When is the right time to buy a new/emerging technology?

If you're looking for a purely financial answer (ignoring the social/environmental aspects) there are a few different ways you can look at it.

For these types of improvements the simplest is a payback calculation. How long would it take you to recoup the initial costs?

For example, if the entire installation cost $5,000 (including any tax credits), and you save $100 per month (I'm making both numbers up), you'll pay back your investment in 50 months, or about 4 years.

(Note that if you borrow money to do the improvement, then your payback period is longer because you're reducing the amount that you're saving each month by paying interest.)

If you're deciding between different uses for the money (like investing, or paying down other debt) then you can look at the return that you're getting. Using the same example, you are spending $5,000 and getting $100 per month back, for a 24% annual return ($1,200 / $5,000), which is better than you can get on almost anything but a 401(k) match (meaning don't stop your 401(k) contributions to do this either).

The decision on whether or wait or not then becomes - will the price drop faster than the amount of savings you will realize. So if you will save $100 per month in your electric bill, is the price of the complete installation going down by more than $100 each month? If not, you'd be better off buying now and start paying back the investment sooner.