Are Hybrid Cars Energy Efficient?

The car

The car is a Volkswagen Golf GTE 2017 plug-in hybrid electric vehicle (PHEV)

Battery/Range 8.7 kWh (pure EV range ~30miiles)
Battery weight 120kg
Petrol engine 1.4L turbo
Official fuel economy 166 mpg (this compares with real world driving figure ~75-90mpg)*
Official emissions 38 gCO2/km

*The real-world figures (see below) are nowhere near as good as the official values. As far as I know the official test is only over ~80 km (50miles) so if driven on a full battery then ~60% will be full-EV and they are allowed to claim ZERO emissions for EV-mode.

Charging matters

The Golf GTE has a 8.7kWh battery which is stated as providing 30 miles range. However, when fully charged it typically indicates 29 miles of range and the rate at which those miles go down depends on how carefully you drive the car – 29 miles seems to be pretty much the maximum you can get out of the battery.

When I am at home I charge the car using a standard 13A socket in our backyard. This feeds power at 2.4kW so a full charge usually takes just over 3hrs. The only problem for us when charging at home is that we don’t have off-street parking (or a drive etc), hence the location we use is round the back of the house on a back-alley. This is OK most of the time, but because the back-alley is not 2 cars wide it is possible for the alley to be blocked at both ends by other cars.

On those occasions we can’t charge the car without knocking at somebodies house to ask them to please move their car to let us in. This practical issue was one of the main reasons we chose a plug-in hybrid car rather than a full-electric vehicle. In practice we have only had 2 or 3 occasions in 100 days when we haven’t been able to charge the car because of this issue.

The car can also be charged up from public charge points such as the GMEV point shown below.

The time taken to fully charge the car depends on the type of charge point you are using. The GMEV point shown provides 7kW of power so can charge the car from flat in just over 1hr. The nearest public charge point to our house is 1 mile away close to the shops in the centre of Prestwich. It would of course be more convenient if there were more charge points dotted around the local area. Local authorities should work with utility companies and businesses to set up lamp-post charge points as these would seem to be an ideal way forward for terraced streets and other areas where people don’t have access to off-street parking.

Everyday driving

One thing that has a big impact on the energy efficiency of the car is the profile of miles driven per day which largely determines the relative usage of the electric only and petrol ICE modes. I have summarised this usage profile below.

On average we are driving 23miles/day, but looking at the histogram it can be seen that the most frequent distance travelled is typically 30miles/day. It is also important to note that for 6 out of the 100 days we drove over 120miles. Conversely for 16 out of 100 days we drove less than 7.5miles/day. The routes we drive are quite variable; for 2 weeks (10 working days) I have been commuting a distance of 6.5miles each way or 13miles per day (excluding other mileage). Other day to day driving includes taking our 11yo to football training or matches; typically in the 5-15miles range. The longer distance driving over the 100 days has been all work related with locations such as Hull and Stoke being destinations where I have needed to take sound testing equipment to site or carry out noise surveys. The result is that for distances of less than 30miles/day it has been fairly easy to charge the car and run it effectively as a full electric vehicle, but when I’ve needed to drive 60miles or more then a lot of those miles have been driven using the petrol engine.

Effect on home electricity consumption

Another thing I want to find out is how much our household electricity consumption is likely to increase and how much the car can be charged from the solar PV on our roof. We have been monitoring the electricity consumed by our Golf GTE via an OpenEnergyMonitor which also records the amount of electricity generated by our rooftop solar PV. The graphs below show a) the daily electricity consumption for our house for 2017 so far, b) the overall monthly electricity balance since 2015 including the solar PV and overall electricity consumption.

Daily electricity use for 2017 with period before and after PHEV purchased shown.

These graphs show the influence of the electricity consumed by the PHEV plug-in at our house. We have also logged the very small amounts of electricity consumed at public charge points in order to calculate the overall number of kWh used to charge the vehicle (overall total = 378.8kWh for the 100day period covered here).

The amount of electricity we use day to day and over a month is quite variable and generally reflects the number of miles driven. The average daily consumption is 3.79kWh/day making the average monthly value ~114kWh/month. So far it looks like approx. 30-33% is likely to come from our PV (but winter data will be needed to be sure). This has fallen to 11% for June because I am now commuting in the car and charging it when I get back after work when there isn’t much sunshine whereas in May I was mainly working from home. This extra commuting has also pushed up the overall electricity consumption as a greater proportion of the miles are being driven on the battery and fewer miles on petrol (a broadly good thing).  This all equates to an additional ~950kWh or approx. 50-55% increase in our grid electricity consumption. 

In terms of carbon accounting it makes more sense (to me) to record the emissions from this additional electricity as “transport” emissions, it also implies a need to keep improving the monitoring tech to keep track of PHEV energy as separate from household electricity.

Initial Energy Efficiency Assessment after 100 days

By collecting the data on the use of the car and the measured electrical consumption above I have made the following assessment of the cars energy consumption and costs.

NUMBER OF MILES DRIVEN 2329
TOTAL PETROL PURCHASED 122.59 LITRES
TOTAL ELECTRICITY CONSUMED 378.8kWh
ESTIMATED PETROL “ONLY” DRIVING 1126 MILES (48%)
ESTIMATED “PURE EV” ONLY DRIVING 1203 MILES (52%)
EFFECTIVE CO2 EMISSION RATE 108 gCO2/KM (ASSUMING 100% GRID ELEC AT 320 gCO2/kWh), OR 99 gCO2/KM (ASSUMING 65% GRID ELEC AT 320 gCO2/kWh + 35% ON-SITE SOLAR PV)
ESTIMATED ANNUAL EMISSIONS SAVED RELATIVE TO OUR OLD RENAULT KANGOO 1.4DCI DIESEL 662 kgCO2 SAVING PER YEAR
ESTIMATED ANNUAL COST OF FUEL + ELECTRICITY £626
COST SAVING RELATIVE TO OUR OLD DIESEL CAR £480 SAVED PER YEAR + £140 VEHICLE EXCISE DUTY

Based on the results tabulated above it is estimated that the average emissions when driving on petrol is 157gCO2/km whereas the emissions running of electric = 60gCO2/km; thus driving on electric effectively reduces the emissions by 62%.

Final thoughts

So would I recommend the Golf GTE? Yes it is a good car. Ideally I would have liked a full electric car, but the restrictions identified above mean that we chose a plug-in hybrid car. Getting the most out of the plug-in aspect of the Golf GTE means plugging it in fairly frequently as you can only travel 29 miles before you need to recharge it. This is both a disadvantage in terms of the frequency which it needs to be charged and a slight advantage in that the car can be recharged fairly quickly as the battery has a small capacity. Nevertheless it would be great if future versions of this car or other similar plug-in hybrids came with a bigger battery (giving say 60 miles electric-only range) and a smaller more energy efficient petrol engine (say 1.2L). Finally it should go without saying that this is still a car and it is always going to be more environmentally friendly to use most forms of public transport and to walk or cycle wherever possible.

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