Why not get the roof before the solar install, if that's what you want. Though they claim only $500 for a temporary remove of a 5kw system.

Look at your power bill and see how many kwh you use per year. The power companies often give you such details, or have it on their web site. Divide by 2, that's how many peak watts you need to meet your needs with solar. A typical house might use 8,000 kwh per year and thus need about 4,000 peak watts here in California. More in less sunny places.

My personal vote would be not to get full capacity, though. Hedge a bit on solar getting cheaper. Excellent odds that you can add more later. With CitizenRe you must buy all the power their panels produce, no option. So buy a little from the power company today, and add more solar (for less money) later. Plus I would not try to get so much as to totally fill all your south roof space, etc.

They only "sell" power in the day (you are required to buy all the power your pv system produces.) You buy the power at night. You actually "sell" your excess during the day to your power company to power your neighbours. CR does not do that. Strictly it's net metering -- your meter runs backwards during the day, and forwards at night, though they never let you go to zero, even if you provide more power back in the day than you take at night, unless the law forces them to do so in your state.