# Number of solar panels to power a level-2 EV charger

I am working on a project for my intro to engineering class. We are theoretically designing a EV charging station powered by solar panels. I found an example level-2 charger that says it is 9.2 kW at 240 V AC (Lowes), and a solar panel which is 400 W at 43 V (A1 Solar).

How many solar panels would I need to power this charger? Is it basic math 9200W/400W? Or is it more complicated?

• Depends if you want it to work when there's anything other than full sunlight. Commented Mar 27 at 22:01
• Should the charger work 24/7? Then you need a battery buffer that will only be charged during daylight. In conditions other than full sun, you need more panels. A converter is not 100% efficient. So yes, more complicated. Commented Mar 27 at 23:34
• web search for : gaisma yourlocation. eg gaisma san diego gives gaisma.com/en/location/san-diego-california.html. || Find the chart line starting "kWh/m^2/day". That gives the hours a day AVERAGE per month of equivalent full sun. For San Diego its 7+ hours/day in June to about 2.6 hours in December. So your 400W panel will on average produce about 1kWh to 2.8 kWh/day if CLEAN and pointed at the sun always. ie say 500 - 2500 Wh/day. IOf you want a 24/7 rate that's about 20 to 100 Watts out every hour 24/7 at 100% storage efficiency. Go from there. Commented May 2 at 13:48

Here's the output from the 12kWp array on my roof.

It is quite variable: on sunny days it peaks close to rated power at noon, on bad days it produces almost nothing (2/22). Average power can be estimated depending on location and orientation of PV array, there are websites to do that, depending on country.

If you want guaranteed available power at a reasonable cost, you have to use the grid.

You can't connect solar panels (which produce DC and are quite finicky about load requirements) to an EVSE (which requires AC not DC). Besides, the charger in the car wouldn't know what to do with solar panels, since it is not programmed for that.

You can add a PV array and a grid tied inverter, which will handle the PV array, turn DC into AC, and inject power from PV into the grid. If no car is plugged in, this can be monetized depending on local regulation and feed in tariffs. If a car is plugged in, then power injected by the inverter will be used to charge the car. If the car needs more power than PV can produce, the remainder will be pulled from the grid.

So if your charger uses 9kW you could put 10-12kWp of PV, on sunny days around noon it will output enough power to avoid pulling from the grid. In other conditions a fraction of charging power will come from the grid. Of course you can always add more panels to widen the time interval and weather conditions when it will produce enough to charge the car on its own, but then it will produce too much when there's a lot of sun. This is not a technical problem, if the inverter is properly sized it will work fine. But it is an economic problem: you're paying for the panels and installation, but you're not using the power, so the money is wasted. So the installed PV power should be the result of an economic optimization.

Now the signals on the SAE J1772 connector allow the EVSE to control the charging power used by the car. So when the panels do not produce enough to charge the car at full power, by using appropriate signaling you can ajust the car's charging current so it consumes all the PV production but not more.

If you want guaranteed available power at an unreasonable cost, then yes, you need an offgrid installation.

Ideally would want this going 24/7. Say there is battery storage units installed also

Again it is an economic calculation: if you want 9kW guaranteed 24/7 from solar, which means 216 kWh/day, even if you get one week of bad weather, you'll need somewhere around 200kWp PV array (about 200 panels) as a starting point, and enough battery capacity to store this energy for about a week. You will also need to wipe the snow off the panels.

Offgrid installations are neither economical nor environmentally friendly, because the PV array and batteries need to be huge, but most of the power capacity is almost never used. So the energy return on investment is terrible.

• Wow thank you so much for your input. Some parts go a little over my head. This is a freshman intro to engineering group project and wondering how this theoretical charging station is even possible lol. So with the "10-12kWp of PV" do you mean something like this? sunwatts.com/12-3kw-solar-kit-sonali-440-all-black-ge-inverter Then additionally need a 216kWh x7 battery capacity for a bad week. Also need to be hooked to grid if all else fails? Commented Mar 28 at 0:29
• Wp means "watts peak" so for your 400 "Watts" panel it really is 400Wp, which means if you hold it in the perfect orientation in midsummer on a clear day you can get 400W. In all other conditions you get less. You need to price the huge battery and array, you'll see that connecting to the grid is a lot more economical XD. Commented Mar 28 at 0:56

It's more complicated.

You need to allow for losses in the inverter. It will never be 100% efficient at converting the DC from the solar panels into AC. So that will need a few extra panels.

But your biggest problem in trying to calculate the number of panels you need is that the output of a solar panel depends on the insolation - the amount of sunlight hitting it. Even on a sunny day, the sunlight needs to be perpendicular to the panel to get the full rated output. If the sunlight hits the panels at an angle, the output will be less. And the output will be a lot less if it's cloudy. Maybe less than a tenth of the rated power. Maybe only a hundredth in heavy cloud.

• Just trying to find a rough, rough estimate. Ideally would want this going 24/7. Say there is battery storage units installed also. Could 25 panels per level-2 charger be a decent starting point? Commented Mar 27 at 23:48