Prius Plug-in Hybrid Part Six - Running Costs & Emissions

Wednesday 21st September, 2011
Examining the total cost of ownership of a vehicle can be hard. When you add in the fact that this car has two different energy sources, it gets very difficult. So instead of trying to estimate the true running costs, based on unknown servicing prices and unpredictable residual values, I'll simply look at running costs and related emissions.

The easiest car to compare the Plug-in Prius to is its own hybrid predecessor. They are, after all, the same car - the plug-in variant just has a bigger battery pack that uses Lithium instead of NiMH in the older Prius. It also requires an on-board charger and a few software tweaks.

We know from our measurements that the Prius Plug-in Hybrid uses around 3.3kWh of electricity for a full charge. For technical reasons, related to battery management, it's rumoured that this model actually carries around 5kWh of Li-ion battery cells on board. This, using our ballpark figure of £500 per kWh of battery modules, equates to a wholesale battery pack cost of around £2500. This pack also replaces the smaller NiMh battery pack in the normal hybrid Prius - so we're saving on that cost too. It's probably fair to say that the cost saving of removing the NiMh battery pack probably covers the cost of the new charging unit in the plug-in model, so the only real additional cost in this car is the £2500 Li-ion pack. No surprise then, that Toyota claim the Plug-in Prius will go on sale in the UK for under £26k (after subsidy) - about £2000 more than the current Prius T-Spirit model.

So what extra do you get for your money? Well, you should get lower running costs. If you frequently do short journeys, the 13 miles of plug-in range will put a significant dent in your fuel bill. Compared to the 60mpg you will likely get in the standard Prius, Toyota are claiming a combined fuel consumption of 108.6mpg in this prototype model. As with the Ampera, this is based on a highly unrealistic testing cycle and your results will vary considerably. Counting miles that don't actually come from the gallons is a bit misleading and pointless. I could pull the car with husky dogs and that would also increase my mpg, it's all nonsense.
In real terms, the hybrid Prius will use around a litre of fuel when travelling 13 miles. Instead of burning that litre of fuel, at a current cost of around £1.35, the plug-in hybrid Prius allows you to use electricity instead; at a rather cheaper cost of 35 pence. You then need to charge for a couple of hours, after which you can save another litre of petrol and another pound in cash. Every time you charge, you save a quid. If you can only charge at home, you may only save a quid a day. If you're fortunate enough to be able to charge at work, you may save another quid - or more if you don't pay the electricity at that end either. If the saving really is only a quid, in the pouring rain you really aren't going to go rooting around in the boot looking for your charging cable and plugging in. If you're in your suit for work, you'd likely rather spend the extra quid under a canopy at the petrol pump next time you fill up.

So, over the course of the year, you may manage say 400 charges - a saving of £400. Over the 3-5 years you might own this car, this equates to £1200-2000. How much are you paying for this privilege? About £2000. Like a lot of current plug-in car maths,you may not see a clear benefit.

Why would I want to pay an extra £2k for the privilege of messing around with a charging cable in bad weather if the overall saving was only £2000? That's no benefit and we've not even considered the ever increasing cost of domestic fuel, which is rising faster than petrol prices.
Of course, you may be interested in this car as you are an environmentalist - so let's look at the CO2 footprint.

The standard hybrid Prius has an official CO2 rating of 89g/km - this is already the lowest VED band (A) and exempt from car tax. Toyota estimate 49g/km for this plug-in model but, as with most things electric car, this negates the CO2 footprint from the national grid whilst you charge it.

We know from our measurements that 3.3kWh of electricity are used to go 13 miles. Using the NEF CO2 calculator, used for all the comparisons on this site, this emits 1.731kg of CO2 or (1.731/13*1000) 133g/mile or (133/1.6) an equivalent figure of 83g/km. It's a marginal improvement over the standard hybrid but, as it needs to lug around a petrol engine, not quite as efficient as the Nissan LEAF's estimated 81g/km. It is massively less than the highly inefficient 113g/km we estimated for the Vauxhall Ampera on EV mode though. The Ampera also only achieves around 40mpg in petrol mode when the batteries run out, making the Prius approx. 50% cheaper to run on the petrol cycle and giving the Ampera a carbon footprint around 160g/km - almost double.

In real terms, though, these cars have very differing charging capacities and range. On a return journey of up to 100miles, the Leaf will always be the cheapest to run and likely have the lowest CO2 footprint. The only real issue is that it won't go further than that without the hassle of recharging which puts off some higher mileage buyers and pushes them towards plug-in models like the Ampera and Prius plug-in.
You'll clearly see from the graph above that the Ampera can't touch the other two for overall CO2 footprint. The Leaf is lowest of them all but runs out of steam at the 100 mile marker. The Plug-in Prius will achieve 5 times this distance without needing to recharge. From a CO2 perspective, clearly things just don't add up for the Ampera, even for short journeys it emits significantly more than the competition.

Look at journey costs, though, and it's a slightly different picture.
The Leaf still has the lowest cost per mile, making it more appealing for short journeys. For the first 13 miles, the Plug-in Prius takes second place, but quickly gets left behind by the Ampera when its batteries run flat and the petrol engine kicks in. This continues until the 50 mile market when the Ampera runs out of charge and its petrol generator kicks in. Graph based on a best case scenario of 45mpg for the Ampera vs. 60mpg for the Prius.

Never before has the old phrase YMMV (your mileage may vary) been more apt. The value of each vehicle will vary considerably based on the journeys you do. If the car never goes further than around 80 miles, the Leaf is arguably the best car for the job. If you regularly do journeys under 15 miles and  frequent journeys of over 100 miles, the plug-in Prius is your friend. For regular journeys of 50-100 miles with intermittent long journeys under 150 miles, the Ampera may fit. With cost differences reaching around £5 per journey, choosing the right car for the job is key.

The original purchase price of the car also counts for a lot here. With LEAF pricing at £25,990 and Ampera at £28,995 there's a whole £3000 gap between them and another ~£500 in interest charges if you buy the car on finance. At the announced sub £26k price, the Plug-in Prius could save the buyer £3000 over the Ampera - an equivalent saving of around 30,000 miles in fuel - effectively making the Prius cheaper than the Ampera for any journey under 13 miles and also those over 40 miles, for three years of typical ownership. If you do less than 10,000 miles per year, the Prius will always be cheaper over the first 3 years, even if you never drive the Ampera, as you're effectively saving your entire fuel cost in the purchase price.

We're back on costs again and we already said that it appeared no cheaper than the standard hybrid Prius anyway. So why build the plug-in Prius? Why sell it? Who's going to buy it? I'll cover that in the next part.

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