Hybrid versus Plug-In Hybrid

I have implied many times on this web site that I do not consider the hybrid car to be a sensible option when the technology can be optimised by the simple expediency of plug-in functionality. With a change in my work, I now have the ideal opportunity to test this theory on a daily basis using my Mitsubishi Outlander PHEV. My commute is 33 miles and I set off from home on a full charge. Arriving at my place of work, the battery is depleted and so the engine has been required to generate a charge for a portion of that journey. The return journey is similar, but starts with a flat or near-flat battery, so the entire journey is covered from electricity that has been generated by the engine.

The First Week


Every day I simply entered the car’s economy readings at the end of each journey. As can be seen, Tuesday’s is missing. The reason is simple: I forgot! I shall list how the calculations are made, in the amber and dark green cells, later. For now, it can be seen that there is a wide variance in economy during the morning, and it steadily got worse during the week. I cannot give a reason for the variations as leaving at not long after 6:00am means that the traffic was reasonable throughout; there were no traffic jams to cloud any issues.

Mitsubishi Outlander PHEV
Mitsubishi Outlander PHEV

The return journeys were a very different proposition, the difference between the best economy of 32.4mpg being only a single mpg away from the worst of 31.4mpg. Strangely, there was more variance in the afternoon conditions with one traffic jam due to an accident, and half the remaining days involving crawling for mile upon mile.

Part of the cause may be down to the weather, which was so atrocious on a couple of mornings that I engaged four-wheel drive. But I don’t think so, as that mode was also used on a couple of return journeys. Four-Wheel drive was engaged on Tuesday (no recordings) and Wednesday, where I achieved 64.1mpg. So did this mode result in the two motors sharing the work? I don’t think so, as Monday’s 75.5mpg was in two-wheel drive, as was the following Monday’s journey, which achieved 70.3mpg.

Perhaps it was due to the charger, as I still haven’t got round to installing a rapid-charge system. The reasons for this are two-fold: first, I do not intend on using the rapid mode on a regular basis as I believe that doing so will reduce the life of the battery pack; secondly I am looking into other activities, including fitting solar panels that will be a part of a future article. I am going to have to get a move on though as, aside from the desire to write yet another article, I do not recommend leaving the charger plugged into the mains for long periods, as an error indication can often result. I have more opportunity to disconnect the charger during the weekend, so maybe it is cooler and so can deliver more charge.

Since writing this article, I have ordered the charger and so will be writing an article following its mid-February, 2016 installation. Note that the grant, whilst being extended, is also being reduced, as reported here:


Week One Costs

The proportional differences that I have provided are quite varied, with Wednesday’s being the most interesting. Here, the journey that starts with the battery pack fully charged provides almost exactly twice the economy that the home-bound journey provides. The final Average column is also very close to this figure. The worst proportional difference of 1.68 is itself very high. The 2.40 from Monday is, frankly, amazing.

Of course, this all translates first of all to fuel usage, and secondly to the costs including charging. I will use the average for the week’s recordings to provide the fuel cost for the whole week. In other words, I will multiply the average by five.

So, the outbound journey provides an average economy of 61.88mpg with a fuel usage of 2.65 gallons. Taking the cost of petrol at £1.10 per litre, that equates to:

2.65 * 4.51 * £1.10 + £5.00 = £13.15 (the ‘+ £5.00’ is for each of the home charges).

and the home-bound journey provides an average economy of 31.83mpg with a fuel usage of 5.20 gallons. Again, taking the cost of petrol at £1.10 per litre, that equates to:

5.20 * 4.51 * £1.10 = £25.80

Therefore, using very simple maths and making wild assumptions about the equality of the journeys, the following overall costs can be provided:

Plug In Hybrid Weekly Cost = £25.80 + £13.15 = £38.95

Hybrid Weekly Cost = £25.80 * 2 = £51.60

Giving a cost difference between the two of:

£51.60 – £38.95 = £12.65

Not a lot! Hold on though; don’t forget to add in the non-commuting costs: visiting family and friends, shopping sports and hobbies, doctors, and so it goes on. I reckon I will do around twenty miles a week. Those are costed at:

Plug In Hybrid Weekly Cost = 20 / 16.03 = £1.25 (all electric at £1 per full charge)

Adding to the commuting cost of £38.95 gives £40.20

Hybrid Weekly Cost = 20 / 31.83 * 4.51 * £1.10 = £3.11

Adding to the commuting cost of £51.60 gives £54.71

This all gives a difference in cost of £14.51 per week or around £725 per year. Alright, I admit that is not a huge difference for me. But for families with active kids it will certainly widen the gap.

The Plug-In Hybrid Maths

Note that:

  • C5 = Home to Work mpg

  • C6 = Work to Home mpg

  • C9 = Fuel Used on the Home To Work Journey

  • $O$14 = journey distance

The Average is simply the average of the two daily recorded figures:

= (C5+C6)/2

Proportion Difference is just the outbound mpg divided by the inbound mpg:

= C5/C6

Home To Work – Fuel Used is a calculation of how many gallons of fuel were used during the morning journey, when fully charged, with a journey distance of $O$14:

= $O$14/C5

Work To Home – Fuel Used is a calculation of how many gallons of fuel were used during the afternoon journey, when fully discharged, with a journey distance of $O$14:


Estimated Electric Miles

Multiply the Fuel Used on the Home to Work Journey by the Work to Home mpg and subtract the result from the journey distance.

= $O$14-(C9*C6)

Week Two Onwards

I have started car sharing, which means that the journey will be slightly longer, by three miles. This will have a significant effect on the economy as the engine use will rise by 25 to 35 percent. Also there will not be as many journeys; for example, for Week Two I drove in on my own on Monday and Friday, was chauffeured on Tuesday and did the driving on Wednesday and Thursday. I expect two to three journeys per week.

Monday’s journey can be directly compared against the previous week, with another exceptional first drive to work providing 70.3mpg, whereas the return journey was again around the same as the previous week even though it was a record low of 31.3mpg. With such a small sample, the word ‘record’ is not really relevant.

Christmas is now stopping further recordings, so expect a follow-up report around early spring, when much more data will have been collated.

As a tempter to wait for the follow-on article, I shall mention that one early New Year recording gives a single journey figure of over 80mpg, which rises in the following week to a figure of over 90mpg.

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