Spreadsheet to Calculate Payback (For PG&E in Bay Area but Can Be Modified)

[sms]

This is the spreadsheet I did. It's pretty complex to do all the formulas for the different tiers and for the compounded degradation. I used the PG&E tiers and rates for my area (Silicon Valley). But I tried to do it so it's easy to modify.

See https://docs.google.com/spreadsheets...it?usp=sharing

If there are any glaring errors in formulas please post them, but I think they are all correct. Arguing about the degradation rate, rate increase percentage, and production estimates are expected, but I always used the worst case numbers.

Assumptions:

1. Used worst case degradation per year (based on panel manufacturer's data sheet and guarantees).
2. Used lowest predicted KWH production (there are several production estimate calculators that give differences of up to 20%).
3. Used 2% annual increase estimate from PG&E (2014 increase was 3.8% so I think an average of 2% is pretty conservative).
4. Averaged the different winter and summer tier break-points (but the difference is actually very minor).

The break-even for the 5520 watt system I am considering is about year 7.5. Could be shorter if the production tends toward the higher estimates and if the degradation is better than the maximum. Could be longer if the production is lower than the worst case prediction.

The time when the cost per PG&E Tier 1 KWH is greater than the cost per generated KWH is a little before year 11 so I can definitely see why some people don't want to offset Tier 1 KWH. But remember, the increase in net cost is not linear for each extra panel, so it's less of savings than some people may realize to put up fewer panels, or to use less efficient panels.

[silversaver]

Very nice.

Cell B24 needs to correct B22 * B23

B30, everyone were difference.....

[sms]

Very nice.

Cell B24 needs to correct B22 * B23

B30, everyone were difference.....

I don't know what happened there, it was correct in the Excel spreadsheet. Something got screwed up in the upload to Google Docs.

I moved the tier percentages out so they can be modified more easily. though I don't think any other utility has such a crazy system.

[sensij]

I've added a couple of rows to the spreadsheet to track what the annual and cumulative electricity expenses are per year. I also added another set of data representing what a system of 16 panels of 260 W each, bought for $3.60 / W pre-incentive, would look like. You can see that up through year 13, the cheaper system has the lower total cost. In year 14 and beyond, the Sunpower system will finally result in the lowest cumulative electricity cost (using the OP's preferred 0% discount rate).

There are a lot of details about how this spreadsheet is constructed that I would object to, some more serious than others. Among other things, looking at monthly consumption and generation will give better results than the annual averages used here (and also used in the quick spreadsheet I threw together in the other thread).

Link to revised spreadsheet here.

[sms]

I've added a couple of rows to the spreadsheet to track what the annual and cumulative electricity expenses are per year. I also added another set of data representing what a system of 16 panels of 260 W each, bought for $3.60 / W pre-incentive, would look like. You can see that up through year 13, the cheaper system has the lower total cost. In year 14 and beyond, the Sunpower system will finally result in the lowest cumulative electricity cost (using the OP's preferred 0% discount rate).

There are a lot of details about how this spreadsheet is constructed that I would object to, some more serious than others. Among other things, looking at monthly consumption and generation will give better results than the annual averages used here (and also used in the quick spreadsheet I threw together in the other thread).

Link to revised spreadsheet here.

I think everyone agrees that you pay a higher price per watt for the higher-efficiency, higher build-quality, panels and that if you have enough space for the lower efficiency panels you're better off (at least in terms of initial cost).

But you should also look at the data sheets for each panel and see what the maximum guaranteed degradation is for the different panels. I.e. 0.7% per annum versus 0.25% per annum may not seem like much difference, but compounded over a 20 year period it's a 5.1% versus 15.0% decrease in output and each year you're paying more per KWH for the electricity that your panels are no longer generating. The worst thing is those lease agreements where your lease price goes up 3% a year while your solar production is decreasing 0.7% per year and the cost of what you're not offsetting is going up 2% or more per year and at the end of the lease you don't even own the system. You probably won't have a lease salesperson explaining that when they do their pitch.

Also pay attention to the accuracy of panel ratings. It's all in how the manufacturers bin the cells after they are fabricated. I.e. is it "Analysed data demonstrates that 99.7% of panels produced have current and voltage tolerance of ±3% from nominal values" or "Power Tolerance +5/–0%." In the latter they are not using any cells that will result in a power rating lower than specified. This is how honest companies rate semiconductors. You wouldn't want to buy an Intel CPU with a speed rating that was +/- 3%, you want +x%/-0%.

Net exports and usage are credited/charged at the customer’s applicable electric rate schedule every month. But since solar production and electricity use track pretty well (in homes that use natural gas for heating) the six month averages I used are going to be very close to the same as if I did each month individually. In the winter you generate less electricity but you're not running the A/C and if you have a pool you're running the pump for shorter times. If you have electric heat, which is extremely rare in California then it's a different story.