Natural gas furnace. I probably would have gone electric if we had installed the furnace after the solar panels went in. But, our natural gas bill is so small that I don’t really worry about it.

If that furnace is "smart" enough, you could add electric heater as additional external heat source. My wood pellet furnace has such options, but I think that if I simply added some electric heater into main loop, it would just work less (it can change it's output power from 24kW down to 7kW or just turn off burner if main loop is hot enough).