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Discussion Starter #1
I recently p[osted this to the Vauxhall Ampera Facebook page in response to someone claiming the Ampera is not efficient nor is it a good car based on its range and running costs:

The amount of energy in a litre of petrol is about 35 MJ/L or 9.7 KWh/L. A normal petrol car nowadays can go about 50mpg (many cars of this size and performance claim more but very few acheive it!). That is 11 miles on 1 litre of petrol or 11 miles on 9.7KWh of energy. That equates to 0.88KWh per mile.

Ben is suggesting that the Ampera uses a lot of electricity. Let us compare just how much energy the Ampera uses in electric mode then.

The Ampera has a 15KWh battery and if the owners in the USA of the Volt are to be believed this will get you about 35-40 miles. For the sake of argument, and to give Ben the benefit of the doubt, I will use 35 miles. So that is 35 miles from 15KWh of electricity. That equates to 0.43KWh per mile.

Now, I might not be a maths genius but unless you strongly disagree with the numbers it seems pretty clear. The Ampera in electric mode uses about 1/2 of the energy of a normal petrol car that does 50mpg. I fail to see how anyone can argue that the Ampera is not economical regarding energy use. When it is in electric only mode it is needing 0.43KWh/mile and in ER mode it is no worse that a normal car that does 50mpg and needs 0.88KWh/mile. Given that most people will be using the car mostly in electric mode the car is very much more efficient in energy use than any other normal petrol car of its type and size.

So,now we can calculate the energy use for my 250 mile trip and convert that into a theoretical "mpg" equivalent. So using these figures it gives us the week use of 200 miles at the electricity power use of 0.43KWh/mile or 86KWh plus the gallon of petrol at 9.7KWh/l or 44KWh. That gives us an energy requirement of 130KWh.

Converting that back to petrol energy at 9.7KWh/l is 13.4ll or 2.95g.

So our Ampera should easily be able to acheive 250 miles on the equivalent of 2.95g of energy or an equivalent of 84mpg. I do not know of any petrol car of this size and performance that could acheive that... do you?

Now, what about cost to run? We all agree, it is expensive to buy and I am not going to pretend anything else. It is unlikely that at this price that the extra purchase price could be recouped in under 5 years... possibly longer. However, as far as fuel costs are concerned it is clear.

In ER mode, at 50mpg the car costs about 13p/mile in petrol. In electric mode at 16p/KWHh of normal grid power it works out at about 6.9p/mile. Again, in electric mode it is about 1/2 the cost of running a comparable petrol car doing 50mpg and no worse in ER mode.

Not only that, but the CO2 per mile produced at the tailpipe for a normal petrol car is way more than the CO2 per mile of an Ampera in electric mode. Not only is the Ampera using less energy but the efficiency of burning stuff to produce that energy at the power station along with savings resulting from not needing to transport and refine the fuel makes any electric car, whether 100% electric such as the Leaf, or part electric such as the Ampera, way better than any existing petrol or diesel car on the road today in these respects.

If I have made any errors in the sums here please just say so and I will correct it. however, I have gone through it pretty carefully and I am pretty sure of the figures.
 

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Paul Churchley said:
The Ampera has a 15KWh battery and if the owners in the USA of the Volt are to be believed this will get you about 35-40 miles. For the sake of argument, and to give Ben the benefit of the doubt, I will use 35 miles. So that is 35 miles from 15KWh of electricity. That equates to 0.43KWh per mile.
You've made a mistake here.

Yes, the Ampera does have a 15kWh battery, but it doesn't use all 15kWh: assuming it is the same as the Volt in that respect, it never uses more than 9.6kWh, i.e. 65%. This is done to maximise battery life: the battery is never charged above 90% capacity and never discharges below about 25% though as the battery ages I think those buffer zones may reduce and they may not be hard limits: if you charge on top of a mountain then you'll have more than 90% when you get to the bottom!

However you can't just use 35 miles from 9.6kWh electricity either as there are charging losses. According to the US forum a full charge takes about 13kWh on a 110V charger but rather less with a 240V charger (I'm not sure how much less exactly but if I see the number anywhere I'll let you know).

Edit: According to http://www.mychevroletvolt.com/chevy-volt-energy-usage-charging-on-240v-vs-110v, 11.9kWh to charge on 240V, 13.5kWh on 110V
 

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Discussion Starter #3
Thanks Duncan.

I only found out about the fact that it doesn't use all the 15KWh yesterday after researching some more on the gmvolt.com site. This seems like a great idea. As the battery life starts to go and range decreases the computer senses this and starts to use a bit more of the battery to compensate so you see less reduction in range over time.

As for the loses you are correct. The Leaf has a 24KWh batter and takes about 25.5KWh to charge from empty so I pro-rated the difference and decided not to include the loses in my calcs. Perhaps I should have! :oops:

The end result of the two items you have highlighted is that the 35 miles in from about 10KWh instead of 15KWh making the efficiency 50% better than I had suggested! :shock:

Adding in the loses when charging would add a little to the running costs but not a lot. I have to say that 13KWh to charge to 9.6KWh seems a rather high amount of loss. I would expect around 5% from what I have read so that would mean about 11KWh at tops.

I will do my own measurements when I get my Ampera but for now I will leave it that the running costs are definitely going to be a bit higher than I suggested due to losses when charging. If it is as I expect then it won't be by any significant amount. I'll post again in a few weeks on this topic.

Thanks Duncan.

If anyone else thinks I have got this wrong or just made a mistake please post. I love being proven wrong... it means we then get to the real truth! ;)
 
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