Hi Everyone,
I was inspired to do my first post on this forum after reading trevdawg122 thread (http://www.greenbuildingtalk.com/Forums/tabid/53/afv/post/aff/15/Default.aspx).

I already have 6KW PV system installed which covers most of my yearly electric bill and I would like to expand the system to provide all my water and possibly offset some space heating bill.

I know SWH are the most efficient for water heating but if I design the biggest SWH system I possibly can based on my budget to get the most hot water in the winter I will be under utilizing the system in the summer (specially since the efficiency of SWH is lower in the winter). So my question is how can I calculate the best ratio for SWH and PV considering I use net metering?

My idea is to design the SWH to supply all my hot water needs for the fall/spring (there is no point in heating more water than I need for summer/fall/spring since I can't store for winter use). And for my remaining budget install additional panels which will collect energy all year that can be used during the winter since I have net metering. This way I'll have the best average.
What do you guys think? Does it makes sense my logic here?

My considerations are:
-Available roof area = 1200 SQFT (west faced 33 degrees)
-$15K budget (which based on my current installation cost I could install extra 12KWp or a combination of SWH and PV)
-Water heateis not decided yet. I could go with drainback for SWH and electric boiler for PV system but it will depend on the system ration.
-Location is Long Island - NY

Based on my calculations it would take me 6 years to payoff the investment if I were to install 12KWp.
I have not done the calculation for a hybrid system since I don't know the ration yet.

Thanks,
Sbre

There are some simple rules of thumb. One from Europe is that 1m2 of panel area will give 50% of the DHW for one person annually. Since we typically use more hot water then they do and since we don't buy panels that way, you could size for 2- 4x8' panels (3m2 each) will do for 4-5 people. Your area is much like mine and this is the standard installation, barring doing a proper simulation (which is quite easy).

Since you have a roof full of PV, you could retrofit some of those panels to get 40-50% of your annual DHW with a PVT panel and not lose any PV output. 2kw of PVT will do. Also, instead of an electric boiler, a heat pump would cost less to run.

http://analogenergy.com/

MikeSolar thanks for your feedback. It is good to have feedback on an actual installation very similar to what mine would be.
Please do you perhaps know the model of your collector? I'd like to use its SCRR rating to calculate how much BTU I can expect.

So if I read correctly 2- 4x8' would be enough for 50% of my DHW annually which means it will likely be higher than 50% DHW in the summer and less than 50% DHW in the winter.

I'm not planning on retrofit my current panels since they are on a different roof on the east side. The 1200SQFT are free roof area than I plan for upgrading the system so I'm thinking based on your input to do the following:

- Install 3 - 4x8' ETC panel which should give me 75% of my DHW annually
- Add 6 to 10 KWp to my PV system. (this is extra electricity that can be stored in the warmer days and used in winter for space heating, likely 6-8 MW/year)

My goal is to have close to 100% of my DHW annual average to come from SWH and use the extra PV to offset space heating bill.
I'll look into the heat pump to complement the DHW use but for space heat I think I have to go with an electric boiler since I don't imagine heat pump is good for space heating.