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How Many Solar Panels Do I Need to Charge my EV?

DIY Solar, Solar 101/
How Many Solar Panels Do I Need to Charge my EV?

We are still miles away from being able to produce a practical, self-contained solar powered car but that does not mean you have to give up your dream of driving on sunshine. If you have an electric vehicle, you are halfway there. All you would need to add is some solar panels to your home charging station.

Before we get into system sizing, many people have the idea that the solar electric vehicle charging station must be a stand-alone system, but that is not the best configuration. If the solar panels are only connected to the EV charger and you don’t drive anywhere that day, the solar power is wasted. The way to avoid that is to install a standard grid-tied system or a system with a SolarEdge grid-tied inverter with an integrated EV charger. With either of these options, the solar power produced when you don’t need to charge your car will be used to run other loads in your house or backfeed to the grid so it would never be wasted.

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So how many solar panels do you need to charge your EV? That depends on how much you drive. The average EV uses about .3 kwh per mile. This means if you average 50 miles per day, you would use about 15 kwh per day to keep it charged.

With solar, it always best to talk about annual production because solar panels will always produce more energy in the long summer days than they will during the winter months. So let’s look at the annual usage for the car which is just the daily number multiplied by 365 days. In our example above, 15 kwh multiplied by 365 days is 5,475 kwh. That is the amount your solar would need to generate to off set the electricity used by the car annually.

We have discussed system sizing in previous articles, but to give a short summary, you can start with PV Watts at https://pvwatts.nrel.gov/. Follow the prompts which start with entering your zip code so your production numbers will be location specific.

It will also ask for a system size in kilowatts (kw). A good starting point is to take that kwh number you want to reach and divide it by 1,500. In our example above where you wanted to generate 5,475 kwh it would be 5,475/1,500 = 3.65 kw. Once you see the output on that system size, you can adjust the size up or down to get to the goal.

You will also need to enter the tilt and Azimuth of the system. Tilt is just the angle from horizontal. If the system will be installed on a 5:12 pitch roof that will be a 24 degree tilt angle. Azimuth is the number on the compass that corresponds to the direction the solar panels will be facing. If you have a compass handy, that’s great. If not, you can download a free compass app on your smartphone or you can use the ruler tool on Google Earth (the “heading” is the Azimuth).

Solar Tilt and Azimuth

You also need to watch out for shade. If you have an unshaded roof area it requires no adjustment in the PV Watts calculator. Removing trees in your yard is a meager sacrifice to make for the overall gains you will make in reducing your carbon footprint. Driving a solar powered car trumps the carbon advantage of more than an acre of forest so axe down the trees shading your roof guilt free.

Early morning and late evening shade aren’t a big deal, but if there is a lot of shade in the middle of the day that a chainsaw can’t fix, you may want to have a solar professional do a shade assessment. These can often be done remotely and will be free for a potential customer. The resulting shade percentage can be entered in PV Watts under “System Losses”.

After working with the PV Watts calculator you will know how many watts of solar you will need. Remember in PV Watts the system size is in kw so you will multiply that by 1,000 to get back to watts. Using our example above, if 3.65 kw was the right system size you would do 3.65 x 1000 = 3,650 watts. The last step in the process is figuring out how many solar panels that will be. The factor here is solar panel efficiency.

The standard 60 cell solar panel will be about 5.5’ x 3.5’ and these range from 260 watts to 330 watts. Take the amount of watts you need and divide it by the wattage of the panel and that is the number of panels you need. Continuing with our example it would be 3,650/260 = 14 or 3,650/320 = 11.4. This means you would need 14 of the 260 watt panels or 11 of the 320 watt panels. Keep in mind that the price per watt will be higher for the 320 watt panels so if you have enough roof space to do it with the 260s, you will save money.

If you have even more roof space, you could make the system a little bigger and off-set more than just your vehicle’s usage. When you drive your solar powered car into the garage, you could then turn on your solar powered lights, cook dinner in your solar powered oven and then do some laundry in your solar powered washing machine. After all, when it comes to reducing your carbon footprint why should your car have all the sun?

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