In case anybody is still interested, I have done a lot of research in the past few days.

First, the idea of putting up a kilowatt of PVs for $20,000 pesos (about $1700 USD) was a pipe dream. I can do it for a about 89K pesos, $7500 USD. And guess what! With CFE's convoluted and very pricey to the heavy user billing, it makes total sense to do it.

Some of the information I posted earlier is wrong because I misunderstood how the billing works. In much simplified form, it goes like this: up to 150 KWH/month is at the basic rate. The next 100 KWH (up to 250) is at the intermediate rate. Anything above 250 is at the excess rate. If your 12-month average is above the intermediate cutoff, you are then into DAC, in which your ENTIRE bill is at the excess rate, rather than tiered. Because (contrary to what I said before) the billing IS tiered unless you are on DAC, the curve of billing as usage increases is much smoother than I showed previously.

I said before that there was no air conditioning. That is going to change. I can tolerate the heat quite well, but SWMBO is absolutely miserable whenever the temperature inside climbs above 85 degrees. So, a power-hungry 12,000 BTU air conditioner is in the plans. I'm figuring that with A/C there is no conceivable way I can get out of DAC where I will pay on a six-year average about 36 cents for every KWH I consume. Unless I put those PVs on the roof.

So... the PVs will save me more than $17,000 pesos per year on my electric bill, factoring in historically-anticipated rate increases, the increased consumption due to air conditioning, and the loss of income from the money put into the solar array. Another way to look at it it: My electric bill will change from the anticipated $132 USD per month (on DAC with A/C) to $11 USD per month with the PVs. It will take 5.78 years for the electric bill savings to pay for the cost of the system, after that it is pure savings.

I wrote an Excel spreadsheet attached to this post where you can play with the parameters and see how they affect savings and amortization time. Not only is it comprehensive, it's reely purty, too! Highly recommended.

tanstaafl.