Solar Hot Water Heater

Hello all you solar guru's :-)
Im in Phoenix AZ and am about to take the plunge and replace my standard electric hot water heater with a solar system. I think I have decided on the system manufactured by Fafco sungrabber.com for those that don't know about them. I have had two companies come out so far to give me estimates and the differ by about $2000 and they are both Fafco systems. For those of you that are running a solar setup like this I have a couple questions. The first quote I got was for a 4 panel system and a 80 Gal tank and the second quote I got was for a 2 panel system and a 60 Gal tank. This is a family of four and we currently have a 50 Gal electric hot water heater. Would you say a 2 panel is enough or are we going to need the 4 panel setup? Secondly do any of you have a system by this company and would you recommend them? Lastly on figuring out the yearly savings of a system like this would my calculations be pretty close? I have APS for electric and over the last year I averaged that we pay .10/kWh. My Current hot water heater based on the Energyguide sticker uses about 5100kW per year. If the new system will only use about 2300kW per year then I should save about $280 per year
Cheers, Chris