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Vehicle fuel performance

Getting the record straight

One of the frustrations of doing road-transport energy audits is the inadequacy of the data that vehicle operators record. Unless they can download consumption and distance data from the vehicle itself, they are unlikely to have much more than fuel-purchase data complemented by unusable mileage readings (the term ‘mileage’ here includes odometer readings in kilometres).

Unusable mileage figures are a major problem. For example, fuel bought on fuel cards should be accompanied by mileage records, but everyone knows that these are usually missing or incorrect. Some organisations make efforts to collect the right data but fall short: among my recent clients Company ‘S’ for example had a rigorous process for recording the daily start and finish mileages for their HGVs, along with fuel purchased during the shift, but that left huge uncertainty because a vehicle could have been refuelled at any point in a shift which could have covered hundreds of miles. And even if the mileage had been recorded at each refuelling, the driver would need to  consistently brim the tank on each occasion to make the record accurate. As a result, their MPG reports were wildly erratic. In Figure 1 I compare a typical vehicle from Company S’s fleet (left) with one from Company H which collects data from its vehicles’ onboard systems:

Figure 1: weekly fuel versus distance for freight vehicles. Left: based on daily fuelling and end-of-shift mileages; right: derived from onboard consumption and distance records

Company S (on the left) can probably get a reasonable estimate of annual MPG for each vehicle but not much more. Company H, on the other hand, can detect deviations from expected fuel economy on a weekly basis and intervene promptly wherever it has deviated significantly.  For sustained deviations they can also discern the nature of the change and discriminate between fixed and mileage-related excess consumption.

For vehicles where telemetry data are not available, the user needs to adopt a recording protocol that will yield accurate consumption and distance data at regular intervals. Of course this is not as straighforward as it might be for metered consumptions, because you cannot get a fuel ‘reading’ at the end of each period (week or month). The trick I have settled on is this: the data points for each period consist of (a) the sum of all fuel taken during the period and (b) the mileage between the last fill of the period and the last fill of the previous period.

Figure 2 illustrates a spreadsheet in which this is method is used for a monthly review.

Figure 2

Note it is crucially important that drivers consistently fill their tanks to the brim. Actual refuelling data go in Table A, columns B, C and D, while the regularised monthly litres and mileage are calculated in Table B. What links the tables is a column of ‘Period numbers’. They are defined in Column G as simply the sequential row number of the table, and in column A they are calculated by looking up the Table A date in column B. Thus by way of an example the Table-A entries for 03/08/23 and 14/08/23 belong to period 10 .

Meanwhile in Table B…

  • the logic for calculating the incremental distance just looks up, in Table A, the finish mileages for the current and previous periods based on the period-end dates (and takes the difference); while
  • the litres value uses a SUMIF() function to summate all the values in Column D that have the matching period-number in Column A.

There is a copy of the example worksheet here for anyone who wants it as a starting point for their own projects. It can be made to work on a weekly cycle simply by setting the period ends in Column F at seven-day intervals.

Finally Figure 3  shows analysis of monthly fuel consumption for the passenger car whose data appeared in the example above:

Figure 3: the data used in the example

Despite the random refuelling pattern, the correlation is reasonable and in fact would have been better were it not for the fact that performance was worse prior to July 2023 and from January to April 2024.

Thanks are due…

With Bill Kent of the Association of Energy Engineers after being presented with their Lifetime Achievement Award. In my acceptance speech I acknowledged just a few of the many people to whom I owe a debt of gratitude for the advice and inspiration they have given me over the years.

I’d start with Mike Horsley, a lecturer at Portsmouth Polytechnic (as it then was) and chairman of one of committees which I was responsible for servicing during my time as education officer and deputy secretary at the Institute of Energy (as it then was) in 1979-81. Mike encouraged me to render myself unemployed so that I could enrol on his 5-month full-time energy supervisors’ course, which in turn opened the door to my first job as an energy manager at Lambeth Council.

Colin Ashford was my opposite number at Hackney Borough Council and an enthusiast for personal computers, which in those days were almost unknown in the workplace. Colin invited me over to see how desktop computers could be applied to energy management, and convinced me to get one at Lambeth (where I think for a while I may have been the only person using one).

From there I moved across to Gloucestershire County Council to work with Mike Simpson, a highly experienced and competent energy manager who helped me enormously in developing my craft, and John Willoughby, our colleague who was energy manager for the local technical college campuses. John was not only an expert in energy saving in buildings (and incidentally the founding editor of Energy in Buildings magazine) but also a former lecturer who got me interested in training. It was John’s example that led to my use of models and physical aids on training courses.

While I was at Gloucestershire I had a phone call from Andrew Buckley, in more recent times director of the Major Energy Users’ Council but in those days the owner of a publishing and training business specialising in energy subjects. I’d originally encountered Andrew in my Institute of Energy days when his company had published the Directory of Qualified Energy Consultants which I had developed as an in-house reference document. Now, however, he became animated when I said I was applying desktop computers to energy management. He asked if I could do a book on the subject (which I did) and run some training courses for him… Which I did, after resigning from my job and going to my bank for a loan (£15,000 in today’s money) to buy ten computers.

Andrew had meanwhile introduced me to his colleague Dr Peter Harris who was presenting training courses on energy monitoring and targeting. It was Peter who introduced me to degree-days and cusum analysis, which completely transformed the way I went about analysing and managing energy, and became the foundation of what I subsequently did as a self-employed energy consultant. Combining the analysis techniques with the newly-emerging personal computer got me into selling energy monitoring and targeting software and providing expert advice in that domain.

For a while in the mid-1990s my brother-in-law Joe O’Keefe worked with me as a (somewhat older than average) intern because he wanted a radical change of career path. Not having much to employ him on, I gave him free rein to tinker with computers and it’s thanks to Joe that the website appeared in 1996… The perfect gift for an ardent self-publicist like me.

Sadly some mis-steps ensued and just short of my tenth anniversary as a self-employed consultant I came close to going bust. Luckily I was rescued by Roger Hawes who was running the energy team at Torpy and Partners in Bristol and took me on when I asked for a job, something I will always be grateful for. Likewise John Mulholland who persuaded NIFES to take me on after Torpy’s became part of the Enron empire (I bailed out because I thought Enron’s European business was doomed to fail; I did not suspect the true scale of the looming disaster). John ran NIFES’s training division and was another person who helped me hone my training skills. He looked after his team exceptionally well and I remember he always had my back with the directors when I overstepped the mark.

In due course, and prompted by the financial crash of 2008/9, I left John’s team to strike out on my own once more; and while looking for collaborators and associates I renewed my acquaintance with Andrew West, who lives a few miles away and had helped me with monitoring and targeting projects in the 1990s. In the intervening years Andy had changed his career path radically and become a book-keeper. To cut a long story short he agreed to come on board and take care of not just book-keeping but other administrative tasks. It’s fair to say the business was never more successful than when Andy was there to deal with back-office distractions.

The provision of free degree-day statistics has been a cornerstone  of my marketing for 35 years so it would be churlish not to acknowledge the help I got from The Thatcher Government in 1991 when they discontinued the supply of official degree-day data. It created a vacuum that I was able to fill with prominent monthly exposure in the print media, and ultimately even became a source of revenue when they were shamed into reinstating it and found that they weren’t allowed to accept a single tender from the Met.Office. But that’s a different story.

The full list of those who’ve helped me along the way would be long and in some cases complicated to explain. So with apologies to those that I haven’t mentioned I would just like to thank my loyal clients over the years and all the bulletin readers who sent in the awkward technical questions on which subsequent issues were based.


Impact of proposed changes to ESOS

Dateline 5 June 2023


Under the terms of the Energy Savings Opportunity Scheme (ESOS) Regulations 2014, the UK’s large private-sector undertakings have to assess their major energy uses every four years to identify cost-effective energy-saving opportunities. They are then supposed to notify compliance through the government-appointed scheme administrator, the Environment Agency (EA), before 5 December 2023. Compliance must be assessed and ‘signed off’ by a registered lead assessor.

We’re currently in the third compliance period, reporting on energy audits—which may have been carried out at any time since December 2019—related to the participants’ assets and corporate structures as they stood on 31 December 2022.

The issues

A number of participants have completed their assessments, and had them signed off, but the EA has failed to open a notification system[1] because it plans to change some of the requirements. Some of the proposed amendments to ESOS are potentially onerous, notably the extension—from 90% to 95%–of the proportion of energy use that a participant must audit. For many, that will mean suddenly having to add marginal minor energy uses where the cost of assessment is disproportionate to any possible gains. Vehicle fuel is the classic example.

The EA has been issuing guidance to participants (most recently on 24 May 2023) which assumes that the ESOS regulations will indeed change before 5 December. Although likely, it is not certain that they will change[2], which could potentially lead to participants undertaking needless extra work. This incidentally puts participants’ energy auditors and lead assessors in a difficult contractual and commercial position. Strictly speaking the EA has no legal basis for the guidance it is currently issuing because it cannot guarantee that the regulations will change nor, if they do change, that they will be applicable to participants certified as compliant before the new regulations take effect.

As a separate issue, part of the guidance issued by the EA is a template listing information that participants will need to supply during the notification process once it becomes available. A substantial part of the information relates to matters such as the savings achieved since previous ESOS cycles, and assorted (in some cases dubious) performance metrics. This amounts to market research or impact assessment and responses to these questions ought to be voluntary since they are not at present stipulated as requirements of ESOS and there appears to be no provision for them to become requirements in any new regulations[3]. If treated as mandatory, they will further add to compliance costs.

Suggested actions

The prudent approach is to assume that Parliament will approve new ESOS regulations before 5 December and EA will force the issue, making them in effect retroactive and incidentally making the market-research questions mandatory part of the notification process.  Whatever happens it is almost certain that new ESOS requirements will be in effect for the next compliance period, so the effort of setting up the necessary record-keeping and reporting now will not be entirely wasted.

However, to minimise the impact during the current ‘phase’ of ESOS I would suggest:

  1. Lobby against any revised regulations taking effect before 5 December 2023;
  2. If they do take effect, lobby for them not to apply to participants certified as having complied before the date that the new regulations become law;
  3. Lobby for the exclusion from the Notification System of questions related to market research or impact assessment.


[1] ESOS Regulation 8(1) obliges the Scheme Administrator to establish a Notification System but Regulation 8(2) allows it to make it available to participants only when they decide it is ‘reasonable’.

[2] The Secretary of State cannot at the moment even lay new ESOS regulations before Parliament; he lost the power when the European Communities Act 1972 was repealed. The Government is relying on the successful passage through Parliament of the Energy Bill (currently with the Public Bills Committee) to restore those powers and enable new regulations to be laid before Parliament and voted into law.

[3] The writer was unable to identify any specific provision in the relevant part (Part 10) of the Energy Bill

Link: Main article on ESOS

Reverse rotation

Here’s an aspect of energy saving in motor-driven systems that had never occurred to me until I went on a training course about industrial dust extraction systems. Our instructor, Christoph Ritter of Osprey Corporation (pictured on the training rig), guaranteed his audience that if he went to their factories he would find that some of their vacuum fans would be running backwards This may sound crazy, but it can and does happen. It only needs two of the motor power connections to be swapped accidentally. Centrifugal fans do still work in reverse but their efficiency becomes diabolical. If they have straight radial blades the fan-wheel itself is no less efficient but the air leaving the volute has to turn through 180 degrees, with the consequent loss of head. If the fan has backward-curved blades (normally more efficient) these are forward-curved when reversed, introducing even more loss.

The problem tends to be masked in direct-coupled fans with variable-frequency drives. One reason is that you cannot easily see the direction of rotation when there are no belts to observe; the other is that the drive system will compensate by speeding up the fan (if it can) drawing much more power to deliver the required air flow. On Christoph’s course he uses a rig to demonstrate this and a fan current of 5 amps had to go up to 22 amps to deliver the same flow when the fan motor was running backwards.

Don’t assume it cannot happen to you.


DATELINE 1 APRIL, 2023: At our last transport energy course the closing discussion took an interesting turn when a delegate raised the question of energy conservation in fishing fleets. After the course I dived in and did a bit of research on the net. I was soon hooked. One company, Dover Solar, proposes electric trawlers towing PV arrays. Another outfit, Energy Fish-in-Sea, has floated the idea of towing sonic emitters to drive the fish forwards. They have yet to demonstrate it at scale and I wonder whether there would be a catch; it could flounder. Could the UK make its fishing fleet net zero? I’ll leave you to mullet over.

Net and gross calorific values

In the UK when estimating fleet energy consumption from vehicle mileages,  we may choose to use the tables published in official guidance (extract illustrated below) which convert distances in miles or kilometres to kWh consumptions for different classes of vehicle.

Figure 1: extract from the tables in UK Government guidance

As indicated in Figure 1, the conversion factors are stated on a net calorific value (NCV) basis. However, this is not compatible with the way we normally account for energy in the UK. We actually use gross calorific values (natural gas, which of course is our predominant fuel for static applications, is billed on a GCV basis). For consistency we should account for transport fuels on a GCV basis as well.

What difference does it make? A fuel’s gross calorific value is a measure of its total energy content, whereas NCV ignores that fraction of energy which will be lost as latent heat in water vapour in the exhaust. The higher the hydrogen content of a fuel, the greater the discrepancy. Forecourt diesel’s GCV is 6.3% higher than its NCV; for natural gas the difference is 10.8%.

To convert kWh quantities based on NCV to their GCV equivalent you need to multiply by the following factors:

Aviation Spirit 1.053
Aviation Turbine Fuel 1.053
Burning Oil 1.053
Butane 1.084
Coal (domestic) 1.053
Coal (electricity generation) 1.053
Coal (electricity generation – home produced coal only) 1.053
Coal (industrial) 1.053
Coking Coal 1.053
Diesel (100% mineral diesel) 1.064
Diesel (average biofuel blend) 1.063
Fuel Oil 1.064
Gas Oil 1.064
Lubricants 1.064
LPG 1.074
Naphtha 1.053
Natural Gas 1.108
Natural Gas (100% mineral blend) 1.108
Other petroleum gas 1.087
Petroleum coke 1.053
Petrol (100% mineral petrol) 1.053
Petrol (average biofuel blend) 1.055
Propane 1.086
Waste oils 1.071



Discounted cash flow

HOW SHOULD energy-saving investment opportunities be evaluated? I recommend discounted cash-flow (DCF) rather than the commonplace, but somewhat crude, metric of simple payback period. DCF will give you two measures of a project’s value:

  1. Internal rate of return (IRR) is the rate of interest you’d need to get from investing your cash in something else, to make that the more profitable choice; and
  2. Net present value (NPV) shows you what lump sum today would equal the lifetime profit from the project, assuming that you place relatively less weight on future savings the further off they are.

To carry out a DCF calculation for an energy-saving project you start with year-by-year estimates of costs and savings. In the simplest model there is a single cost item in the first year and equal annual savings thereafter; but with some projects there will be costs in future years, or the savings may be predicted to vary in future. The screenshot below shows an annotated example in an Excel spreadsheet. It evaluates a heat-recovery system which, as well as an up-front investment cost, will incur annual electricity costs and maintenance charges (all outgoings being shown in red):

The IRR in this example is 54.7%.

The NPV depends on what you choose as a ‘test rate of discount’. In theory this would be the interest you pay on borrowings, although in practice it is often set a lot higher as a hedge against perceived risk. The example above uses 16% and you can see in row F the discount factors that result. In Year 3 the factor is 0.641, meaning that the savings of £31,332 expected that year are only worth £20,073 in today’s terms.  In aggregate the NPV is £91,444. That’s how much better off you would be than the option of doing nothing.

Note that with a lower test rate of discount, the discount factors in row F increase, which raises the net present value.

A DCF workbook is available to download here. It includes the annotated example shown above, a live calculation that you can use for experimentation and familiarisation, and an unlocked version which you can copy and adapt.



‘Average’ and ‘Standard’ degree day figures

EFFECTIVE MANAGEMENT AND ANALYSIS of buildings’ energy consumptions for heating or cooling calls for summary data about how cold or hot the weather has been. Weekly or monthly degree-day statistics provide that information in a convenient manner. But as well as the current values (against which your weekly or monthly energy consumption can be gauged), in the United Kingdom[1] we have two kinds of long-term aggregate value that can be useful for other purposes.

Standard degree-day values

For normalising consumptions we need degree-day values for a ‘reference year’, which allow actual consumptions for buildings in different locations (and possibly measured at different times) to be adjusted back to a comparable basis. Historically, the UK government recommended a standard value of 2,463 heating degree days (to base 15.5C). The source of this number is unknown; it is, in effect, arbitrary but for the purpose of weather-adjustment it does not matter what the number is. This single point of reference was later developed into the following table providing corresponding reference values for different base temperatures, for cooling as well as heating, and disaggregated to individual months:

|     |        Heating       |       Cooling        |
|Month| 18.5'C 15.5'C 10.0'C | 15.5'C 5.0'C -20.0'C |
|     |                      |                      |
| Jan |    488    395    226 |      0    17     705 |
| Feb |    426    342    189 |      0    23     652 |
| Mar |    390    297    134 |      0    64     803 |
| Apr |    319    233     96 |      5   114     837 |
| May |    235    151     39 |     14   192     963 |
| Jun |    148     77      9 |     26   265    1015 |
| Jul |     88     42      4 |     96   380    1155 |
| Aug |    100     45      5 |     57   338    1113 |
| Sep |    162     83     10 |     14   245     995 |
| Oct |    268    177     48 |      1   158     925 |
| Nov |    359    275    124 |      0    53     719 |
| Dec |    439    346    176 |      0    29     755 |
|     |                      |                      | 
|Total|  3,422  2,463  1,060 |    213 1,878  10,637 |

Table 1: standard degree day values

It is important to appreciate that these standard values are fixed and not related to any particular geographical area.

Average degree-day values

If we are forecasting consumption we need to know what future degree-day values we can expect month by month. For this purpose we use 20-year average degree-day values. So the expected degree-day value for next February (for example) is the average of the last 20 Februaries. You can download the UK regional average figures via this link.

In contrast to ‘standard’ degree-day values, ‘average’ values differ from region to region and tend to vary with time thanks to the changing climate. I usually update the table once a year.

[1] Subject to the availability of suitable historical data, the same principles could be applied in other regions.