Please check my solar/powerwall calculations

[schan1w]

Hi

I'm looking into if solar/powerwall makes sense and having a hard determining if my assumptions are correct.

I'm in the bay area under the new EV-A rates where peak is 4pm-9pm and off peak is during most of solar operating hours.

Due to this limitation, it appears I can only earn mostly $0.16 per kWh I send back to PG&E from morning till 4pm.

I use about 25 kWh per day on average over 12 months, 50% of which is at peak rates ($0.75 per kWh). Hence non-peak cost per year are about $1K and my peak costs are $2.5K per year. If I install a medium system $15K to handle the 12.5 kWH of non-peak/part peak usage, I would recoup the cost in about 13 years. If I install the same medium system and add powerwalls to store the energy for peak times, I'd recoup the $30K cost (15K solar/15K PW), it would take about 11 years.

Is my math correct? I think PG&E screwed new solar owners with the non-peak hours during most of the solar collection times.

Thank you.

 

[Ampster]

The peak PG&E rate is only for about 4 months of summer. Even then, are you sure it is $0.75 per kWh? .My summer EV A rate with PG&E is around $0.55 per kWh.

 

[schan1w]

You're

The peak PG&E rate is only for about 4 months of summer. Even then, are you sure it is $0.75 per kWh? .My summer EV A rate with PG&E is around $0.55 per kWh.

You're right, I added the generation and distribution charges. I should just look at the distribution charge since the generation credit offsets the generation charge for the most part. It is about $0.52 per kWh in the summer and $0.35 per kWh in the winter.

 

[Ampster]

You must be on a CCA. My bill is the same. Too bad PG&E can't make it easier to understand.

 

[miimura]

Hi

I'm looking into if solar/powerwall makes sense and having a hard determining if my assumptions are correct.

I'm in the bay area under the new EV-A rates where peak is 4pm-9pm and off peak is during most of solar operating hours.

Due to this limitation, it appears I can only earn mostly $0.16 per kWh I send back to PG&E from morning till 4pm.

I use about 25 kWh per day on average over 12 months, 50% of which is at peak rates ($0.75 per kWh). Hence non-peak cost per year are about $1K and my peak costs are $2.5K per year. If I install a medium system $15K to handle the 12.5 kWH of non-peak/part peak usage, I would recoup the cost in about 13 years. If I install the same medium system and add powerwalls to store the energy for peak times, I'd recoup the $30K cost (15K solar/15K PW), it would take about 11 years.

Is my math correct? I think PG&E screwed new solar owners with the non-peak hours during most of the solar collection times.

Thank you.

You must be talking about the new EV2-A rates since you mention the Off-Peak during the generating hours.
Here are the prices as of January 1, 2020. These are the total rates. If you are on CCA, there is a different breakdown, but the total is about the same.

The part where they really screwed us is that EV-A has Part-Peak from 7am-2pm while EV2-A only has Part Peak from 3-4pm before the Peak hours.
Summer is only 4 months June 1 - September 30. The older rates were 6 months long from May 1 - October 31.

 

[wwhitney]

The interconnect agreement would allow the use of energy storage for solar generation time shifting. With a sufficiently flexible energy storage system, you could charge the batteries during Off-Peak from solar and then export power during Peak from the batteries in excess of your actual usage.

Unfortunately Powerwalls do not currently support this. So you'd either need to convince Tesla to implement this feature, or implement your own workaround.

Cheers, Wayne

 

[Merrill]

You must be on a CCA. My bill is the same. Too bad PG&E can't make it easier to understand.

Why would they want you to understand, make it complicated and then penalize you for having solar.

 

[getakey]

I'm on EV-A
Peak 2pm to 9pm
Off Peak 11pm to 7am
Part peak 7am to 2pm and 9pm to 11pm

a lot of solar production is during Peak in the summer.
A good solar company can take your bill and run it through their software to show you savings. I've tracked my actual against their prediction model and it is almost always within 5%

 

[schan1w]

My ROI is as follows: my peak usage is about 10 kWH per day. @ ~$0.40 per kHW, that's about $4 per day X 365 = ~ $1460 per year.

I can get a small solar system + 1 PW for about $15K. So about 10 year to recoup the cost. The assumption is that all the solar generated is stored in the PW and then entirely consumed during peak hours 4pm-9pm. I assume my peak usage is < 3 kWh instantaneous, in order to stay within the output capability of a single PW for 5 hours.

With EV2, the only ROI that makes sense is to use solar during peak hours. And because peak is 4-9pm, this means you must store the solar and use it later (e.g. you need a PW). Off-peak (midnight - 3pm) is way too cheap to be using solar. There is no ROI on solar during off-peak.

Please let me know if I missed anything or if I don't understand something as I just started researching to see if solar/PW make sense fo rme. Thank you!

 

[schan1w]

The interconnect agreement would allow the use of energy storage for solar generation time shifting. With a sufficiently flexible energy storage system, you could charge the batteries during Off-Peak from solar and then export power during Peak from the batteries in excess of your actual usage.

Unfortunately Powerwalls do not currently support this. So you'd either need to convince Tesla to implement this feature, or implement your own workaround.

Cheers, Wayne

I would not expect to export as I would size it such that the system approximately matches my peak usage.

 

[Dan123]

Hi

I'm looking into if solar/powerwall makes sense and having a hard determining if my assumptions are correct.

I'm in the bay area under the new EV-A rates where peak is 4pm-9pm and off peak is during most of solar operating hours.

Due to this limitation, it appears I can only earn mostly $0.16 per kWh I send back to PG&E from morning till 4pm.

I use about 25 kWh per day on average over 12 months, 50% of which is at peak rates ($0.75 per kWh). Hence non-peak cost per year are about $1K and my peak costs are $2.5K per year. If I install a medium system $15K to handle the 12.5 kWH of non-peak/part peak usage, I would recoup the cost in about 13 years. If I install the same medium system and add powerwalls to store the energy for peak times, I'd recoup the $30K cost (15K solar/15K PW), it would take about 11 years.

Is my math correct? I think PG&E screwed new solar owners with the non-peak hours during most of the solar collection times.

Thank you.

There are some easy things you can do to improve the economics a little, even without a powerwall.

1. Panels facing west will generate more power during the afternoon, including from 4-6pm. you can go to PVWatts Calculator and do an hourly calculation to see the difference.

2. What's using 12.5kWh of energy during 4-9 time? Is there a way to shift it before 4 and after 9?

3. Why is your PW - $15k. Is this before the tax credit? I think it should be significantly cheaper, after tax credit. If all you are doing is load shifting, there are other battery products available, that may cost less. For example, a 3.2 kWh battery will make a big dent in your on-peak usage.

PHI 3.2™ kWh 160 Amp High Output Deep-Cycle Lithium Ferro Phosphate (LFP) Battery