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Comment by imh | Hacker News Reader

I expected that 1kW would be a drop in the bucket for a car, since moving that huge hunk of metal from A to B seems like it should totally dominate. But I worked it from first principles, and sanity checked against Tesla anecdotes [0]. At usual speeds and 2.5-3 mi/kWh [0], that's a power consumption in the tens of kW [1], so call 1kW a 5% bump. According to CNN[2], that could be around ~$20/mo from super rough back-of-the-envelope math.

If that estimate is on the money or high, I'd totally pay <= $20/mo for my car to drive itself. If it's low, I probably wouldn't pay $100+/mo for it.

[0] https://forums.tesla.com/forum/forums/miles-kwh

[1] X mph / (Y mi / kWh) = X/Y kW

[2] http://money.cnn.com/2011/05/05/news/economy/gas_prices_inco...

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You might be willing to pay $20/mo, but how much do you think Tesla would be willing to pay to increase their range by 5%?

I wrote out the numbers I worked out for a tesla, but I had also done it from basic physics (plus referenced efficiencies) for a gas car, and it's not horribly different. I didn't include it because I have no idea what so ever how big of a deal adding in a 1kW power system to a gas car that still drew power from the gas engine, or what the losses look like. The order of magnitude remains about the same. I drive a prius, and if my estimate is close enough, the range isn't a factor for cars like mine. I was mostly curious in order of magnitude kW stuff, and only tangentially $ stuff.

Also consider whatever you value your hourly rate. You can work/do whatever you want for an extra hour in a driverless car on your way to work but you cant do that in a traditional car. There are other cost factors here besides electricity/gas.

So you put extra 1kW heater somewhere in the car. Now how much energy you spend on cooling?

Internal combustion engines are 20-45% efficient. That means a 150 hp car dissipates 55 KW at peak. Assuming half load that's over 25 KW to dissipate continuously. Cars usually dissipate heat by way of a ~100 kg radiator, ram air and a couple of 20W fans. Brakes dissipate far larger amounts of heat in short bursts, and are passively cooled by ram air. For EVs, I'm not sure, but I think 100KW li-ion pack will continuously emit something between 1-5KW in heat when charging or discharging.

1KW is a small fraction of a typical car's heat budget.

I agree with your general point, but to sanity check your numbers: car radiators don't even weigh a tenth of that. The entire cooling system, including fluid, is in the sub-20kg range. The entire drivetrain of a common 4-banger is in the 150-160 kilo range.

Except it only takes about 20hp to keep a car at highway speed. The rest is for acceleration, hills, headwinds and looking cool.

Driving autonomously is pretty cool don't you think...

So the first self-driving car should be a 20hp Volkswagen with a computer under the hood.

... I'd buy it!

Well, in a world where all the cars were automated and could talk to each other (coordinated merges, for instance), you'd probably be fine with 60 hp motors. Where all the vehicles are hybrid, 30 might be enough for highways.

Hilly neighborhoods might be something else entirely.

The brain can drive a car and uses a fraction of that amount of energy.

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