One Electric Vehicle = A lot of microwave ovens

When is an electric vehicle like a microwave? When people plug them in at home. To get technical, a microwave is typically rated at 1-1.6 kW (110V, 15 A). A charger for a car on a 220V circuit at 30 A is 6.6 kW. A 50 A charger would be rated at 11 kW. If you take just the 6.6 kW case, that would be like turning on 3-4 microwaves in your house, and running them for about four to eight hours.

On my street, which features a lot of 1920’s era Craftsman style homes and bungalows, there is a transformer in my alley. I walked up and down the alley one day and counted the connections, and estimated about 12-15 houses on that transformer. If one of my neighbors bought a Tesla S or X, and charged it consistently at home, the resulting increase in load would be like stuffing a couple more houses into my block. If two neighbors had Teslas, it would be like adding 3-4 houses to the block. Given that all the automakers are releasing EV’s with 200+ mile range, it doesn’t have to be Teslas. It can be Chevrolets, Nissans, Fords, BMWs, or Volkswagens too.

Because it takes nine months to build a house and one day to buy a car, you can see the electricity load growth rate with EV’s is terrifyingly quick. When I drop my kids of at school, there are not one, but two Teslas in the drop off line, as well as a Nissan Leaf and two Chevrolet Volts. I expect there to be more soon. This is a very real and prescient issue.

Here’s another benchmark, using my own small Craftsman house as an example: My present daily demand is about 30 kWh average. I’d probably need to charge a Tesla P100D once per week. That would add 100 kWh/7 to my daily demand = 14.2 kWh. That increases my household demand by 150%. Adoption of EVs presents a transformer overload challenge, but eventually it produces a wires challenge.

A major concern for utilities is predicting the uptake of EVs and making grid upgrades in lockstep to meet increasing demand. Peak loads are more costly to utilities than baseloads. Anticipating and mitigating peak loads will become more and more important as EVs become more common.

You can’t drive on solar

What if your car could generate its own power? The best solar panels today are about 22% efficient. Peak sun is 1000 W per square meter. A typical car footprint is about 8*15 ft = 120 ft^2 = 11m^2. This means if you covered a car with solar you’d never get more than 2.4 kW out of it. A 100 kWh car with 300 miles of range can consume 100 kWh within 5 hours (at 60 mph); the equivalent power of 20 kW The solar on your car would therefore only achieve 10% of your power needs. You’d have to park it in full sun for 41 hours to get a full charge.

What does this mean in practice? Well, obviously a better place to put the solar is on your house. To charge your car only with solar, you’d need something that offsets your daily electricity consumption from driving, provided you are using solar to power your home as well. If you are like the average American and drive 30 miles per day, assuming your EV can achieve 3 miles per kWh, you consume about 10 kWh per day. To generate this electricity with solar, you’d need a 4 kW solar array in peak sun for 2-3 hours. You’d have to oversize for winter, which might mean 2-3 times bigger, depending on where you live. And that’s just for your car, never mind your microwave.

If everyone had solar on their entire roof

There are a number of ways to put solar on roofs, but Tesla has developed roofing materials that mimic shingles that will cover the entire roof, generate electricity, and store any excess in a Powerwall. ECD Ovonics and Owens Corning had similar designs a decade ago, though Tesla has somehow made them physically attractive. For a 625 square foot Tesla roof at $21.58/square foot, it will cost $13,487 for the Tesla roof materials, plus installation costs. Assuming these are 16% efficient, I estimated 14.8 W/ft^2 of solar power could be generated. The 625 ft^2 roof supports 9.2kW nameplate. Since it’s unlikely for the entire roof to be south facing, and you assume an average of three hours of sun per day year-round, this roof will produce 27.6 kWh/d = 201, 480 kWh over a 20 year life. At $0.12/kWh that power is worth $24,174. My present daily household demand is about 30 kWh. If I covered my whole roof with Tesla shingles I’d barely offset my entire existing demand, not including an EV. However, the same conventional roof replacement is $4500. In other words, if you replace your roof with solar, you’d pay it back in about 12 years, but it won’t power your house and your EV.

This isn’t awesome, but it’s not bad. However, an asphalt roof has no payback, other than avoided leaks. So yes, in essence, over the life of the roof, a Tesla roof is certainly cheaper than a conventional roof. This could also be financed (without a power purchase agreement) with storage. Tesla uses a certified installer to give you a new roof and a Powerwall in the basement. If we assume the 9.2 kW array is purchased and installed for an all-in $18,000 cost, which is then financed over 20 years for a mere $100/month, you never have a utility bill or an outage again. Is that worth it to you?

When it comes to the increasing penetration of EVs, it behooves utilities to work with partners working on EVs, charging solutions, and fleet software to smooth out peak load when everyone comes home to charge up their cars. EVs, solar, and storage are all part of an interconnected ecosystem that has tremendous financial and technical ramifications.

Davion Hill is organizing and leading the panel “Transportation: hitting the electric gas pedal” at the 2018 DNV GL Energy Executive Forum on May 17th. Register today to meet Davion at the Forum to continue the discussion.

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