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Sunday, February 13, 2022

2021 On Peak - A Year w/ Tesla Powerwalls

We just completed our first year with Tesla Powerwalls installed on our home. I thought this would be a good opportunity to review the year and see how they performed and how they impacted our energy use and electricity bill.

Some highlights from the year: Powerwall kept the lights on when the grid when out in February, in the summer we were under a heat dome and our AC ran for 5-days straight, we put our Powerwalls in our utility's SmartBattery VPP, our Powerwalls cycled about 94 times during the year (timeshifting our energy use), we saved far more on our electricity bill than with solar alone, we even had a negative bill one month. 

Peak Hours - Time-of-Use (ToU) 

With our electric utility, ToU is optional. If you sign up for it, then during peak you pay 5.3¢ more per kWh than the standard rate and during off-peak you pay 11¢ less per kWh. If you can use more off-peak than peak, this trade-off is a net win.

Below is a nice visual explanation of ToU. This is not from our utility but it's a good illustration of the concept. 

Our solar panels generate energy during mid-peak and on-peak times (reducing our peak load) and our EVs charge at off-peak times. This alone makes ToU worth it. Additionally, the Powerwalls allow us to load-shift by discharging during peak hours further reducing our peak load. Collectively, this should mean that we save money with ToU. How much? We shall see below. 

Solar Production and Energy use

We have 12 kW of solar panels on our roof. We covered every inch that we could. Sadly, we are not at net-zero. The panels only provide about 60% of the energy that we need on an annual basis. So we are definitely still dependent on our grid connection (especially during the winter months). Here's a graph of our 2021 energy production and use: 

Powerwall Usage 

During the year, we ran about 3.8 MWhs through our Powerwalls. This is about 94 full cycles of the batteries. If you look closely at the chart above, you can see a flat spot in late May. This is when the Powerwalls were in "Stormwatch" mode for due to fire danger in our area. Whenever a potential disaster is in your region, the Powerwalls charge up to 100% and go into standby mode in case you need the energy to keep the lights on.

Ironically, as I write this in January of 2022, the system is again in Stormwatch mode for high winds in the area. Stormwatch mode was used off and on during December due to snow and ice storms. It was reassuring to know that if we were hit by a regional blackout, our home would be spared; our frig and furnace would continue to operate.

In 2021, we did experience one blackout and the Powerwalls came through for us. On Valentine's Day, we had a 3-hour outage in our neighborhood. Everyone's lights were out except for ours. The Powerwalls kept our light on and our internet connection even continued to work. When the event ended, we still had an 80% charge. The dryer was running when the outage started and since the cycle was nearly done, we decided to let it complete; otherwise, we'd have had even more charge remaining.

2021 Grid Energy Usage

Ok, so let's finally get to the point of this article: how much grid peak-energy usage did we have in 2021? 

You can see in the chart above that the combination of solar, Powerwalls, and off-peak EV charging has moved the bulk of our billed usage to the preferred off-peak rate. For eight months (Mar - Oct), we had zero net usage at peak hours. I was surprised to see this since our air conditioner ran 24 hours a day for 5-days during the record-breaking heatwave

According to my calculations, if we were on the basic rate plan (paying the same amount for every kWh), we would have paid over one thousand dollars more in electricity bills last year. A couple more calculations show that with ToU, on average, we're paying less than half the typical rate per kWh.

So while we do still use the grid (and pay connection fees, taxes...), Powerwalls help us use it far more efficiently. The bulk of our grid usage (76% actually) is during off-peak times. This is when the grid has surplus capacity and when rates are cheaper. This means that we're helping to better utilize the existing infrastructure and saving money.

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