Seems like both solar and heat pumps are rather marginal where you are. How many hours of sunlight do you get on Dec 20?
5 hours from sunrise to sunset but obviously at such low angle that even small trees and buildings,
anything not a completely flat horizon, make it 2-3 hours and that is quite low angle as well.
Last December was 3 kWh of production for the whole month. The same I produce in
one hour in a sunny day at good angle. But this is because last December only had TWO even remotely sunny days. Rest was all cloudy and foggy all the time; very typical December. The combination of cloud, fog and low solar angle is the killer. Now if I have a
similarly cloudy and foggy day in May, I get maybe 3-4 kWh per such day. In December it's 0. It's also depressing!
But the total yearly insolation in Finland is not bad at all, it's only marginally worse than middle Europe or northern USA for example. It's just that we get little to no production for two months, quite limited production for another two, the rest is just fine or even very good.
So yes, part of the time it's marginal, basically ROI lengthens by some 30% compared to middle Europe for example. But nowadays, ROI for solar installations is so good this isn't a problem IMHO. I'm definitely going to install more solar despite it being useless part of the year. Probably south facing at steep angle to boost winter time production as suggested by richard.cs.
Air source heat pumping isn't marginal in Southern Finland if you buy machines capable of low-temperature operation; temperatures below -20degC (down to -30degC) are reality but the number of such days per year is usually just 5-10, so just use alternative sources for those days, direct electric heating being simplest, burn wood or even untrendy oil, no big deal in total cost nor total CO2 because it's a few % of yearly energy. Air-source heat pumping works
very well to about -10 to -15degC, and most of the heating energy is spent approximately in such conditions.
BTW, one of the biggest challenges in designing a cold environment air source heat pump is the defrost algorithm, in particular how the unit decides when the evaporator is blocked due to freezing and needs defrosting. Freezing itself lowers both COP and power output, but because defrosting wastes energy, false positives also totally kill the COP. If you ignore the freezing issue, designing an unit to operate with large dT is fairly trivial, just some component and refrigerant optimization engineers do all the time. But buggy defrost algorithm can totally kill an otherwise well designed pump as evidenced by recent problems in some top-of-the-line Mitsubishi Electric air-to-air heatpump models that enter endless defrost-after-defrost loop and just stop working.
Would you do better with vertical panels on a South facing wall, would that be enough to be self-clearing?
Not only self-clearing, but also in a pretty good angle for those winter days. Such installations can be seen here, it's generally a good idea if self-sustainability (i.e., energy production for oneself
when it is needed) is the target. Yearly total production suffers significantly, but OTOH, does it make sense to produce excess energy in summer when everyone else is also producing in excess, and sell it for peanuts?