I will have to look into this more on monday, but one very odd *financial* reason to (at the moment) heat our PV grid-intertied house with air-source heat pumps (electricity, but at ~250-300% efficient, counteracting mostly the ~33% efficiency from a fossil fuel usage standpoint for heating) is that since our PVs are connected to the grid, even if we have a 0 KWh use (and this would actually be a miracle since there is no net metering, so it would mean I don’t use the computer and we don’t use lights when it is dark or cloudy)… The “at the moment” part above is that there is always hope that we will some day have some net metering. I assume it will happen eventually.
So anyway, even if we used 0 KWh/month in electricity by only using electricity when our PVs are outputting enough juice, our bill from our municipal electricity company is still going to be ~$20/month for any usage < ~200KWh. Including 0KWh. (I'll know the details on mon). IOW… I bet it will be unlikely, even with no net metering that we will use more than the minimum payment for heat, hot water, and electricity use. We'll see. Vs… if we had a propane hot water heater or Rinnai-style direct vent heater, we would be paying real money for the usage (and on top of that… the price per gallon would be highish since our annual usage would be low and all the propane companies have higher prices for less usage). And by highish I do mean in a KWh of source fossil-fuel-based energy sense since I always multiply electricity use by three because it's ~33% efficient from the mostly kinda dirty power plants around here.
Is this making any sense?
My logic here and always is that you have to think holistically about PVs. They are really just an offset for other usage. IOW, if you are just looking at energy used by the world, it doesn't really matter if YOUR use the electricity pumped out out by your PVs, or your neighbors. So the very best way to heat your house if you are not in a dense area and bugging neighbors, even if you have PVs and/or a net-zero or passivhaus/passive House is still wood. Well, solar heating I should say comes first. But most "traditional" solar heating systems are expensive vs wood stoves. (I should note that I think a re-birth of TAP / "solar siding" / low-mass sunspaces will bring back lower-cost solar heating. Go join the "Solar Heat" discussion list at http://tech.groups.yahoo.com/group/SolarHeat/)
Air-Source Heat Pumps are next, especially if you live in an area with lots of hydro electricity, or have a greenup option on your bill, but even then I suppose it is questionable, since this greener electricity could probably be “exported” farther afield if one wasn’t using it. But heat pumps generally are only tied with fossil fuel options like propane or natural gas — IOW, the global fossil fuel usage is probably going to be the same in either case. Break out excel and do the math. I’m pretty sure I’ve got this right, even if assuming the best numbers out there: a 98% efficient Navien hot water heater being used for hot water and radiant heating VS a heat pump with a COP of ~3 like a Fujitsu 9RLS. Since 33% * 300% (heat pump) ~= 100% * 98% (propane)
Hot tip if you are looking at heat pump manuals: COP = HSPF / 3.413
So the Fujitsu is 12 / 3.413 = 3.51. But that’s for a more southerly climate zone (like NC?), so the adjustment I usually use for MA/Massachusetts is the 15% adjustment I think I’ve seen Canada uses for heat pump ratings. So 3.51 / 1.15 = 3.05. Voila.