In reply to ProDarwin :
I think you have some valid points. Out of curiosity, for a electricity bill of $X, what should a system cost?
I dont understand this last part:
1) its a near $0 outlay, so ROI isn't calculated off a $35k investment. Similar to Keith's situation, only solar output isn't as favorable here in NC.
2)what solar system will pay for itself within 25 years? Many systems I have seen require a major cash outlay up front, which messes with all ROI calcs, because ROI of the stock market is always going to outperform that.
It's not linear, there are a lot of variables to the cost and ROI of a solar system.
First you have the fixed costs. The fixed costs are a greater percentage of the cost on small systems Vs. big systems, so smaller systems have longer paybacks.
Your usage and energy rates directly impact your return on investment. If you don't have much useage to offset, and/or your rates are low, it takes longer to payback the cost of the system with energy savings. How your utility charges for tiers of usage is very important. If it uses a tiered approach, you get the fastest payback when you knock out the higher tiers of usage. Say you are charged .15 per kWh for tier 1 and .30 for tier 2, and your bill happens to be split evenly between the two tiers. Say you need 30 modules to zero your bill. The first 15 have a higher ROI and shorter payback because they are knocking out expensive power. The next 15 are knocking out cheap power, and will take twice as long to pay for. That is why is often not cost effective to try to zero your bill, but to shoot for knocking out 90% or so of your useage.
The average payback on a solar system in the US is between 6-12 years. CA is closer to 6, anything over 10 really isn't a good candidate. Payback is when your system pays for itself in savings. ROI is what it pays back over the life of the system. Both are important, but can be adjusted independently. Let's use our hypothetical example above. If 15 modules knock out our tier 2 bill, just installing 15 panels would have a quick payback. Ignoring the fixed cost, let's say that undersized system pays back in 5 years since each panel is offsetting expensive electricity. If we add 15 more panels to knock out tier 1, those modules take 10 years to pay back, since they are eliminating half cost electricity. So the overall payback is 7.5 years. In other words, only installing 15 modules has a payback 2.5 years earlier. It looks like the most bang for the buck solution. But let's look at total ROI. The first 15 panels pay for themselves in 5 years, and will pay for themselves 5 times over the life of the system. The next 15 panels will pay for themselves 2.5 times over the life of the system. So lower return rate, but still more dollars overall over 25 years. And a bit better after you figure the fixed costs for the initial 15 panels that you do don't need for the other 15.
In your example, a system with a 8 year payback at $115 per month would cost just $11k total. Now, you aren't likely to find that, because your bill is below the lower end that can support a solar install.