Best Greenhouse Heater Compared: BTU, Running Cost, and Safety

Updated August 2026 · Editorial team · Topic: greenhouses

The 1,500-watt heater that dominates greenhouse listings puts out 5,118 BTU per hour. An 8×10 house in twin-wall polycarbonate needs about 8,400 BTU/hr to hold 45°F when the yard sits at 5°F, and 14,800 in single-layer glazing. The category's most-recommended heater covers a third of what a typical structure demands on the night that matters. Sizing is the whole purchase decision, and it runs on one line of arithmetic no listing prints.

Sizing: the number the box never prints

Heat leaves through the skin, so the input that matters is glazing area — on a gable-roofed structure, roughly 3.5 times the floor. Multiply it by the glazing's U-value (heat lost per sq ft per hour per degree of difference), then by ΔT, the gap between target and design low. Add 10% for leakage past door seams and vent flaps.

Rows assume ΔT 40 — a 45°F target against a 5°F low — with infiltration folded in. Drop the target to 35°F and demand falls a quarter; raise it to 65°F and ΔT hits 60, adding 50% to heater size and bill. Glazing is the other lever: twin-wall nearly halves every row, which is why insulating beats a bigger burner. Material trade-offs sit in the polycarbonate versus glass comparison.

Seven heater types, compared

Two rows deserve a flag: 175 W of tubular bar is 600 BTU, a small frame held a few degrees over ambient and nothing more, while unvented propane has the opposite problem — plenty of output, delivered with everything else combustion makes.

Running cost is a fuel question, not a heater question

Compare fuels on delivered energy — dollars per million BTU after burner efficiency — and the ranking barely shifts.

The right-hand column assumes 2.5 million BTU across a hard month: 8,400 BTU/hr peak cycling near 45% load. Propane looks cheap until the flue enters the math — venting exhaust outdoors costs a fifth of the fuel, the whole gap between the $96 row and the $119 one. A 20 lb exchange cylinder holds 4.6 gallons — six swaps that month, and rack pricing near $5.50 a gallon pushes it past $150. Electricity at $0.11/kWh drops the bottom row to $32; at $0.30 it climbs to $88. Your own rate is worked through in the winter heating cost guide.

Safety: what separates electric from anything that burns

Combustion inside a sealed humid box people walk into produces four things besides heat.

• Carbon monoxide. An unvented burner needs a CO alarm at $20–35 and 1 square inch of permanent opening per 1,000 BTU/hr of input — a 30,000 BTU heater wants a 30 sq in gap, roughly 5×6 inches, never taped shut. Occupational guidance sets the 8-hour ceiling at 35 ppm.
• Water. A gallon of propane releases about 0.8 gallons of water as vapour, so six cylinders add near 25 gallons of moisture indoors — a Botrytis problem first. Pair any burner with the airflow in the ventilation guide.
• Exhaust gases. Incomplete combustion releases ethylene and sulphur dioxide. Ethylene damages plants well under 1 ppm: leaves curl downward, buds drop, and growers blame disease first.
• Electrical load. A 15 A circuit is rated for 1,440 W continuous under the 80% rule, so one 1,500 W heater already exceeds it. Use a dedicated 20 A circuit, a GFCI outlet, a damp-rated unit, no extension cord.

Keep any heater 3 ft from foliage and film, and mount the thermostat at canopy height — built-in sensors read 5–9°F off the air plants sit in.

Common mistakes, in numbers

• Sizing on floor area. An 8×10 has 80 sq ft of floor and 292 sq ft of skin: a 3.6× error that turns a correct heater into a third of one.
• Trusting "heats up to 300 sq ft". That rating describes an insulated room at ΔT 20; the same unit covers about 90 sq ft of twin-wall glazing at ΔT 40.
• Targeting 65°F out of habit. Going from 45°F to 65°F lifts ΔT from 40 to 60 and adds 50% to the bill — near $62 a month on the 8×10 electric example.
• Heating a bare structure. Bubble insulation at $20–50 pulls single-layer U from 1.15 toward 0.80 and cuts fuel near 30%, beating any heater upgrade.
• Skipping the thermostat. An always-on 1,500 W heater burns 1,080 kWh a month, $184 at $0.17 — double the same unit on a $20 controller.

FAQ

General educational information, not professional horticultural advice. Prices, energy costs, plant hardiness and local climate vary by region and season; check figures against current listings and your local extension office before spending. Fuel-burning appliances and greenhouse wiring are governed by local codes — have gas and electrical work inspected.