Natural gas vs electric heating
I have alway wondered that the real monthly costs would be if we replaced our natural gas 80% furnace with an electric heat pump system. With all else equal, and assuming the equipment costs are not in play, I don’t think it would be as easy as converting my therm usage to kWh. If I do that, I get some crazy costs for a sample winter monthly bill, but that would probably be a standard electric resistance heat.
I used about 132 therms last month @ about $0.618 per therm with fees. My electric is about $0.15 per kWh with fees. I am estimating that with a mini split system with a COP of 2.6, and my existing gas furnace at about .75 COP, my cost would be about double when compared to a single gas bill. Does that sound right?
Here is my math:
COP ratio is 2.6 / .75 = 3.7
Therm conversion is 132 therms = 3867 kWh
3867 kWh * .15 = $580 (probably my cost for an electric resistance heat system)
$580 / 3.7 COP ratio = $157 added to my electric bill.
compare this to the gas bill of $82.
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If the all-in delivered cost of natural gas & electricity is 62 cents/therm and 15 cents/kwh respectively it's going to be tough to make an economic case for any heat pump vs. gas.
Even with buck fifty gas and twenty cent electricity (like my neighborhood) it's cheaper to heat with gas.
It's a much easier case with $2.50/gallon propane.
There’s more too it than that. As it gets colder outside, it takes more energy to pump the heat inside. You’re essentially scavenging heat from outside. Your effective cost per therm of heat delivered to your house will rise as the outside temperature drops, with a big jump at whatever temperature your auxiliary heat kicks in (which essentially puts you on full electric resistance heat).
The actual costs per therm of heat delivered is complex and varies with different equipment. The specific operating parameters (outdoor temp and indoor temp) result in different operating costs at the specific time you’re running your heat pump.
You can make educated guesses to get close, but it is very difficult to be specific.
Bill
I'd check the numbers using the heat pump only when it's not too cold (and using the existing furnace when it is). And there is usually some value to cooling.
We replaced our gas furnace with a heat pump last summer. There's only been one electricity bill since we starting using significant quantities of heat (we get electric bills every other month, but gas every month, so we had a mid-October to mid-December bill and won't have another until mid-February). So far, after backing out the monthly minimum gas charge and the approximate amount we usually use for the water heater, and similarly backing out the typical electricity use from the same two months last year, it appears we have thus far saved (IIRC, don't have all the calculations in front of me) about $35 a month compared to the bills with our elderly gas furnace. (Note: our December gas bill from last winter was about 138 therms, so not too dissimilar to yours.) But that's with gas running just under a dollar a therm and electricity about eleven cents a kWh. However, our electricity source is pretty clean (will be still cleaner if we get solar panels), and starting to get away from fracked gas (which is artificially cheap) and other problems with fossil fuels is something we felt we had to do if possible. (We did try for a heat pump water heater, but for various reasons threw in the towel and put in another gas one for now.) In any case we certainly aren't going to make up the difference in overt cost unless gas gets way more expensive and/or the heat pump lasts an unexpectedly long time. Even if we'd been able to get off gas entirely we likely wouldn't do that.
4000KWhr/month seems like a lot of energy.
My "pretty good house" of 4000 sq ft (incl basement) with 4 heat pumps, all 9' ceilings, used total 1700kwhr ($240 cdn in Ontario) from dec 18 to jan 17. There were some nasty cold spells in that time (-20C), and the the house was wrapping up construction (lots of in/out trips for contractors/us), we had a move-in day where the door stayed open for about 2hours straight, we have 2 electric in floor heat mats in kitchen and en-suite bathroom. We moved early Jan, so 30% of that bill is regular household use. Hot water tank was on, but not much hot water useage. Lots of vacuuming as well.
I just went and checked the meter and the last electric bill, and in the past month we've used about 1180 kWh. Last February's two-month bill was for 1152 kWh, so half of that is 576, which would put the likely heat pump usage at about 600 kWh. Last year at this time the gas bill was for ~153 therms: this year it's ~17 therms, so the furnace usage was likely about 135 therms.