Plug Bug: Power/Energy Required

Electric Bug

This is an older post I never ended up publishing. Basically it is my process of how selection and research — especially with my particular commute and the hills I have to tackle.

How much energy will my bug need for what I want it to do? How do I figure this out? I started out on the DIYElectricCar Forums/Wiki page – How Much Power Will I need? This left a lot of unanswered questions, mainly because my physics is now 10 years old and rusty. So, I dug through some of my old books and read a bunch of stuff online and started making a spreadsheet in Numbers on my Mac. Strangely, I was re-reading a page from my old physics book and then ran into that same exact article on the web: “Topic 2: Automotive Power”.

I also discovered a few other people had made good spreadsheets, but it was still good practice to make my own. The ones I found:

The Electric Auto Association of Nevada has one made by tomofreno. Awesome.

Cameron Software has the first one I came across (click on “Drive Power”).

EVConvert – an online spreadsheet calculator.

Using these as guides, I made my own; I’ll post it when I finally stop tweaking on it.

Now, I have about a 20 mile commute from the mountains to Apple. (The bottom of the chart to B on the left of the chart). But, I frequently go to Planet Granite to climb and want to be able to add that into my daily driving. I want to do this complete route, without charing. But, I’m hoping that normally I will charge at work. The total distance is nearly 52 miles (normally 40 miles if I don’t go to the gym). That’s pretty tough for a homebuilt EV with the giant hill.

map to work small.jpeg


Part of the problem is Highway 17. It is steep roughly a 4.5% average grade from Los Gatos to the top. I am upping it to 5% to round up for my number estimates. I want to be able to maintain the normal speed, about 50-55 mph (I’m shooting for 55mph).

I mapped it with iMapMyRide:

Elevation profile:
Screen shot 2010-04-23 at 10.31.35 PM.png

It takes 33kw (44hp) of power to get up that hill, and has to sustain it for at least 7 minutes at 55 MPH to get me to the top. I need a motor rated for at least this. That eliminates a lot of options — especially the lower priced AC options like the Azure Dynamics AC24 and HPEV’s AC50. One of the troubles I had with DC motor selection is that they usually have charts at really low voltages (72V), so I don’t know how they will perform at higher voltages (ie: 154V). The WarP 9 is rated up to 170V.


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John C. Randolph

If only the altitude of your home and office were swapped, you could always get home because that would be mostly downhill.



Ah! But I’m hoping to only charge at work, so it might be to my advantage to have the hill on the way home!

Jason Beaver

Have you done the computation if you have to drive home in stop and go traffic? Or is the power consumption basically constant because the work done is roughly constant?

Paul Rehrig

I didn’t look hard, but did you post your custom spreadsheet? If not, would you be willing to share?

Thanks in advance,
Paul (hasn’t been updated in a while)

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