Powerwall Sizing: How Do I Determine Peak Usage Based on PGE Data

[Janus]

I'm looking to get some rough numbers worked up for a solar+Powerwall install. My goal is to ensure the system can cover 100% of my usage, and provides decent coverage during an extended grid outage.

I can easy see what my monthly kWh usage is from my PGE bill. From there, I can ensure the solar system generates a total yearly kWh that is slightly more than the aggregate of the last 12 months.

But my question is related to the Powerwall and how many I'd need. Each PW can only output 5 kW continuously, and 7 kW peak. So in order to backup things like a HPWC, dryer, AC, etc, I'd need several PWs. In theory, I could just add up my breaker box, and call it done. But I was wondering if there's a way to glean from PGE what my all-time max kW was? Is there some hidden corner of the bill/website that can give me that data?

Obviously, during a grid outage, I'd minimize the use of dryers, HPWCs, etc. But I'm worried that at the moment of the outage, if I have several heavy loads going, I might overload the PW system.

 

[SoundDaTrumpet]

For non-motor loads, usually just walking outside and looking at your meter is all you need to do. Do 2 PWs minimum and ideally 3 or 4.

The LRA (locked rotor amperage) is way too high for me, AND I do not have a scroll-type compressor to add a soft-start. The A/C are not backed up during outage, but my 2PWs can easily offset both 4ton & 3ton units. IMO, backing up A/C is the most challenging on average.

The HPWC uses 11kW when in use. HPWC is not backed and not monitored by the gateway CTs; therefore, not offset by the PWs.

 

[dandrewk]

There will be a limit as to how much power you are allowed to generate, based on your current usage. The installer would know the maximum, as well as how many panels you will need to satisfy your requirement.

The number of PWs you need depends entirely on how much you might need if the power goes out at night. You can set a minimum charge state (ours is at 20%) to keep a safe reserve. Then it's a matter of deciding how many PWs you need.

Keep in mind, it's a lot cheaper to install as many units as you need now, rather than going back later. There is also a sizable federal tax credit, which I believe is set to expire by the end of the year. We have a couple of EVs, so we ended up getting three PWs.

 

[miimura]

There are two quantities that you want to know when planning for a Powerwall.

1. What is my peak draw in kW.
2. How much is my consumption during the Peak rate period in kWh.

With the exception of motor startup power mentioned above, these values can be extracted from your smart meter data, downloadable from PG&E. I created an Excel spreadsheet that "bins" the data according to various rate schedules. Download your data in CSV and paste it into the spreadsheet and the totals for each period are found. Finding the Max for a given sorted column is easy.

PGE Electric Rate Calculator v1.9b.xlsx

Just bear in mind that this is about a year and a half old, so the actual rates calculated are not up to date. PG&E changes the rates 4-9 times per year.

 

[NuShrike]

Aim for 200%+ oversizing of your current demands. You'll only regret undersizing like range anxiety.

 

[colorbynumber]

Similar to miimura, I created a spreadsheet to estimate:
1. What is my peak draw (kWh)
2. How much energy I need during off peak (3-9pm)
3. How much energy I need after sun sets

Based on this, I estimated I need 2x PW due max usage (after sun down) of 24 kWh during summer months. or max 14kWh peak usage.

To download data from PGE site, go to "Energy Usage Details" and look for the "Green Button"