In the first part of this series, we discussed why building a DIY Powerwall is a smart move to offset peak demand costs. In this part, we’ll focus on how to determine the appropriate battery size for your needs using historical usage data from your SRP bills. The Python program below will output these charts and recommendations.

Output

The max you used during any 2-8 PM period is 58.90 kWh on 2021-09-05. Based on historical usage, this battery size would eliminate your electricity usage during peak hours.

The average you used during all 2-8 PM periods is 10.92 kWh. Based on historical usage, a battery of this size would eliminate your peak electricity usage on average.

If you did not have solar, you would need a battery with a size of 87.20 kWh to eliminate the maximum demand during peak hours. This maximum was recorded on 2021-09-05.

If you did not have solar, you would need a battery with a size of 39.04 kWh to eliminate the average demand during peak hours.

If you did not have solar, you would need a battery with a size of 232.80 kWh to power your house for a full day based on the maximum daily consumption. This maximum was recorded on 2021-08-03.

If you did not have solar, you would need a battery with a size of 116.08 kWh to power your house for a full day based on the average daily consumption.

Because you have solar, this reduces your battery size by 32.45% to eliminate all your peak energy.

Because you have solar, this reduces your battery size by 72.02% to eliminate on average all of your peak energy.

Collecting your data

To use this program, you need to download your hourly usage data from SRP:

1. Log in to Your SRP Account:
   • Navigate to the usage section.
   • Select hourly usage and choose your date range.
   • Export the data to Excel for the categories: net energy, generation, and usage. If you do not have solar, you may only have usage data, which is fine.
2. Data Export:
   • Download the data for May through October to focus on summer consumption months, as winter usage without AC is significantly lower.
   • Export data for multiple past years to allow the program to determine maximum and average values accurately.

You should now have some files that look like this.

With your data files ready, you can now run the Python program:

Time to buy the equipment

Now that you know your required battery size, you can begin shopping for components and building your DIY Powerwall. Stay tuned for the next part, where we’ll dive into the construction process!

Recently, I decided to embark on a DIY Powerwall project for my house. The first challenge I faced was determining the appropriate size for the system. Living in Arizona, where APS and SRP are the primary utilities, my goal was to eliminate or shift peak demand. Specifically, I aimed to build a battery that discharges between the peak hours of 2 PM to 8 PM and recharges at night when energy is less expensive.

Why Build a DIY Powerwall?

The primary reason for this project is that demand charges and peak energy costs are incredibly expensive. SRP’s peak demand charges are calculated by breaking down your entire month into 30-minute increments. They then determine the highest 30-minute increment of kW usage for that month and apply it to the chart below to determine your peak demand cost. This cost is a one-time charge applied to your monthly bill but can be significant.

In addition to peak demand costs, you still have to pay per kWh for on-peak and off-peak usage. Here is the breakdown of SRP’s E27 plan:

You can see the prices above are a lot cheaper than SRP’s traditional plan

• Winter 42% cheaper
• Summer 58% cheaper
• Summer peak 56% cheaper

Understanding the Costs

The prices on the E27 plan are significantly lower than SRP’s traditional plan. However, to achieve these savings, you need to shift your electricity usage to off-peak hours. If you don’t offset your 2 PM to 8 PM usage on this plan, you will incur substantial demand charges, which can easily exceed $1,000 in a single month. This highlights the importance of avoiding peak demand charges to benefit from cheaper electricity rates.

The Goal

Our objective is to eliminate electricity usage from 2 PM to 8 PM by discharging a battery and then recharging it at night. The next step is to determine how much energy I have historically used during these hours.

In part two of this article, I will share a Python program I wrote that analyzes all your SRP hourly data to recommend an appropriate battery size.