Saturday, 4 March 2017

Toyota & Panasonic just announced a solar roof option for the Prius

… & quantified why you won’t want it.

Prius with new optional solar roof
Credit: Toyota/HybridCars

Solar cells on the roof of an electric car is a popular idea.

Panasonic’s solar roof for the Prius is described as having an output of 180 Watts.

Cruising at 100 kph (62 mph) on a flat road in an electric car requires about 14 kiloWatts. I.e. 14,000 Watts. That’s just maintaining speed, not accelerating, not climbing a hill.

Can you see where this is going?

As a fraction of 14 kiloWatts, 180 Watts is

180/14,000 = 0.013 = 1.3%

Putting that another way, if your drive 100 kilometres in one hour, on a flat, straight road, in good sunlight all the way, the solar roof will have added 1.3 kilometres. That assumes no losses in the system due to inefficiencies, & depending on how Panasonic/Toyota are rating their system, it probably means around mid-day, in summer, & not at high latitudes.

If the car is parked in sunlight, the solar roof could charge the battery. Ignoring the angle of the sun as it varies through the day & any inefficiencies in the charging system, in 8 hours at 180 Watts:

180 * 8 = 1,440 Watt hours = 1.4 kiloWatt hours

Driving for economy, at around-town speeds, one kiloWatt hour will propel an electric car about 6 kilometres (3.73 miles). So if you park your car in the sun at work, & live less than 8.4 kilometres (1.4 * 6 = 8.4 k = 5.2 miles) from work, you may just make it home on the power generated through the day.

If Toyota is offering its solar roof at no cost to the consumer, it may be marginally useful. On the other hand, they’re not running a charity ...

Neither is Tesla – the article linked below claims that a solar roof will be optional on the Model 3.

Electrek – “Tesla partner Panasonic unveils new 180W solar roof product for cars after Elon Musk said Model 3 could have the option”

With much more roof area than is available in a sedan, say a classic VW bus, & solar panels with a generous overhang front & rear, it is possible to generate enough power for, “short trips within the city”.

Credit: Green Car Reports

HitchHiker's Guide 2 Tech: “Classic 1966 VW bus with a solar powered drivetrain”
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