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Sunday, October 2, 2022

Smart Thermostat vs Smart Battery

Our local electric utility has two programs with similar intentions to reduce grid load during times of high demand. We are participating in both and, sometimes, the two don't really work well together. 

Program 1: Energy Rush Hour

Our utility, Portland General, offers a program they call "Energy Rush Hour." This program attempts to alleviate the strain that air conditioning places on the grid during hot summer days. 

When people arrive home from work, they typically turn on the AC and with everyone turning on their AC around the same time, the grid load is very high during these hours (4 to 7 PM). Energy Rush Hour reduces this strain by pre-cooling participating homes. Pre-cooling typically starts at 3PM. This means that by 4PM when other homes are turning on their AC, the pre-cooled homes can shut off their AC. This spreads out the load. 

If the load wasn't spread out, then the utility might need to use peaker plants to supply the excessive short term energy need. Spreading out the load allows the existing generation infrastructure to fulfill the cooling power demand. Avoiding peaker use is good for the customers, the utility, and the environment. Peaker plants are among the most polluting energy sources and the most expensive source to operate. The cost to run these peakers is certainly passed on to customers in one form or another.

Any month that you participate in an Energy Rush Hour, you receive a $25 incentive bonus on your electricity bill. This more than makes up for an extra electricity use that you may have for having the AC kick on an hour or two early. 

So far, so good; let's look at the second program. 

Program 2: Home Battery w/ Time-of-Use

By default, residential PGE customers are on a flat rate fee structure. With a flat rate, it does not matter what time of day you use electricity; it's the same cost at 3PM as it is at 3AM. This is convenient since you can run your appliances at anytime of day or night and not get hit with higher fees. However, this default flat rate schedule is blind to the ebb and flow of grid demands.  

If you want people to understand something, put a price on it. This is what a Time-of-Use (TOU) rate schedule does. When there's typically surplus supply on the grid (such as overnight), you make the price cheaper to encourage use. And when demand is expected to be high (when hordes of people come home from work and turn on their AC), you make prices higher. PGE's TOU schedule has three rates: peak, mid-peak, and off-peak.

TOU nudges people to push loads into non-peak times. Much like the Energy Rush Hour program, this reduces the demand during peak hours and can avoid or reduce the need for peaker generation. 

We signed up for the TOU program because the off-peak rate is less than half of the flat rate fee. Our solar panels are able to offset most of our usage that's not off-peak and with the home batteries, we can time-shift our solar to make sure that we are effectively off grid during peak times. So we're reducing grid strain and saving on our electricity bill.

Where it Goes Wrong - The Interaction - 

Now you understand the two programs. On their own they work great, but mixing the two doesn't always provide the desired result. Since we're using our home battery to effectively go off-grid during peak hours (3PM till 8PM), tweaking our AC settings to reduce energy use from 4PM till 7PM has absolutely no impact on the grid. In fact the pre-cooling could result in over-all more energy use. This is fine when the power is coming from the grid. Using a little more during mid-peak to reduce the peak usage is certainly worth it. However, when our home battery is the energy source, optimal operation is more important than when it's used.

This is a subtle, second-order effect and I'm sure there are very few people in the PGE service region with batteries, on TOU, and enrolled in the Energy Rush Hour; so I'm not suggesting any immediate changes to either of the programs, but as more homes install battery systems, it will be something for the folks at PGE to consider. Perhaps the next generation Energy Rush Hour program will be aware of customers that have home batteries and have slightly different behaviors when homes that are battery-powered rather than grid-powered. If both the battery and the AC were managed under the same program, the coordination of the two could work much better. 


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