Hi, first time poster, would appreciate your advice...

We are in the process of having a good sized grid-tied solar PV system installed, and have planned to intentionally over-size it to make room for further increases in electric usage (an electric car, HVAC).

I wanted to explain my thinking on the HVAC side and see if I'm doing anything stupid.


The house is was built in early 1980s and is wired & plumbed for Air Conditioning but doesn't have it. The gas furnace is original. Natural gas is used for heating, hot water, clothes dryer, and the fireplace (which is rarely used). The house is fairly well insulated.

Climate is near-coastal San Diego, so AC usage is typically only a week or two a year. However this summer was really hot -- since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (We put in a crummy window AC and it did ok at keeping one room cool, but it was noisy and added about 100KWH to our bill.)

Our gas furnace is probably 65% efficient (based on its age). Our comfort heating load looks to be 30 therms per month, or 880KWHt/month. So if we just ran electric space heaters, we'd go broke. However, modern heat pumps can do about 3.0 COP (ratio of heat out to electric in) in a mild climate. So we'd need 880*.65= 650KWHt of heat, which would require only 215KWHe of electricity for several months of the year (if my calcs are correct)

Price-wise this works out to about $30/month for gas vs. $30-60 for electricity at today's prices. Thus if we were paying SDGE for electricity there'd be no financial benefit to buying a heat pump and using it for heat. But with solar, the balance changes. (for reference SDGE charges about $1/therm for natural gas and $0.14 to $0.33/KWh. Our solar system should be big enough to just zero out our bill on average year round, we believe)

For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year (that's a WAG number).

* are my calculations roughly correct? any big errors?
* I assume that we'd get rid of the gas fired furnace entirely (our HVAC closet is pretty cramped) - is that dumb? Typical winter low temperatures here are about 40F, perhaps 35F on a couple days.
* any other issues with heating with a heat pump? As I understand it, they tend to deliver air that isn't very hot, so they run more often.
* I'm a big fan of humidity control (I can stand 80F when it's dry, but if it's 70F and humid I'm not happy), and we are concerned about noise - does that mean we should look into a 2-stage system? Any recommendations?

Thanks in advance