I wonder about this - does the mini-split efficiency drop below 1:1 ,ie the efficiency of the electric resistive heaters?
Or do you find it is not efficiency so much as just incapable of extracting heat at those temperatures?
When it is cold, it runs thru defrost often. The Pioneer I purchased does not have a pan heater so it runs a timed defrost cycle. I've looked for the efficiency curve but never could find it. I will say this, when it hits defrost cycle it will take heat from the room to put heat to the outside unit. It may even blow cold air from the inside unit. I know when it was -10°F last night, it would hit defrost about every 20 minutes. That takes power plus it takes heat out of the room. Outside unit is on the west side of the house near the SW corner so after noon it gets some heating from the sun and will be more efficient. At some point the mini only puts out 1:1 efficiency when it is pumping heat but the defrost cycle will kill that and make it net negative.
I reached full charge by about 1 pm and have 2 resistive heaters putting out full bore since 7 am and had turned the mini split on around 11:30 am as I realized it would hit full charge today early. I just checked and it is 80°F in the house, I haven't burned any propane since 6:30 am. High today was around 8°F.
The mini split will extract heat in below 0°F but it doesn't heat as well as the resistive heaters at that outside temp due to the defrost cycles. It can about hold room temp with the mini split during the day but it won't increase very fast. The resistive heaters will raise temp quite well. It's amazing how well I can heat the house during the day with excess PV when temps are 0°F and below when the sun is shining.
I've mentioned it before that I will be installing a wood gasification boiler long term with over 1,000 gallons of thermal storage. The thermal storage will at some point be hooked to an air to water heat pump to heat the thermal storage if I'm not burning wood or lengthen the time between batch burns. It will just be a large dump load to use for heating and cut down on wood usage with less boiler batch burns. I'm using the excess PV energy now for heating, this allows using it during the night more without draining the batteries. I'm adding more PV to the house this summer, about 4Kw and will need somewhere for any excess PV to go.
Batteries come in different forms, some are solid and some can be liquid. The cost of thermal storage actually isn't that high if you look for deals. I'm revamping the house heating system anyway, the high efficiency propane furnace is over 20 years old and I decided I want radiant floor like my shop. I run a propane condensing boiler in the shop and will install one for the house as backup.