How to charge your electric car from the sun, at home

With the big upturn in electric-car adoption driven by the Tesla Model 3, incoming Kia and Hyundai options, and the highly competitive Chevrolet Bolt, many drivers will be moving to vehicles that will be primarily charged at home.

This means that their household electric bills are likely to show a significant increase—a big and obvious increase. Many of these new electric vehicle (EV) adopters, however, have the option of embracing solar (or sometimes even wind) for their household electricity.

Back in 2009, as I was putting in my first order for an EV, a Nissan Leaf, I already had a decent-sized solar photovoltaic system on my home—one that just balanced the annual costs for the house electricity demands. Doing a little extra estimating, I wanted to keep my electricity payments essentially zero annually. Even back then, there had been both local and regional horrific events that suggested to me how negligent my utility seemed to be in terms of system upkeep.

George Parrott's 2018 Tesla Model 3 with orange wrap [CREDIT: George Parrott]

Anyway, I had put my order in for the incoming Leaf and Chevrolet Volt to move our two-car household from hybrids to plug-in vehicles and I added a second solar panel system to that house’s rooftop with seven  220 watt panels. I had to add a full second system since my first inverter was maxed out and could not handle the addition of more panels.

Go solar!

My encouragement is to  go solar as soon as possible. There is now a 30-percent tax credit for your solar install, but given the current political climate, support for environmental and green energy action may not be continued.

The first "lesson" for solar planning is what was alluded to above: 

Make sure your inverter can handle at least six to seven more panels than you are installing. Even if you aren't expecting that first (or additional) electric vehicle for a few years, this will save you considerable money.

George Parrot's home solar installation [Credit: George Parrot]

On the other hand, micro-inverters will cost more up front and might be slightly less reliable. Overall solar costs for 2019 compared to  my first install in 2008 are about 50 percent less or perhaps even better value with a bigger Federal Tax Credit now too. Amortization time for a PG&E customer to recoup installation costs now are about 6 years. For more-affordable-utility customers that recovery time stretches to around 10.5 years.

George Parrot's Tesla Wall Charger in his garage with his Tesla Model 3 [Credit: George Parrot]

Getting your home charger:

My Tesla Wall Connector, on a 100-amp dedicated circuit, can charge at 50-plus miles per hour of charge, but virtually none of us need that rate for overnight charging, and the big negative with the Tesla charger is that none of my friends with electric cars from other makers can use my charger when they visit for a boost charge. Every Tesla comes with an adapter for the generic Level 2 charger, so having a more “generic” home charger allows you to charge any vehicle, and even those of us current “Tesla fans” might want to consider another option sometime in the future.

George Parrott's 2018 Tesla Model 3 with orange wrap [CREDIT: George Parrott]

Sizing Your Solar Panel System:

How much extra solar power output will you need to cover totally the power cost for driving an electric car?

As a typical example of power draw, let’s use 300 watt-hours per mile for your new EV. My last Tesla Model S P85D averaged 297 wh per mile for 59,000 miles and my current Model 3 is averaging 278 wh per mile. Lighter Model 3 variants without the “Performance” configuration are averaging under 250 wh per mile as are most of the EVs from competing brands.

At 300 wh per mile your car goes 3.3 miles per kilowatt-hour which is the unit that utility companies use for their billing. At that, if you drive 15,000 miles per year, you will need 4,545 kwh to cover that driving total. So how do you get 4,545 kwh into your annual solar panel output?

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Your home system will operate 365 days a year and an average of about 7 full hours per day (in sunny climates), so using those values, we can compute our per-hour output needs by dividing 4,545 kwh by 365 days and then by 7 hours to get needed output per hour. This equals 1.77 kw per hour required to totally cover the annual electric charging needs for your new EV.

Today’s solar panels vary in output from around 250 watts per panel to 340 watts per panel and the higher the panel output the higher each panel costs. Staying away from “top of the line” panel options, let’s focus on a mid-range panel of 300 watts. How many of these panels do we then need to give you your required 1.77 kw? Divide 1,770 watts by 300 and the outcome is 5.9 panels, 

So you would need to add six panels (or 7 if you wanted to be a bit generous) to cover charging a single EV driving 15,000 miles per year using only home charging and be able to do all that driving without any actual “utility expense.” 

READ THIS: Here's Why Electric Cars Will Succeed: Owners Just Adore Them (2014)

There are some caveats: You will still have a token monthly fee to your electricity provider for your grid connection service. This will typically be around $12-15 per month. 

Contact your utility company

As soon as you get the vehicle identification number (VIN) for your incoming electric car, call your electric utility company or check their customer website to determine if they offer special "time of use" rates for electric vehicle owners, which typically run from around 11 p.m. to 7 a.m. and can sometimes discount 50 percent from typical household rates.  

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Getting this special discount rate usually involves submitting a form to your utility company which confirms ownership of an EV and is separate from any incentive for EV ownership that many electric utility companies actually provide in the form of a direct cash rebate. (Check for that too. Sacramento area utilities near me offer $500-$599 direct cash rebates on top of state and federal incentives).

My significant savings are based on my home in relatively temperate California. I used to face electricity costs in the summer using extensive air conditioning of over $300 a month and in winter gas costs that approached $300 a month. Now using electric space heaters for 95 percent of my winter heating and my large solar array covering all that cost, my annual gas charges are less than $200 and my electricity usage charges—zero, even charging my Model 3 totally in my home garage. I have gone from an annual utility cost (gas plus electric) of over $3,500 to less than $350 per year.