If you get to -20C a couple of times a year but normally it's -5, it doesn't matter that it's inefficient for 2 or 3 days a year, as it's very efficient for 360 days a year
It doesn't matter for you personally, but it kind of matters globally. If everyone hits that same inefficiency at the same time then there needs to be a lot of extra capacity in the electric grid that can be spun up during a small part of the year.
There's already plenty of spare capacity in winter time that is necessary to support peak demand in summer time. Loads like electric heat pumps are also perfect candidates for automated management to smooth out load spikes, so there would be no "at the same time" to worry about.
> There's already plenty of spare capacity in winter time that is necessary to support peak demand in summer time
That's not the case at all in very cold climates. Here in Alberta we get grid alerts when it gets extremely cold and that's with the vast majority of houses being heated by forced-air natural gas.
Those are general demand curves, and are not necessarily representative of the coolest days of the season. What we care about is the gap between supply and demand.
From experience buying large industrial quantities of natural gas, the larger market can see bad effects at low temperatures. Even in the upper Midwest you can get force majeure events, particularly when temperatures drop below 0 F for a couple days.
The winter months bring planned shutdowns for power plants to perform maintenance, trying to prevent downtime during the summer heat. The non-linear nature is a concern, though in cases almost up to the 1:1 point, it's more energy efficient to burn natural gas to create electricity and use a heat pump compared to burning natural gas in the home.
The issue I’ve seen is need for heat goes up a LOT in older houses when it gets that cold, and the heat pump without resistive heat performs a lot worse (like 2x+ worse).
Because the labor to properly insulate a home can be quite extensive, while the labor to install a heating unit is typically less than half a day.
Take a structural brick house built with no cavity insulation, plaster walls, and a finished interior and try to get any amount of additional insulation into the walls. You end up having to destroy and repair the finished plaster walls, which is obviously prohibitive from a labor cost standpoint. Those projects can be (and usually are) done when remodeling/redecorating is already planned (so you don't double-pay for finish work), but are economically unrealistic to do just to save on HVAC.
>economically unrealistic to do just to save on HVAC
That depends on how much the HVAC costs... I saw some videos from last USA freeze where people had ice (!) inside of their homes - I doubt heating that is in any way economical.
There are plenty of insulation firms that specialize in insulating old homes without ruining the finishes. Typically you cut a small plug, either from the outside or inside depending on the construction method and mechanically force (typically blow) cellulose fibres into the wall cavity. You need one plug per stud bay, but repairing those is a pretty simple job compared with tearing down all the finishes.
Air sealing is tougher, but techniques like aerobarrier where an aerosolized polymer is sprayed throughout the home while it's under positive pressure has made air sealing fairly simple. Stuff like that can plug up to multiple inch gaps
The "cut a small plug into each stud bay" works very poorly when the wall material is 9" thick structural brick and there are not studs, but rather lath and plaster (and often wallpaper) on furring strips.
Lath and plaster can be extremely expensive to repair. Sealing old houses has problems in terms of moisture- many old houses depend on evaporation in various ways- list goes on
No, that's about the capacity of the heating system at -20, not efficiency. It does mean that the system will be oversized for the days it doesn't get that cold.
There is no lie / part skipped, what that number is meant to tell you is that it reaches its 1:1 point there, so it "works", with diminishing returns, down to that point. Which is where it's as "efficient" (to put it that way) as a resistive heater.
Obviously you need to take that into account - is the amount of heat you get out of it sufficient for nominal winter temperatures, at its coldest, for where you live? (geographic location and building conditions). As for myself, I have a backup in my wood stove, but I only need it under special circumstances (like last week when it was particularly cold and no electricity for parts of two whole days because I had electricians doing major rewiring in my home).
* https://www.mitsubishielectric.ca/en/hvac/professionals/fs-s...
More advanced unit (primarily for hydronic heating) that goes down to -20F / -29C:
* https://www.spacepak.com/solstice-inverter-extreme
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