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Saturday, May 22, 2021

Better Together: Solar, EVs, Powerwalls, & Time-of-Use: Positive Feedback

It Started with an Electric Vehicle (EV)

We lucked upon a 1998 Chevy S10e EV after its fleet life at Disneyland had concluded. Driving an EV was fun and filling batteries with electricity was far cheaper than filling a gas tank. Annually our electricity bill was about $300, but we were saving about $2400 per year that we were previously spending on gas.

Charging the EV with Solar Photovoltaic (PV)

Then we wanted to make sure that the EV was powered by renewables, so we installed solar photovoltaic (PV) panels. This reduced our monthly electricity bill.

Time-of-Use Saved Us Even More

Now with PV panels reducing our daytime grid uses, we switched from the tiered flat-rate energy plan to a time-of-use (TOU) billing plan. The peak rates were higher than our old flat-rate plan, but the cheaper off-peak rates more than made up for it since we were now charging up the EV overnight. This further reduced our electricity bill.

More EVs Meant More PV

We had the bug. EVs were so fun. They were clearly the future of personal transportation and we wanted to be part of it. The Chevy was traded in for a Nissan Leaf and our Honda Passport was traded in for a Model X. We were now a two EV household. With another EV, we needed more solar panels. This time, not just to offset the new EV but also to reduce our grid use even further. 

Powerwalls Peak Shaver - Super Saver

Tesla Powerwalls were the last thing we added to this mix. The Powerwalls allow us to time-shift our solar usage. On TOU, the peak rate is about 3 times more than the off-peak rate. The peak times happen just before and just after typical solar production hours, so the solar alone was not helping us to avoid paying these high rates.

Our utility has a split peak during winter months, meaning there are morning peak hours (6AM - 10AM) and evening peak hours (5PM - 8PM). The solar panels are not producing significantly until 7AM and not much, if any, past 7PM. That meant that we were paying full peak rate for at least 2 hours each day, and anytime it was a cloudy day, you could add several additional hours.

However, Powerwalls changed all of that. Our grid energy usage during peak hours is now *negative*. 

EV, PV, TOU, and Powerwalls Working Better Together

The EVs charge overnight at the cheapest TOU rate. The Powerwalls discharge during the peak rate hours. Any solar power generated during peak hours is sent to the grid, running our meter backward during the most expensive electricity rate times. The mid-day sun recharges the battery. This means that we'll use some mid-day grid energy, but the mid-day TOU rate happens to be the same as our old flat-rate fee schedule. 

The graph above shows that electricity rates increase, our usage goes down. Other than a couple of hours mid-day while the batteries recharge, on even moderately sunny days almost all of our energy usage is now during off-peak hours. Here's an actual example, from March: 

There you have it. Despite having the significant electric demand of 2 EVs charging daily, air conditioning during the summer, and all the other normal household demands, the bulk of our electric bill (if not all) is for the cheapest rate. During off-peak hours (around here) the grid typically has surplus wind energy available so by shifting the bulk of our demand to hours that the grid has a surplus, we're helping to stabilize the grid while reducing our electricity bill.

If you want solar with Powerwalls, use our referral
Disclosure: I'm long TSLA


  1. Nice. Solar came first for us, we ended up with 3 EVs, and I haven't jumped on the powerwall yet.
    I thought that you can't use powerwalls for arbitration, though, if you're also using solar with feed-in tariff - did that change, or am I remembering that wrong?

    1. Thanks for the comment! The only restriction (at least here), that I know of, is that you can not charge the Powerwalls from the grid. Other than that, you can do all the arbitrage / load-shifting that you want to.

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