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Wednesday, December 20, 2017

How Many Solar Panels To Power A Tesla Semi Truck

I've seen a couple attempts to figure out how solar energy would be needed to "fuel" a Tesla Semi Truck. The analyses that I've seen have several flaws. They assume things like the panels must provide power at the same level as the charging system or that charging at night would cause problems. I'll show that neither of these are problems.

The primary failure of these analyses is that they confuse power and energy. I even conflated them in the title of this article (depending on how you read it). If you want to understand the difference between power and energy, you can read about it here. When you are looking at how far the semi can go, what matters for our purposes is energy.

The Tesla Megacharger stations will have onsite batteries and they will be grid-tied. This means that energy use can be time-shifted from the energy generation and the system could use net metering as well as the batteries as a form of energy storage.

If the energy can be generated and stored in onsite batteries, then the power levels that are needed can be supplied without taxing the grid. The level of power that is drawn from the grid does matter for demand charges and we'll cover methods to mitigate this in our next article. This just leaves us with one question:

Can solar panels generate enough energy to significantly supply a Tesla Semi?

To be clear, I don't mean solar panels on the semi truck itself. We're talking about stationary rooftop or carport solar panels. Will it take 4000 homes worth of rooftop solar installations or just a handful of rooftops PV panels worth?

Several years ago, we wrote an article that detailed how you could calculate how big of a solar array you'd need to supply your own EV. Now, we'll use the same tools and apply them to the Tesla Semi.

This is a two-step process: One, determine the energy need. Two, size the solar array to meet that need.

One: Determining the Semi Energy Needs

According to Tesla, their Semi uses less than 2 kWh per mile. To cover the worst-case, we'll use 2 kWh in our calculations. According to the Federal Highway Administration, a typical semi drives about 45,000 miles each year on average. Long-distance trucks travel upwards of 100,000 miles a year.

Using these datapoints, we can determine that a Tesla Semi driving a typical 45,000 miles annually will use about 90,000 kWh each year. A road warrior Semi would need 200,000 kWh. For context, in 2016, the average annual electricity consumption of a U.S. residential customer was 10,766 kWh. So a typical Tesla Semi will use the energy of about 8.4 typical U.S. homes.

Two: Determining Solar Array Size

Now that we know the typical and extreme energy use, we can size a solar PV system to meet this need. How much energy is produced by solar panels depends on where they are installed. Not surprisingly, sunny locations produce more energy. Since Tesla Megachargers will be installed all around the world. To get some idea of the impact we'll look at sunny San Diego and rainy Portland, Oregon.

Using the calculator at the National Renewable Energy Lab, you can determine that in sunny San Diego, to generate 90,000 kWh, you would need a 53 kW system. We have a 12kW system on our house. Assuming a similar rooftop size, 5 San Diego home rooftops could drive a Tesla Semi more than 45,000 miles.

Looking at rainy Portland, running the numbers, it would take a 78kW (6.5 rooftops) PV system. That is big, but not undoable. You can use the link above and plug in your own town and see for yourself how big of a PV system would be needed where you live.

Solar Carport at Intel Jones Farm in Hillsboro, Oregon
Above is a picture of a 400kW SolarCity carport. An installation like this in San Diego would generate 664,000 kWh/year. This would fuel a Tesla Semi for ~332,000 miles. That means each of these carports that covers 40 cars could fuel one of the 100,000-mile road warriors with miles to spare.

In Portland, a carport installation like this would generate 469,000 kWh/year, enough for 235,000 Tesla Semi miles.


Solar carports and rooftops can provide enough energy for hundreds of thousands of miles.

Now, if you hear someone say. "It would take football fields worth of solar panels to power a Tesla Semi", you know the truth of it. You now know that a 40 car solar carport could supply enough energy for 100,000 miles. That means coving a 400-car parking lot at the mega-mall or outlet stores with solar carports would deliver enough energy for over 1 million Tesla Semi truck miles.

If you hear someone say, "It would take the power of 4000 homes," you know that it is energy that matters and that it only takes 5 or 6 houses worth of solar to supply a typical Tesla Semi's annual needs.