insulated building. Adding a second air-gapped poly layer, switching to twin-wall polycarbonate, or using double-pane glass all lower the effective U-value and cut the heating requirement proportionally — a double-poly upgrade alone typically saves 35-45% on heat loss for the same area and temperature target.
Picking a Design Temperature You Can Trust
The outside design temperature should represent a realistic worst-case cold snap for your specific location and season — not the average winter low, and not necessarily the all-time record either. Sizing against a design temperature that's too mild leaves the heater unable to keep up exactly on the coldest nights, when crop or livestock protection matters most; sizing too conservatively adds unnecessary heater cost and oversized equipment that short-cycles on milder nights. Many growers use their region's 99% design temperature (the temperature exceeded 99% of winter hours) as a practical middle ground, available from local agricultural extension offices or ASHRAE climate data.
From Heat Loss to a Heater You Can Buy
The raw BTU/hr heat-loss figure is the minimum a heater must sustain continuously at the design temperature — in practice, add a safety margin (commonly 10%) so the unit isn't running flat-out with zero reserve for wind-driven heat loss, door openings, or a colder-than-expected night, and so it can recover temperature quickly after any disruption. Beyond that margin, match the heater's rated output (not its input rating, which differs by combustion or electrical efficiency) to the padded BTU/hr figure, and consider that very large single heaters may be less efficient than two smaller units that can stage on and off with demand.