Something I worry about with solar is once power is psychologically "free" I'll be tempted to set the thermostat down to 65F or whatever, and end up paying full rate when clouds blow by. I think it would be difficult not to increase power use once its free. I'm not talking about explicitly leaving stuff on for fun, but more like "well, power is free, so I guess I can afford a tropical reef tank instead of a plain fish tank" and there goes 1 KW of lighting continuously plus a chiller whereas right now my fish and I are very happy with ten watts of LEDs. This kind of thing needs to be taken into the financial analysis.
Also in the winter if I have "free" electricity and have to pay for natgas to heat, I'll simply buy a raid array of plug in electrical heaters to eliminate that "free" electricity and eliminate paying for some natgas. Or maybe I'll just install a bigger raid array. Either way, I like free heat in the winter and typical analysis usually doesn't consider heating. If you pay me 3 cents per KWh in the winter, and charge me 15 cents equivalent in natgas energy, I'm not going to sell you a single KWh until my house is above 75F in January.
Your array of inexpensive resistors will heat your house just as much (or probably much less) than a window would.
But you can use heat pumps, and you can do deep storage of summer heat, for use at winter (no idea about how well it works at your place). Both are very efficient.
Anyway, if your marginal price for electricity is zero, and the marginal externalities are also zero, there's not much problem in using it.
Heating your house with electricity is extremely inefficient. 5KW of "excess" electricity won't go very far in heating your house when it's cold outside, and any excess you might have will be less in the winter because of your latitude.
I'm not saying you can't do it, but even with today's cheap solar panels it's not likely to be a good value proposition.
Heating your house _directly_ with electricity is inefficient. Using that electricity to run a heat pump can be very efficient (assuming you have a large enough heat reservoir (e.g. a few hundred feet of pipe buried underground))
Turning electricity into heat isn't all the work you can get out of the energy. Heat pumps get even more, by using the energy to move heat from one place to another (e.g from ambient outside air to the volume to be heated), and the heat from this process also heats the volume.
If the air outside my house is warmer than the air inside my house, I'll just open the windows. Generally when one needs a heater it is because the system surrounding the house is colder, sometimes much colder, than the house itself.
Cold makes it harder on the heat pump, but not impossible. Heat pumps use a compressor (just like AC unit). If the pump can create a mass of gas colder than the outside air, then that mass will be warmed by the outside air (no matter what the absolute temps are).
In the limit of course (too cold outside), a heat pump is just as efficient as using the electricity for heat directly.
If that electrical energy was generated from gas, then it's often more efficient to burn the gas directly for heat. So depends when you take as the starting point for 100%
Also in the winter if I have "free" electricity and have to pay for natgas to heat, I'll simply buy a raid array of plug in electrical heaters to eliminate that "free" electricity and eliminate paying for some natgas. Or maybe I'll just install a bigger raid array. Either way, I like free heat in the winter and typical analysis usually doesn't consider heating. If you pay me 3 cents per KWh in the winter, and charge me 15 cents equivalent in natgas energy, I'm not going to sell you a single KWh until my house is above 75F in January.