Comment by TheGrim-888 | Hacker News Reader

Comment by TheGrim-888 | Hacker News Reader

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Correct me if I'm wrong, and I quite possible could be, but from a quick Google search, I'm seeing numbers of anywhere 200Wh/mile to 350Wh/mile for a Tesla. If we're generous and assume only 200Wh/mile, then 40 miles of range translates to an energy consumption of 8,000Wh.

A 100 watt solar panel, producing the full 100 watts, would take 80 hours of perfect sunlight to produce enough power for an extra 40 miles of range.

Assuming best case (unrealistic) conditions, if you get 8 hours of perfect sunlight in day, and your solar panels produce 100% of their rating for all 8 hours, it would take 1000 Watts worth of solar panels to produce that extra 40 miles of distance over 8 hours. It seems kind of unrealistic to fit 1000 Watts of solar on top of a car. And that's an absolute minimum, under best case conditions.

If you had, say, maybe a more realistic 300 watts worth of panels on top, and they got 4 hours of full sunglight, you'd be producing an extra ((300W * 4h) / 200Wh/mile) = 6 miles. And that's still assuming best case condition for power consumption per mile.

[Edit] - And like the other commenter stated, those few extra miles get cut down when you consider the weight of hundreds of watts of solar panels added onto the vehicle.

Here's a recent analysis: https://teslatap.com/articles/solar-vehicle-roof-analysis/

To me it looks like it will never be a meaningful solution, due simply to physics.

That has several really bad assumptions by assuming all solar roofs need to fit on an existing Tesla‘a roof.

> The usable clear area of a Model S glass roof is 42” x 45”

A Model S is actually 195.9” by 77.3 (ex. mirrors). Assuming a reasonable shaped solar car using 75% of that surface is covered in panels that’s 12x the area. But you also gain from panels covering the sides of the vehicle.

Further “Because the vehicle roof is flat, it collects less light than if it was positioned at the optimum angle to the sun.“ as I said your not limited by the roof. “Lastly, we lose at least 10% more due to the safety glass,” we don’t need glass and that’s already part of panel efficiency numbers. “and the inverter/charging is only 81% efficient.” Solar panels and batteries are both DC so you don’t need an inverter, the charge discharge efficiency of lithium ion can be over 90%.

San Francisco 5.34kWh/m * .3 efficiency * 7.33 square meters = 11.7kW/day /.3kW per mile = 35 to 39.14 miles in San Francisco depending on how much your charging the battery with plenty of areas getting more sunlight. Using the highest efficiency panels currently produced that goes up significantly, but cheap 30% efficient panels seems like a more reasonable mid term prediction.

PS: Example of a flexible 1m panel zero glass required at under 300$/m: https://www.amazon.com/dp/B082FCZ4MD/ref=emc_b_5_t?th=1

> Solar panels and batteries are both DC so you don’t need an inverter, the charge discharge efficiency of lithium ion can be over 90%.

You'll still need a DC-DC converter, which is just an inverter with a rectification step. The 81% number is low, but there is a loss here.

Doing some calculations from another direction:

The maximum daily energy density of sunlight in sunny Los Angeles is about 6.4 kWh / m2 [1] (assuming perfect, moving angle of panels to sun).

If we can turn the entire footprint of a Model X into solar panels, that gives us about 10 m2 (big car!).

The US DoE reports the Model X gets 100 mi / 31 kWh [2]. Or 12.4 kWh for 40 mi.

So those panels would need to get 1.2 kWh / m2 of solar power. Which is about 18% efficiency and pretty reasonable for good consumer panels [3].

But it assumes the car is in sunlight all day at the perfect angle, there is no loss (eg due to weight), in a locale as sunny as LA (eg Seattle gets half of the sunlight as LA), and can be completely coated in efficient panels (the model S solar roof is <1 m2 in comparison). Bumping efficiency to 30% gives some headroom but it still seems pretty impractical.

[1] https://globalsolaratlas.info/detail?c=34.270738,-116.929301...

[2] https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=41196

[3] https://news.energysage.com/what-are-the-most-efficient-sola...

I'd like to see this for large vehicles. Buses, semi, etc.

But you could have a few 100w (or 200w) solar panels installed on the roof of your house which either charge your home battery or offload that to the grid. And just charge your car when you want.

It is true that you lose some energy every time you store or transfer it, but if we install solar panels on every roof (so we get excess energy from solar) and also find a way to store energy cheaply for a long time, that should be enough to completely switch from fossil fuels, (well, mostly)

Yeah I think I did some back of the napkin calculations that this would be feasible if we put solar panels on every roof. One challenge is that installation is still pretty expensive from a labor point of view because roofs are all unique to one degree or another (age, orientation, composition) and the parts and knowledge to DiY aren't yet commonly available at the local big box hardware store. Plus you have to interact and interconnect with the power company so its another layer of complexity and specialization. Adoption could be greatly accelerated if we had something that made installation of panels on houses a priority especially for lower income households in both the developed and developing world as they would in theory benefit the most from really cheap electricity or at least lower electrical bills. As it stands now it can pay for itself over time but typically on the scale of decades and there are a lot of middle men who will install them and then own the panels and try to profit off of the price differential.

just use off-car solar panels.

https://sunelec.com

routinely sells panels for 30 cents/watt.

If you could skip the inverter step, charging your electric car could be very low cost.

5,000 watts of (raw) solar panels for $1500 and would give even the most power-hungry 350wh/mi tesla ~ 70 miles of charge in 5 hours of sun.

(a 200wh/mi car would need ~ $500 of panels for 40 miles)

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