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Electric Vehicles and Solar - Why Filling up your Car with Solar Makes Sense in Silicon Valley

You drive an EV because it’s good for the environment. Or you just hate to spend $100 to fill up your gas tank (I paid $4.50/gallon last week). Or it’s cool and you’re one of those “early adapter” geeks (OK, I resemble that remark).

Regardless of why, now you have to “fill up” from your local utility. Just as you usually drive around looking for the cheapest gas price, you will probably browse around and figure out the cheapest way to recharge your car. This means you need to decipher the various electric rates offered by your local utility. Arrghh. Unfortunately, these rates are fiendishly complicated and very influenced by your electricity usage profile. At Cinnamon Solar, we’ve spent a lot of time (and programming) with our customers to help them simulate their EV costs under various PG&E rate schedules and solar system sizes. Here are some simple electric rate facts and EV charging guidelines that we’ve learned.

First, some facts:

  • You currently have three rate options, the standard E-1 residential rate, the time-of-use E-6 rate, and the special EV charging rate E-9.

  • Depending on how much you drive, your EV will use between 150 kwh and 300 kwh/month. This extra consumption will generally kick you into the higher rate tiers.

  • Top rate tiers are $0.35/kwh for E-1 (day and night), $0.51 day and $0.32 night for E-6, and $0.56 day and $.021 night for E-9.

  • If your EV uses 200 kwh/month and you only charge at night, your “fill up” costs will be $70/month on E-1, $64 on E-6 and $42 on E-9.

Sounds good, so far. But you have a life. That means if you change to the E-6 or E-9 rate your daytime electricity consumption could be billed at the punitively high $0.51/kwh or $0.56/kwh rates. Fortunately, rooftop solar generates electricity during the peak daytime hours – exactly when your rates will be highest. Because solar panel prices have come down so much, electricity generated by a rooftop solar power system ranges from about $0.07/kwh to $0.10/kwh.

Here are some guidelines:

  • The E-9 rate is generally better than the E-1 rate for an EV because you can charge at night at $0.21/kwh — EXCEPT when you use a lot of electricity during the day (air conditioning and swimming pools contribute to high daytime usage).

  • Ten rooftop solar panels will produce all the electricity you need for a typical EV (but YMMV).

  • The E-6 rate does not make sense for an EV.

It’s a good idea to do a rate simulation before you decide to switch from the E-1 rate to the E-9 rate. One of the things you will find is that if you have high daytime consumption, solar makes a lot of sense (4-8 year paybacks). If you get an EV, solar becomes almost a no-brainer.


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