One pound at a time
A pellet appliance is easier to reason about than a wood stove because the fuel arrives at a rate you can look up. The auger feeds a known number of pounds per hour at each setting, the bag tells you the heat value per pound, and the rating plate tells you the efficiency. Multiply the three and you have the output; everything else on the page is that number in different units.
On the defaults: 8,200 BTU a pound at 75 percent efficiency delivers 6,150 BTU per pound to the room. At 2.5 pounds an hour that is 15,375 BTU/hr. A 45 pound hopper lasts 18 hours at that rate, which is the difference between a stove you fill in the evening and one you have to think about at two in the morning.
| Feed rate | Delivered at 8,200 BTU/lb and 75% | 45 lb hopper lasts | Pounds in 24 hours |
|---|---|---|---|
| 1 lb/hr | 6,150 BTU/hr | 45.00 hr | 24 lb |
| 2 lb/hr | 12,300 BTU/hr | 22.50 hr | 48 lb |
| 3 lb/hr | 18,450 BTU/hr | 15.00 hr | 72 lb |
| 4 lb/hr | 24,600 BTU/hr | 11.25 hr | 96 lb |
| 5 lb/hr | 30,750 BTU/hr | 9.00 hr | 120 lb |
The season, and why the ton is not automatically cheaper
Sixteen hours a day at 2.5 pounds an hour is 40 pounds a day. Over a 150 day season that is 6,000 pounds, which is exactly 150 bags of 40 pounds, or 3.00 tons. The heat delivered is 36.90 million BTU.
At the prices in the form, 150 bags at $6.50 is $975.00, which is $26.42 per delivered million BTU. Three tons at $320 is $960.00, which is $26.02. The pallet wins by $15.00 here, but that margin is thin enough that it flips on a quantity that does not divide evenly into tons, and it ignores the two things that decide it in practice: whether the delivery puts the pallet where you can reach it in February, and whether you have dry covered space for three tons of bags. A pallet left under a tarp in the rain is a much more expensive mistake than fifteen dollars.
The bag specification moves more money than the price does
This is the part worth carrying away. Holding everything else constant and changing only the heat value on the bag from 8,200 to 8,700 BTU a pound raises the delivered heat per pound from 6,150 to 6,525, which is 6.1 percent more heat out of the same 6,000 pounds. The season delivered goes from 36.90 to 39.15 million BTU. If both pellets cost $320 a ton, the cost per delivered million BTU falls from $26.02 to $24.52 — a saving of $1.50 per million BTU, which is worth more than the $15 the pallet saved over the bags.
Ash does the same thing to your evenings rather than your wallet. Six thousand pounds at 0.7 percent ash is 42 pounds of ash over the season. The same weight of a 1.5 percent pellet is 90 pounds, and every pound of it has to come out of a pan by hand while the burn pot fouls faster in between. That is the reason to read the bag rather than the price tag.
Matching the rate to the building
If you know the heat loss of the space from the heat loss calculator, dividing it by the delivered BTU per pound gives the feed rate the building corresponds to. On the defaults, 24,000 BTU/hr over 6,150 delivered BTU a pound is 3.90 pounds an hour, against the 2.5 entered. That is the arithmetic of a stove being asked to run near the top of its range on a design day, and the same appliance being run far below its low setting in a mild week. The wood stove sizing calculator lays that relationship out in full, and the mechanism is the same for a pellet appliance even though the fuel is metered.
If you want to compare the whole thing against cordwood, run the firewood BTU calculator for your species and moisture and compare the two cost per delivered million BTU figures directly. That comparison is the only fair one, because a cord and a ton are units of completely different things.
Questions people ask
What burn rate should I enter?
The one in the manual for the setting you actually run, which is usually given in pounds per hour for each heat level. If your manual gives only a range for the whole appliance, run the calculator at both ends and treat the answer as a band, because the spread is large. The most reliable method is to weigh the hopper contents, run the stove for a measured number of hours at one setting, and weigh what is left; that gives you your fuel, your appliance and your settings rather than a specification.
Why does the calculator round bags up?
Because you cannot buy 150.4 bags, and rounding a purchase down produces a number that looks tidy and leaves you short in March. The exact figure is shown next to the rounded one so you can see how close it was. The same applies to tons: buying by the pallet means buying up to a whole ton more than the season needs, and the results show exactly how many pounds of that you are paying for and not burning.
Are pellets cheaper than cordwood?
It depends entirely on what both cost where you are, and the only fair comparison is cost per delivered million BTU rather than per ton against per cord. Run this page for your pellets and the firewood BTU calculator for your wood, and compare those two figures. A ton of pellets on the defaults here delivers 12.30 million BTU. A cord of seasoned red oak in a good stove delivers something in the high teens. The prices that go with those two numbers are local and this page quotes none of them.
The stove uses more than the calculator says. What am I missing?
Usually hours. Sixteen hours a day of burning is not the same as sixteen hours of the stove being switched on, and a thermostat-controlled unit spends part of its day idling on a low feed rather than off. Beyond that, the feed rate at a given setting is a nominal figure, cold weather pushes the stove to a higher setting than the one you entered, and a fouled burn pot changes the picture. Weighing a measured run settles it in an evening.
What happens in a power cut?
The auger, the combustion fan and the room fan all stop, and the appliance stops being a heat source. What happens to the fuel already in the burn pot is specific to the appliance and is covered in its manual, which is the only place to get that answer. The hazard to name here is that a venting system that depends on a fan behaves differently when the fan stops, and carbon monoxide has no smell. Battery backup, and what if anything is appropriate for a particular appliance, is a question for the manufacturer instructions and a certified installer.