Dribbling Solar into an EV or How Much Solar Needed to Charge an EV

By Paul Gipe

Recently friends in Spain asked a question about EVs that I hadn’t given much thought. The question was in two parts. First could they “dribble” energy from rooftop solar into an EV and second how many more solar panels would they need?

How Many kWh per Year are Needed

Let’s take the “how many panels would you need for an EV” first. That’s relatively straightforward. Our driving efficiency is typically 4 miles/kWh. (And yes, I’ll get to the metric system shortly.)  Charging on AC (as opposed to DC Fast Charging) is about 85% efficient—that is, you don’t put everything in the traction battery that you pull from the charger or EVSE to be correct.

So, let’s say that on average you get about 3.5 miles per kWh that you draw from the house or about 6 km per kWh.

The question then becomes how many miles do you drive per year where you’re charging at home. We typically drive 10,000 miles per year. The average American drives about 12,000 miles per year. I don’t know what our friends drive, probably much less than an American so I’ll just provide some round numbers in both systems.

10,000 mi/yr @ 3.5 mi/kWh ~ 3,000 kWh/yr

15,000 km/yr @ 6 km/kWh ~ 2,500 kWh/yr

If you drive more or less than this, simply adjust the amount of kWh per year you need proportionally.

How Many Solar Panels Needed

We’re going to assume that the insolation or yield of solar panels in Granada, Spain is similar to that here in Bakersfield, California.

Our yield of installed solar capacity is about 1,500 kWh/kWDC/yr. In Spain, the yield can be less than 1,000 kWh/kWDC/yr, depending upon location. Still, this gives us something to work with.

3,000 kWh/yr ÷ 1,500 kWh/kWDC/yr ~ 2 kW of installed solar photovoltaics.

How many solar panels is that? The answer depends on the rated power of each panel. Today, panels are rated from 300 to nearly 400 WDC. For simplicity, let’s assume 350 W/panel.

2 kWDC ÷ 0.350 kW/panel ~ 6 panels

This is what we have here. We have 12 panels. Half are used to generate electricity for our electric car. The other half is used to offset our annual electricity consumption. (We use much less electricity than the typical American. Our consumption is similar to that of the typical European.)

Can You “Dribble” Solar Energy Directly into an EV?

Technically, no, it doesn’t work that way. An array of solar panels is typically not big enough even at midday sun to charge an EV at the full power of the Electrical Vehicle Supply Equipment (EVSE) or charger used.

Of course, it’s far more complicated than that. Nearly all homes here and in Spain are connected to the grid. While technically you could “dribble” solar-generated electricity into an EV using a Level 1 EVSE in the states, you can’t do that in Europe.

Here, our homes use a mix of 120 V and 240 V circuits. So we can use a Level 1 EVSE to charge at 120 V often at no more than 1.4 kW when plugged into a standard outlet. Most of the EVSEs that come with EVs in the US work at Level 1 voltages and current.

Not so in Spain. Household circuits in Spain are 230 V so an EVSE plugged into a standard outlet in Spain will be feeding ~3 kW into the EV. Wall-mounted EVSEs in Spain can charge up to 22 kW. This is far more than the typical residential solar system in Spain—even during peak sun.

Typically, the EV draws from the home charger what it requires in kW and what the EVSE is capable of providing in kW. This is part of the negotiations that take place every time you plug an EV into an EVSE.

If your EV is capable of using 22 kW in Spain and you’re wall-mounted EVSE is capable of 22 kW that’s what is provided to the car. Often the car has software that can be programmed to draw less from the grid, but most users are not sophisticated enough to use this feature. By the nature of the question that started this essay, I assume our friends would never use this feature—if it existed on the car they buy.

How Then Does it Work to Charge an EV with Solar?

This gets into the messy area of electric-utility regulation. It’s a morass. Our friend is an attorney and that’s what you need to figure this part out.

In California we use a complicated system of “net-metering.” When our solar system generates excess electricity, we feed it into the grid and “bank” it with the utility company. When we consume more electricity than our solar panels provide, we draw what we need from the grid, including the amount that we banked. This simple concept is complicated by when we feed electricity into the grid and when we draw electricity from the grid. When we draw from the grid during peak periods, we pay more than when we draw electricity during off-peak periods. The utility keeps track of all this and bills us at the end of the year after summing all this up. On top of that, we pay a fee for the privilege of doing this.

Spain has a simpler but less favorable system. Electricity banked with the utility is valued at 1/3 to 1/2 the retail rate. As a result, the residential solar market in Spain is largely untapped despite the excellent insolation across much of the country. There’s less residential solar in sunny Spain than cloudy Great Britain.

As with most things in life, “your performance may vary,” and our friends living in Spain should do their legal homework before trying to do the right thing by installing solar to meet their EV charging needs.