What are your goals for heating the pool? Do you want the pool to be 85 year round every single day? In your location the heat pump as well as solar are both good options. You said you can only put the panels on the North side of the roof. Is your roof pitch steep or shallow, shallow would be better with a northfacing panel, you will have to increase the number of the panels as opposed to a south facing installation. I can not comment on what it will do to your roof but it must not be that bad as many houses use them. With respect to the heat pump they work best with an outdoor temp. above 65 degrees, they CRANK out much more heat when the outdoor temp starts to hit the 80's. I have a heat pump up hear in PA and love the thing but we mainly use it to extend the season and during the summer keep the pool at 85 degress where the wife likes it. It does not cost me more than $150/month to do that and run the pumps.