# How many solar panels would one need to carry to fully charge Tesla model S or S in 20 minutes?

Some would say carrying solar panels to charge a Tesla would be inefficient, and would take up all the space, however towing a trailer would. Solar panels plus inverter are also not that heavy.

Would carrying a folding device that stacks solar panels in a 3 meter by 2 meter 2 wheel trailer be efficient, especially in a country that does not have fast charging yet, but 14 hours of sunlight in summer and 11 in winter? It is possible to get the fast charger that charges batteries to 80% or more in 20 minutes.

Finally, assuming 400 Watt panels (which I assume are the largest would one have to carry), how much space would it take when charging? Of course one would find a very large open space on the side of the highway, where the device unfolds and charges.

• We don't do your calculations here but we will help you do them. I'll start: The information still lacking is how much energy (Power in Watt for 20 minutes or in 11 hours) is needed to charge the batteries? Then relate that number to the number of solar panels and there's your answer. Note that that will be best case, 400 W solar panels will only deliver 400 W when correctly angled in full sunlight. – Bimpelrekkie Feb 11 at 13:14
• It looks like your equinox is not 12 hours. Now you must live in a real interesting place with a fairly uneven orbit around the sun. – Andy aka Feb 11 at 13:17
• Im not an expert on equinox but sun rises at 05:30 during summer and sets around 19:30. In Winter it rise around 06:30 and sets aound 17:30 – securitydude5 Feb 11 at 13:24

## 2 Answers

Using the sun as energy source is actually one of the few reasonable ways to overcome the issues we are globally facing. The problem is the efficiency of conversion from sunlight to electricity (the conversion efficiency of photovoltaic modules is about 20%), plus the energy storage (Expensive, heavy batteries with limited lifetime obviously can't be the last word).

Anyway:
According to Wikipedia, the Tesla S battery has a capacity of at least 60kWh. Charging 80% (48kWh) within 20 minutes requires a power of 48kWh * (60/20h) = 144kW.

One of the rare 400W panels I've found is from LG. It has a length of about 2 m and a width of about 1 m, so about 2 m². It weighs about 21 kg.
Assuming that the sun is strong enough for the panels to work at their peak power, this would result in 360 panels, or 720 m². The weight of just the panels is 7.5 tons.

This is just a rough estimation, which doesn't consider many things (efficiency of the charger, weight of the mechanical stuff, etc.), but I believe

Would carrying a folding device that stacks solar panels in a 3 meter by 2 meter 2 wheel trailer be efficient

can be answered quite clearly with No.

• Especially, no because your best-case weight calculation means that a light little Tesla that actually can go quite a streak with 80% of it's batteries means it'll not get very far with the same amount of energy when it has to pull a trailer that weighs a multiple of its own weight. – Marcus Müller Feb 11 at 13:54
• Just to add insult to injury here, it seems like this time is completely unfeasible. Unless there have been some big changes in the last year or so, it seems like the maximum power delivered to a Tesla is 138.7 kW. Plus, the current and voltage required to deliver that power are quite large. You would need a lot of weight for your inverters too I assume. – JMac Feb 11 at 14:39
• And by the time you set up all those panels, the sun will have gone down. – CrossRoads Feb 11 at 14:39
• Plus the cost of all those 400w panels, it would be easier/cheaper to tow a dozen charged Tesla's and just swap them out. – Ron Beyer Feb 11 at 15:09
• And a support frame for 420 panels will need some mounting for wind load etc – Solar Mike Feb 11 at 15:36

Would carrying a [folding] device that stacks solar panels in a 3 meter by 2 meter 2 wheel trailer be [worthwhile] efficient?

It's a great question, but I think that stopping driving and then fast charging is the wrong methodology and makes the solution impractical as shown in the other answer.

However, carrying solar panels that can provide power WHILE driving is IMO a worthwhile option for some scenarios.

I'll cheat and use the solar panel from the other answer, and the trailer with 6m^2 exposed surface from the Ops question.

1. In terms of driving range a tesla achieves about 3 miles per kWh for a total range of about 200 miles.
2. With just 6m^2 of trailer you could get 3 * 400W solar panels on the surface that could provide energy while driving. This would deliver 1.2kW of energy on a continuous basis to charge the battery or propel the vehicle, with a weight of about 140lbs (a small passenger equivalent).

If you calculate your average speed and distance covered around your area of operation you can begin to see the possibilities of charging while driving.

Let's use an example of freeway driving at a relatively constant speed for 1 hour between cities 60 miles apart:

1. The tesla would consume about 20kWh of energy.
2. The solar array would produce about 1.2kWh of energy.

Under these conditions the solar panels offset about 6% of the energy consumed which is a worthwhile gain.
When you then consider a more varied environment such as driving around a suburban area with speeds much lower, stops and go traffic etc, the gains from the solar assist multiply.

Assume a trip schedule of say 20 miles over 1.5 hours (a decent commute):

1. The tesla would consume about 6.7kWH of energy.
2. The solar array would produce about 1.8kWH of energy in this time.

Now the solar array is offsetting about 28% of the energy required (while driving). Or another way of thinking about it is that you increased the range of the Tesla by close to 30%.

If this solar array is collecting energy for 14 hours per day then it can augment the battery or driving by 16.8kWH during that time. In the example I gave above (2 * 6.7kWh) it is clear that the solar array would easily handle the power requirements of a 3h commute cycle and still provide more energy than that required.

Whether it is worthwhile to provide charging in this manner is a somewhat open question. The Tesla battery (depending on model) is about 300Wh/kg, so a 60kWh pack likely weighs in around 200kg (440lbs).
The weight of a light trailer plus panels plus DC-DC converters is likely in the 170kg (350lb) range. During daylight times this is about 100Wh/kg so about one third the power density of the battery.

While for particular operational scenarios towing the solar cells might mean never using a grid charger, it seems unlikely we will see this as a viable solution for most. However if the solar cells were the vehicle skin, it might just be a worthwhile option.

• "offset about 6% of the energy consumed which is a worthwhile gain" that sounds like a terrible gain to me, especially since you've not accounted for the extra power required to pull the trailer. – pjc50 Feb 12 at 9:20
• that 6% gain include peak power from panels and 100% efficiency in converting that power from panels. – Rokta Feb 12 at 10:36