A mat is sized by lift, not by area
The thing a propagation mat has to do is replace the heat a tray of warm wet media loses to its surroundings. That loss is roughly the tray area times a loss coefficient times the difference between the media temperature and the air around it. Everything else follows from those three numbers, and the one that dominates in practice is the difference — the lift.
Take a 107 W mat covering 2.99 square feet, which is 35.8 watts per square foot and 365 BTU an hour flat out. Holding 75 degree media in 60 degree air is a 15 degree lift, which at a loss coefficient of 1.6 costs 71.8 BTU an hour and puts the mat at about 20 percent duty. Holding the same 75 degrees in 45 degree air doubles the lift and doubles the demand to 143.5 BTU an hour, or 39 percent duty. The mat copes with both, but it is working twice as hard for the same result.
Now change the coefficient instead. At 4.0 — an open tray of soaking media on a cold bench with air moving over it — the night demand becomes 359 BTU an hour against 365 available, so the mat is running continuously with essentially nothing in reserve. At 6.0 it tops out at about 65 degrees and never reaches the setpoint at all, no matter how long it runs.
| Loss coefficient | Night demand at a 30°F lift | Duty cycle | Highest media temperature it can hold |
|---|---|---|---|
| 1.0 | 89.7 BTU/hr | 25% | 167°F |
| 1.6 | 143.5 BTU/hr | 39% | 121°F |
| 2.4 | 215.3 BTU/hr | 59% | 96°F |
| 4.0 | 358.8 BTU/hr | 98% | 76°F |
| 6.0 | 538.2 BTU/hr | continuous, short | 65°F |
Read the last column rather than the first. The question is almost never how many watts the mat has. It is what the tray is losing, and the two biggest terms in that are the open top and the evaporation from wet media. A humidity dome attacks both at once and is a fraction of the price of a larger mat.
The duty cycle is why the running cost is small
People price a heat mat as though it runs continuously, which is what the label invites. On the numbers the form opens with — four 107 W mats, 75 degree media, 45 degrees at night for twelve hours and 60 by day, coefficient 1.6 — each mat runs 39.3 percent of the night and 19.7 percent of the day. That is 7.08 hours of actual running out of 24, so 0.757 kWh per mat per day. Four mats over 21 days is 63.6 kWh, and at 16 cents that is $10.18 for the whole germination run.
The same four mats running flat out for 21 days would be 215.7 kWh and $34.51. The thermostat is doing two thirds of the work, and that is on a genuinely cold bench. In a heated room the gap is far larger.
Spread over 288 seedlings, $10.18 is about three and a half cents each. That is a real number to set against buying transplants, though it is only the electricity — seed, media, trays and the bench space are all on top, and the bench space is usually the expensive part in April.
Media temperature is not air temperature and not surface temperature
The number that matters for germination is the temperature in the root zone, an inch or so into the mix. Media sitting on a mat is warmer at the bottom than at the top, wet media is cooler than dry media at the same power because evaporation is carrying heat away, and the air an inch above the tray tells you almost nothing about either. A probe pushed into a representative cell is the only reading worth acting on, and it is also the only reading a thermostat should be controlling from.
Without a thermostat, a mat does not hold a temperature at all — it applies a fixed watts per square foot and the media lands wherever the balance puts it. In a 70 degree room that can be well above what any seed wants. In a 40 degree greenhouse it can be short of what any seed needs. The highest-temperature line in the calculator is the honest reading for an unthermostatted mat, and comparing it against the setpoint tells you which of those two failures you are heading for.
Where this sits in the season
The mat is the front end of a schedule, and the schedule is what determines whether the bench heat is needed at all. If the seed starting schedule calculator puts your sowing date in late March rather than mid February, the air on the bench is ten degrees warmer and the mat question largely answers itself. If the house has heat, the greenhouse heat loss calculator will tell you what holding the whole structure warmer costs against the alternative of heating only the trays, and heating only the trays wins by an enormous margin — heating 12 square feet of bench to 75 rather than 7,500 cubic feet of air is not a close comparison.
After germination, warmth stops being the constraint and light takes over. Leggy pale seedlings on a warm mat under a dim window are a light problem, and the daily light integral calculator is where that gets diagnosed.
Questions people ask
What wattage of mat do I need?
Work it the other way round. Decide the media temperature and measure the air the trays will actually sit in, multiply the difference by the tray area and by a loss coefficient, and that is the heat the mat has to supply continuously in the worst case. A mat delivers 3.412 BTU per hour per watt, so divide by that for the minimum wattage. On a 12 square foot bench at a 30 degree lift and a coefficient of 1.6, that is 576 BTU an hour or about 169 W across the whole bench. The point of doing it this way is that it makes clear how much cheaper it is to reduce the lift than to add watts.
Do I really need a thermostat?
A mat without one applies a fixed power and the media settles at whatever temperature the heat balance produces, which is entirely determined by conditions the mat does not know about. The same mat that sits at 72 degrees on a cool bench sits at 90 in a warm room with a dome on, and 90 degree media is actively bad for most seed. The calculator reports both a duty cycle, which assumes a thermostat, and a highest achievable temperature, which is what an unthermostatted mat actually does. If those two numbers are far apart, the thermostat is not optional.
Does a humidity dome really make that much difference?
It attacks the two largest loss terms at once. A dome cuts convection off the media surface and it nearly stops evaporation, and evaporation from a freshly watered tray can be the biggest single path heat takes out of the mix — every pound of water that leaves takes around a thousand BTU with it. The calculator does not put a number on the dome because there is no defensible constant for it, but the sensitivity table shows what moving the coefficient does, and a dome moves it a long way down. The trade is that a dome on too long invites damping off, so it comes off once things are up.
Is it cheaper to heat the greenhouse or just the mats?
The mats, by an enormous margin, and it is not a close call. Bottom heat on a few square feet of bench costs single-digit dollars for a three-week run. Holding the air in a 20 by 48 ft single-film tunnel at germination temperature through March costs hundreds of dollars a month in fuel. The whole point of a propagation mat is that seeds care about the temperature of the inch of media around them and nothing else, so heating that inch and letting the rest of the house do what it likes is the correct answer. Run the numbers on the greenhouse heat loss page if you want the comparison in your own figures.
Can I put a mat under a tray of standing water?
That is a question for the mat instructions and it is answered there, not here. What is worth saying plainly is that propagation benches combine mains electricity, constant condensation, wet trays and people reaching over them with wet hands, and that combination deserves more care than it usually gets. What protection the circuit needs, whether the mat may be covered, and how mats may be connected together are all in the product documentation and, for the installation itself, are matters for a qualified electrician. A mat with a damaged surface or a damaged cord is replaced rather than repaired.