The right greenhouse heater for a 6x8 to a 12x16, with prices checked September 20, 2026: Bio Green Palma $149.99, Phoenix $299.99, Modine HD45 $1,087 and up.
Comparison
Quick answer: Checked September 20, 2026, the best greenhouse heater for most hobby houses up to about 8x12 is the Bio Green Palma, $149.99, a 1,500 watt, 5,120 BTU splash-proof fan heater with a built-in thermostat and a two-year warranty. A 6x8 twin-wall polycarbonate house needs roughly 3,800 BTU per hour to hold 30 degrees above the outside low, an 8x12 about 5,800, a 10x12 about 6,600 and a 12x16 about 9,100, so one 1,500 watt heater covers a 6x8 with margin, just about covers an 8x12, and is short for anything bigger. From 10x12 up, buy the 240 volt Bio Green Phoenix, $299.99 and 9,554 BTU, or the Dr. Infrared DR-218 at 3,000 watts for $129.99 if you have a 240 volt, 20 amp circuit. For a heated 12x16 or larger in a real winter, a vented gas unit like the Modine Hot Dawg HD45, $1,087 to $1,200 with 36,900 BTU output, is the reliable answer. Unvented propane and kerosene heaters are cheap to buy and are not recommended inside a greenhouse by any extension service we found, because incomplete combustion releases ethylene that deforms plants and carbon monoxide that harms you. At the June 2026 US residential rate of 18.34 cents per kWh, a 1,500 watt heater costs 27.5 cents per hour at full duty.
Best for
Owners of a 6x8 to 12x16 hobby greenhouse who need to pick one heater and one circuit before the first frost, and want the wattage tied to their actual square footage.
Wrong fit
Commercial growers sizing boilers, or anyone who has not first decided whether to heat at all; season extension without heat is often the better buy.
Tradeoff
Electric heaters are safe, clean and cheap to buy but limited by the circuit you have, and 1,500 watts is the ceiling on a standard outlet. Gas heaters cover big houses cheaply per BTU, but the only safe ones are vented, and a vented unit costs seven times a plug-in fan heater before the gas line.
Checked September 20, 2026, a splash-proof 1,500 watt greenhouse heater with a thermostat costs $149.99 from Bio Green and puts out 5,120 BTU an hour. That number, not the box's "heats up to 130 square feet" claim, is what decides whether your plants survive January. A 6x8 twin-wall house loses about 3,800 BTU an hour when it is 30 degrees colder outside than in. An 8x12 loses about 5,800. A 12x16 loses more than 9,000. Match the heater's output to that loss and the rest of the decision is about the circuit you have and whether you can vent a flame.
We don't sell greenhouses, and we don't sell heaters. This guide sizes the heater to your square footage with extension-service heat-loss math, then ranks the units we could price today. For what a winter of heat will cost you, read the heating and ownership cost guide. For the cheap methods to try before buying a heater at all, read how to heat a greenhouse, cheapest to most reliable.
Quick Answer: Which Heater for Which House
Your greenhouse
Heat loss at 30F colder outside
Buy
Price found September 20, 2026
Circuit
6x8, twin-wall polycarbonate
about 3,800 BTU/h
Bio Green Palma, 1,500 W, 5,120 BTU
$149.99
120 V outlet, GFCI
8x12
about 5,800 BTU/h
Bio Green Palma, or two 1,500 W units on separate circuits
$149.99 each
120 V, one outlet per heater
10x12
about 6,600 BTU/h
Bio Green Phoenix, 2,800 W, 9,554 BTU, or Dr. Infrared DR-218 3,000 W
$299.99 / $129.99
240 V
12x16
about 9,100 BTU/h
Phoenix at the margin; Modine Hot Dawg HD45 vented gas, 36,900 BTU output, for a real winter
$299.99 / $1,087 to $1,200
240 V, or gas line plus 115 V
Seedlings only, any size
not applicable
Heat mat under the tray, 17 to 20 W
about $15 to $32
120 V
Heat-loss figures use the University of Alaska Fairbanks Extension formula with a U-factor of 0.6 for twin-wall polycarbonate and our own geometry of a 5-foot eave and 7-foot ridge. Single glass loses roughly twice as much; see the sizing section. A 40-degree gap between inside and outside adds a third to every number.
How These Numbers Were Built, and When
Every heater price and specification was read from the manufacturer's page, or a named retailer where the manufacturer does not sell direct, on September 20, 2026. Heat-loss math follows the University of Alaska Fairbanks Extension, Rutgers and University of Georgia formulas, using their published U-factors. The electricity rate is the US Energy Information Administration's June 2026 residential average. The propane price is the EIA's last weekly print of the 2025 to 2026 heating season, from March 30, 2026, because the weekly survey pauses until October. Big-box retailer prices could not be verified because their pages blocked our fetch, so they are not used. We have not run these heaters side by side.
Size the Heater First
The formula every extension service uses is the same: heat loss in BTU per hour equals glazed surface area in square feet, times the glazing U-factor, times the temperature difference between inside and outside in degrees Fahrenheit. Size to the heater's output, which for electric equals its input, and for gas is 80 percent or less.
Glazing
U-factor (BTU per hour per sq ft per degree F)
Source
Single glass
1.1 to 1.2
UGA, Rutgers, UAF
Single polycarbonate
1.2
UAF
Double poly film
0.7 to 0.8
UAF, Arkansas
Twin-wall polycarbonate, 8 to 10mm
0.5 to 0.65
UAF 0.6, Arkansas 0.65, UGA 0.5
Palram publishes its Sunlite twin-wall at 3.3 W/m²K for 8mm and 2.9 for 10mm, which converts, by our arithmetic, to 0.58 and 0.51 in these units, inside the extension range.
Greenhouse
Glazed area (5 ft eave, 7 ft ridge)
Loss at 30F difference, twin-wall (U 0.6)
Loss at 30F, single glass (U 1.2)
6x8
about 210 sq ft
3,780 BTU/h
7,560 BTU/h
8x12
about 323 sq ft
5,814 BTU/h
11,628 BTU/h
10x12
about 369 sq ft
6,642 BTU/h
13,284 BTU/h
12x16
about 506 sq ft
9,108 BTU/h
18,216 BTU/h
Rutgers adds 10 percent for older, leaky glass houses, and both Georgia and Arkansas add an infiltration term for air changes. Treat the twin-wall numbers as a floor. Then convert: 1 kilowatt is 3,412 BTU per hour, so a 1,500 watt heater delivers 5,118 BTU per hour and an 8x12 house at a 30-degree difference needs 1.7 kilowatts. That is why the 8x12 row above says two heaters or a 240 volt unit.
If your house is single glass, glass vs polycarbonate explains why the glazing decision costs more each winter than the heater does.
The Heaters, Ranked
1. Bio Green Palma, 1,500 W: the default for a 6x8 to 8x12
$149.99 at biogreen-products.com, in stock. Switchable 750 or 1,500 watts, 5,120 BTU, a manual thermostat, IPX4 splash rating, two-year warranty, 110 volts. Bio Green says it suits rooms up to 130 square feet, which lines up with our 6x8 and 8x12 rows in a mild climate. The Palma Digital version with a summer and winter thermostat is $199, marked down from $219, and listed as pre-order at check time, so treat it as a backorder.
2. Dr. Infrared DR-218, 1,500 or 3,000 W: the value pick with a 240 V option
$119.99 for the 1,500 watt, 120 volt model and $129.99 for the 3,000 watt version, which needs a 240 volt, 20 amp circuit. IPX4 splash protection and an adjustable thermostat. The manufacturer page does not publish BTU, thermostat range, coverage or warranty, so those are blanks until Dr. Infrared answers. At 3,000 watts the output is 10,236 BTU per hour by conversion, which covers a 12x16 twin-wall house at a 30-degree gap. The DR-968 at $129.99 is an indoor unit with no wet-location rating and belongs in the house, not the greenhouse.
3. Bio Green Phoenix, 2,800 W, 240 V: the 10x12 and 12x16 answer
$299.99, down from $329.99, in stock. Two elements of 1,000 and 1,800 watts for 2,800 watts together, 9,554 BTU, a thermostat from minus 58 to 79 degrees F, IPX4, two-year warranty, and Bio Green's own "up to 215 square feet". You need a 240 volt circuit in the greenhouse, which is the real cost of stepping up.
4. Modine Hot Dawg HD45, vented gas: the reliable big-house unit
45,000 BTU input and 36,900 output, 80 percent efficient, power vented through a 3-inch pipe, natural gas or propane. $1,087 for the propane model in stock at HVACDirect, $1,200 at Greenhouse Megastore where the propane version was sold out and two natural gas units remained. Ten-year heat exchanger and two-year parts warranty; a thermostat is a $40 to $180 add-on. Greenhouse Megastore's "up to 350 square feet" is the retailer's claim, and by our math the unit covers a 12x16 single-glass house at a 40-degree gap with room to spare. Mr. Heater's Big Maxx MHU50, $913.87, 50,000 BTU input, is the equivalent with a propane conversion kit in the box and the thermostat sold separately.
5. Comfort Zone 1,500 W ceramic: the $32 heater with a one-year warranty
$32 at the manufacturer's site, 600 or 1,500 watts, 5,120 BTU, adjustable thermostat, sold out when checked. No wet-location rating and a one-year warranty with receipt. It heats the same air the Palma does for a fifth of the price, on a dry shelf away from the watering can. Fine in a dry house you visit daily; not what you want running unattended over a bench.
6. Heat mats: heat the roots, not the air
A Hydrofarm Jump Start 19.5 by 9 inch mat runs on 17 watts, is waterproof and UL listed, raises the tray 10 to 20 degrees above ambient, and was listed at $31.53, out of stock, at the retailer we could read. Vivosun's 10 by 20.75 inch mat is $14.99 at 20 watts with the same 10 to 20 degree claim. Twenty watts around the clock is 0.48 kWh a day, about 9 cents. If the goal is seedlings in March rather than tomatoes in January, this is the heater. See the seedling heat mat profile and its thermostat.
Not ranked: unvented propane and kerosene
Mr. Heater's Little Buddy is $117.26, 3,800 BTU, with an oxygen-depletion shutoff, and the Sengoku HeatMate kerosene heater is $179, 10,000 BTU, 14 hours on 1.2 gallons. Both are cheap per BTU and both are unvented. The University of Alaska Fairbanks Extension states that "fuel heaters (oil, kerosene, white gas or diesel) are not recommended for use in greenhouses unless adequate ventilation is provided." Alabama Extension names ethylene from incomplete combustion as "the chief culprit" behind twisted leaves, thickened stems and stunted plants, and specifies one square inch of outside-air opening per 2,500 BTU for any burner. A 1983 New York State Flower Industries bulletin adds the human risk: "carbon monoxide can be another problem, especially if someone enters or works in a small or hobby greenhouse heated with this type of unvented kerosene heater." The oxygen sensor on a Buddy protects you from suffocation. It does nothing about ethylene, moisture or carbon monoxide at plant-damaging levels. If you must burn fuel, vent it.
What It Costs to Run
Heater
Output
Cost per hour at full duty
Basis
1,500 W electric
5,118 BTU/h
$0.275
EIA June 2026 residential average, 18.34 cents per kWh
2,800 W electric (Phoenix)
9,554 BTU/h
$0.51
same
3,000 W electric (DR-218)
10,236 BTU/h
$0.55
same
Vented propane at 80 percent (HD45)
36,900 BTU/h
about $1.35
EIA propane $2.674 per gallon, March 30, 2026; 91,452 BTU per gallon
Unvented propane, 9,000 BTU/h
9,000 BTU/h
about $0.26
same, bulk propane, not 1-lb cylinders
A thermostat means full duty is the worst case, not the average. A 1,500 watt heater cycling half the time through a 12-hour night costs about $1.65 for that night at the June 2026 rate. The heating cost guide turns these rates into a winter budget by climate.
Wiring and Placement
Every electric heater here belongs on a GFCI-protected circuit. The 2023 National Electrical Code, section 210.8, requires ground-fault protection for outdoor receptacles and for indoor damp and wet locations, and a greenhouse is both. Confirm the specifics with your inspector. Run the 240 volt units on their own circuit sized to the manufacturer's amperage, which for the DR-218 is 20 amps. Mount fan heaters low, aimed along the length of the house, away from drip lines and the watering path, and put a remote thermometer at plant height on the far end so you know whether the heat is arriving where the plants are. A circulation fan spreads a single heater's output across a 10x12 better than a second heater does.
What We Could Not Verify
Dr. Infrared DR-218 BTU rating, thermostat range, coverage and warranty. Not on the manufacturer page; the warranty page returned an error.
Walmart, Home Depot, Amazon and Tractor Supply street prices for every unit. Pages blocked. Only manufacturer and named specialist retailer prices are used.
The Mr. Heater Big Maxx propane-model price at big-box retailers.
Any US price or stock for Parasene paraffin heaters, a UK product.
The current heating-season propane price. The EIA survey resumes in October 2026.
Manufacturer statements on GFCI for any specific heater; only the code headings were readable.
We did not run these heaters side by side.
Frequently Asked Questions
Should I bubblewrap the sides and top inside my greenhouse?
The owner who asked had a heater that could not keep up and lost a collection. Before adding insulation, do the arithmetic above: if the house loses 5,800 BTU an hour and the heater makes 5,118, no wrap closes that gap on the coldest night. Bubble film helps at the margin and costs light. A second heater on a second circuit, or a 240 volt unit, is the fix that scales.
Is it worth the investment (initial cost + electric heating) or would it be better to just use the grow lights I already have in the basement?
For a few tender plants, the basement wins on cost. A 1,500 watt heater cycling half the night through a 30-night cold spell is roughly $50 in electricity at the June 2026 rate, plus the $150 heater, and that is for a 6x8 that can hold the temperature. For a citrus collection you want to enjoy in the greenhouse, that is cheap. For three pots, it is not.
Do I need grow lights and a heater?
In zone 9a, the question asked, usually neither for hardy plants and a heater only on frost nights for tender ones. Size the heater to the house using the table above and put it on a thermostat set a few degrees above the plants' minimum, so it runs only when it must.
Curious if bubble wrap works as insulation?
It reduces the U-factor a little and the light a lot. The owner who posted measured data found a modest gain. Twin-wall polycarbonate already halves the loss of single glass, which is the insulation decision that matters. Wrap the north wall if you wrap anything, since it gives no useful light.
Is the 10mm wall worth it for insulation and heat compared to the thinner stuff?
By Palram's own published figures, 10mm twin-wall is 2.9 W/m²K against 3.3 for 8mm, roughly a 12 percent lower loss. Over a winter that is a real but small saving. Against single glass at roughly twice the loss, either one is the big win.
These guides are built from manufacturer documentation, public specifications, primary research where health claims matter, and repeated buyer questions that show up in real ownership and installation decisions.
Manufacturer responses can clarify pricing bands, warranty terms, support footprint, or common mistakes. They do not move a page up the shortlist on their own.
