Cheaper than free electricity from your solar panels? Heat pump + timer seems like the cheapest
There are lots of variables, but return on investment in solar is around 14% annually for me. It'll vary from place to place, so you need to do your own math.
It is interesting how the numbers work out in different places. Where I'm at it doesn't make sense at all. So much of my bill is just to have a meter and the per kilowatt rate is crazy low. Because of the high fee just to be hooked up the only way solar wins here is to get off grid - but winter kills that idea.