Converting pellet stove usage to minisplit operating cost
smokey059
| Posted in General Questions on
If I use between 40 to 80lbs of wood pellets a day to heat and the pellets say 8800 btus per lb and the stove is around 80 % efficient, can I tell from this info how much it would cost to run a mini split if electricity is 14 cents a kwh. The zone is 6a. The pellets cost around 5.60 per 40 lbs. How much savings would the mini split be because the pellet stove has to be replaced. Even an approximation would help. Thanks
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Replies
Hi Smokey -
you would need the efficiency of the minisplit and then the simple conversion: 1 kWh = 3412 btus.
Peter
A properly sized cold climate mini-split will have an as-use HSPF of 10-12 BTU/watt-hour (10,000-12,000 BTU/kwh) in zone 6A on a seasonal average, though 6000 BTU/kwh would be more likely when it's below zero outside which is less than 5% of the time.) The average performance in January at an outdoor average +15F would be 8500-9000 BTU/kwh, but when it's north of 35F it'll be about 15,000 BTU/kwh as long as it's still modulating rather than cycling on/off multiple times per hour.
So at $5.60/40lbs the pellets cost 14 cents/lb. and at 80% efficiency x 8800 BTUlb delivers about 7000 BTU for that 14 cents. Since (conveniently enough) your electricity also cost 14cents it looks like the savings would be negative for those hours when it's really cold out, but even in January it would likely be cheaper on average, and in the shoulder seasons it would be half the cost.
This is of course predicated on the notion that you're buying a cold-climate specific minisplit (which has a different compressor design optimized for cold temperatures) and that it's sized correctly for the heating load. If it's oversized and cycling a lot or undersized and running at maximum speed all the time the annual operation cost could be about the same as your pellet stove.
The ease of use and maintenance of a mini-split is a lot simpler than a pellet stove though- those kwh don't have to be lugged from the truck into the house.
If you use 60 lbs/day, that's 60*8800*0.8 = 422 kBTU/day. That's 124 kWh of heat. If the minisplit has a COP of 3, that's 41 kWh. That costs $5.79, vs. $8.40 for the pellets.
But the minisplit COP will depend on the outdoor temperature. And the pellet stove operation probably drives a lot of infiltration will increases the heat load. So my guess is the improvement from switching to the minisplit is more than the raw comparison above indicates.
Thanks for the numbers. I only use 2 bags a day for maybe 25 days a yr otherwise it's closer to 1 bag. Sure would be nice not to haul pellets and have to clean everyday. Seems even at break even it be worth it. Thanks again
Smokey059,
Just to add a bit of fuel to the fire, I'm wondering if you might be overestimating the BTU output of your pellet stove and therefore what the heat pump would need to provide. It's my understanding that wood pellets typically run more on the lines of 7500 to 7900 BTUs per pound. My Harman P43 is rated at ~78% efficiency and I bet that's when it's freshly cleaned. I too am interested in the prospect of using a mini-split especially for the shoulder season when it's nearly impossible to throttle the stove down enough without running it too cool and a heat pump would be running with a very attractive COP range.