[toast0]

I don't get it... system costs scale pretty closely with number of panels.

If a $X system covers 100% of your use, a $X/2 system probably covers 50% of your use.

Ignoring time value of money, if the $X system saves you $X / 5 per year, the $X / 2 system that generates 50% will save you $X / 10 per year. Both systems would have a 5 year ROI.

Even if your half price system generates only 45%, it only brings the ROI out to 5 years 6 months.

There's are some fixed costs to hook into your panel and whatnot, but as long as your system is within a reasonable range, the math should math pretty well until your system generates more than your usage. Assuming the math maths at all... if system costs are high and utility rates are low, it doesn't math. If you roll a roof replacement into your system cost, then yeah, a small system won't get you the ROI, but I don't think the math works to include roofing in the system cost unless your utility power is very expensive per kWh. If you're in a reasonable cost area, and you need a new roof for your system, you need to wait until it's time for a new roof anyway. Or if you can't afford a new roof and a solar system at the same time and don't want to finance the solar system, you could probably have the roofers put in rails for future panels when the roof is installed so that the roof warranty covers the rails. Then you can add solar when you've rebuilt your reserves.

[ramesh31]

>"Ignoring time value of money"

The time value of money is the entire point of what I've said.

And it goes up exponentially when you are just getting by as most of us are. $17k right now is worth incredibly more to me than saving $200/month for the next 20 years.

[toast0]

Time value of money scales with the value too, though. I just wanted to make the math easy. If you the system costs half, and delivers half the savings annually, the ROI is the same. If the system costs half and saves a little less annually, the ROI is a little longer. I'd like to see real numbers where a 100% system gets you a ROI of 5 years and a smaller system gets you a ROI of 10+ years.

> $17k right now is worth incredibly more to me than saving $200/month for the next 20 years.

Would you take a $17k loan today, and pay it back at $200/month for 20 years? That's a 13% loan. Which I guess is a market rate for a personal loan, but those usually have a shorter period. My preferred credit union gives solar loans at 6.25% to 8%.

Would you take a $17k loan, to save $200/month for 20 years, but have to pay $191/month for 10 years to clear the loan? What about if you paid $146/month for 15 years? If not, why not? How about if you paid $331/month for 5 years?

[lowbloodsugar]

Identical solar panels in San Diego produce more KWh than they do in Boston. Same cost, quicker payoff.

[toast0]

Well yeah, if you have the option of where to build your system, it makes sense to put a system where the math works best. There are many reasons why San Diegans were building more solar systems than Bostoners, insolation and utility rates are high among them.

But regardless of where you install, if a system with 100% generation has a 5 year ROI, a smaller system should have a similar ROI, because the system cost mostly scales with the system size. Probably not down to a single panel (although the plug and play systems available in some jurisdictions do work for low scale)