Cost analysis of Solar PPA

[llamafilm]

About 9 months ago I bought a house with Tesla solar panels and I inherited the PPA agreement. I've been tracking my costs in a spreadsheet to figure out how much money (if any) I'm saving thanks to the solar. So far I'm very happy with it, I'm saving an average of 63% on my monthly bills. I don't have a battery, so my excess generation is sold back to the grid (SCE) at TOU-D rates.

I'm only looking at monthly statements from each companies, not the hourly data, so I had to make a few assumptions here:

• Tesla billing periods end on the last calendar day of each month, while SCE ends somewhere between the 4th to the 7th (it varies). I don't want to spend the effort collecting daily usage data from SCE, so I just line them up as close as possible and it will average out over time.
• I'm using the published TOU-D rates but these are not precise. The real rates are the sum of several line items including some NBCs and other taxes which aren't directly tied to actual consumption, and the rates also increase slightly every month. That's why there's a difference between columns AA and AB.
• For calculating hypothetical non-solar TOU-D cost, I assume all the solar generated power happened during the mid-peak rate period. This is of course not true, but hopefully it's a reasonable average.
• Without solar, the tiered Schedule D rate would be cheaper than TOU-D so that's what I compare against.

I started trying to calculate how much I would save by adding a Powerwall. This is a very rough estimate because I'm just looking at monthly data, and this also assumes 100% battery efficiency, but it looks like I could save around $90/month. I don't think that justifies the cost of the Powerwalls.

If anyone is curious in the data, here it is. I welcome any feedback, especially if you find any errors.

 

[Ampster]

I welcome any feedback

It may not pencil out today but looking at the trends, have you considered the value of a hedge against erosion of NEM benefits?

I presume you are in California? How much longer does your NEM agreement go for? What is the impact of the proposed Successor Tariff on your planning assumptions? How long before you will be forced off tiered rate onto a TOU rate? What is your exposure to power outages?? Your PPA costs will also escalate per that agreement?

 

[llamafilm]

Yes I’m in California. I wasn’t aware of the NEM changes, thank you for pointing that out! It seems like that would drastically change my bill.
I’m reading estimates of $.05/kWh exported. I pay Tesla $0.17 which increases 3% annually for 20 years. So without some kind of additional equipment, it seems like it would actually be cheaper to have some panels removed to ensure I never export to the grid.

 

[Ampster]

So without some kind of additional equipment, it seems like it would actually be cheaper to have some panels removed to ensure I never export to the grid.

The PPA may limit your ability to modify the system.

Depending on when the PTO was granted to the original owner you may have 15 years from that date. A more careful read of the proposal may also be required to determine how it applies to a PPA when you do not own the system. The Proposal does incentive storage storage so that may be your only avenue to improve your situation.IThe Proposal is due to be approved or modified by late January and it is hard to predict if and how it will be modified. I do not know of any provisions on the one PPA I have read that prohibits adding storage. Yours may not be the same. These are the issues that I can think of but there may be more.

 

[llamafilm]

In my agreement I am allowed to add a Powerwall but I’m not sure if other brands are allowed. I do have the option to have the system removed at any time at no cost.

 

[llamafilm]

I do have the option to have the system removed at any time at no cost.

Oops, I misread the contract, this is actually not true.

 

[Ampster]

Oops, I misread the contract, this is actually not true.

Thanks for clarifying. My condolences. My daughter assumed a Solar City Lease when she and her husband bought their first home. It has turned out to be onerous but they love the home. That one had a provision that the seller could move the system to another home but they did not want to force the seller to do that.