Introduction To The Persistence Factor Model
Since being set up in 2001, we have worked with thousands of companies – helping them make lifetime carbon savings of over 29.5m tonnes. This tool will
give you an understanding of how we calculate those savings. It will take you through the calculation of persistence factors for different classes of energy
efficiency technologies, alternative energy technologies and behavioural measures.
This page and the User Guide will provide you with an introduction to the Carbon Trust Persistence Factor model however it should be used in conjunction
with the methodology guide which can be downloaded from the Carbon Trust web site (http://www.carbontrust.co.uk/persistencefactors).
Use of Persistence Factors
There are many measures that can be taken to reduce energy, cost and carbon from buildings and industrial processes. The Carbon Trust recommends that
organisations prioritise measures: reduce energy consumption, improve energy efficiency and then look to alternative sources of fuels such as renewable
energy. Simple low and no cost energy reduction and efficiency measures can save 10-20% of an organisation‟s energy bill and carbon footprint.
The Carbon Trust offers support to organisations looking to reduce carbon emissions. For businesses with energy spend over the qualification amount*, our
environmental consultants can help to identify appropriate and feasible measures which may involve no capital outlay whatsoever. Where capital expenditure
is necessary, our consultant will help to calculate the payback period. The consultant will also calculate the carbon savings that can be achieved in the first
year. A persistence factor is then applied to calculate the lifetime savings of a measure for compliance with funding requirements i.e. the total savings over
the period for which the measure is expected to produce savings.
A persistence factor is determined from the useful life of the equipment or behavioural measure multiplied by two percentage degradation factors:
• The inherent degradation of the equipment (i.e. the inherent reduction of performance through time due to wear and tear etc and not addressed by
• The operational degradation caused by use which reflects maintenance policy, skill and frequency of maintenance.
To assign a persistence factor to a measure, the particular technology or behavioural measure has to be identified. A taxonomy has been developed to allow
the measure to be classified by a main technology descriptor and two associated levels of detail. Once the measure has been identified within the taxonomy
the model provides the useful life and associated inherent and operational degradation factors which drive the persistence factor.
The persistence factor model is used to derive the typical expected useful life and the typical degradation of carbon efficiency over that period, assuming a
business-as-usual baseline. It offers a simple and consistent approach that is required to enable the Carbon Trust to report its impact and effectiveness.
Actual economic lives may be shorter or longer than the modelled lives due both to factors specific to the actual technologies implemented and the context in
which they are implemented. Actual inherent and operational degradation rates may also differ from those assumed by the model. Organisations are advised
to seek expert opinion on the life of specific technical solutions chosen and expected degradation rates for use in their organisation if they wish to take
account of project specific issues, as the persistence factor model uses assumptions with wide general applicability.
Please refer to the Carbon Trust web site for a more comprehensive description of the model.
* Refer to our web site to see how the Carbon Trust can help your business: www.carbontrust.co.uk
Release Date: August 2010
General Guidance - How To Use This Model
Make the following selections from the 'Main Tool' page:
1. Technology Classification - use the drop down boxes to select the technology you would like to obtain the persistence factor for. Each drop down box rep resents a level of
technology which allows you to identify a more specific technology type.
2. Maintenance Type - select either Good Practice Maintenance or Basic Maintenance. Refer to the Operational Degradation guidan ce below for further details about these
3. Maximum Life, User Field - refer to Useful Lives guidance below. Maximum of 60 years.
4. Discount Rate - defaults to Government Green Book discount rate of 3.5%.
After the selections have been made the model will return:
1. Persistence Factor
2. Financial Persistence Factor - reduces the persistence factor by the Discount Rate. Is used for calculation of payback period.
3. Useful Life - the useful life of the technology selected as determined by CIBSE Guidance. Refer to Useful Lives guidance bel ow.
4. Operational and Inherent Degradation - of the technology selected as classified in the guidance below.
5. Years Breakdown Table - this displays the persistence factor and financial persistence factor for each year of useful life.
6. Persistence Graph - illustrates the impact of inherent and operational degradation on the expected carbon savings achievable by a measure over its lifetime.
The taxonomy consists of four groups (Alternative Energy, Building Technologies, Process Technologies and Management Measures ), and a further two levels of detail below
each group to enable more specific identification of technologies.
For measures that require investment in new equipment or measures such as insulation, the relevant technology should be selec ted from alternative energy, building
technologies or process technologies. For measures that require behavioural change, such as training staff to use technologi es in a different way, to adjust controls or to
better maintain current technology, the relevant measure should be selected from „management measures‟.
Useful lives are based on indicative / economic lives published by the Chartered Institute of Building Services Engineers in February 2008 (see CIBSE Guide M: Maintenance
Engineering and Management). This is the most comprehensive source currently available. Although not exhaustive, it can be us ed to identify similar equipment or
equipment likely to have a similar failure mode (i.e. prone to similar errors or defects in a process, design, component etc) so that estimates can be made of the appropriate
life. Where this source fails to provide an estimate of useful life, alternative published sources have been used (e.g. Build ing Life Plans‟ Construction Durability Database and
RICS Surveyors‟ Estimates). Estimates are made taking a cautious approach rather than an optimistic or pessimistic one.
A distinction is drawn between the “useful life” of a measure and adjustments to the life, reflected in inherent and operatio nal degradation as separate factors.
For the purpose of Carbon Trust‟s reporting of its impact and effectiveness, useful lives are capped at 30 years. Carbon Trus t considers this to be the limit of the foreseeable
future for the purposes of our energy advice.
For Carbon Trust customers and use by third parties the model is able to report savings over a period of up to 60 years. Use rs can adjust the maximum useful life by using
the field 'Max Life, User Field' in the 'Main Tool'. For example the technology Boilers Steel has a CIBSE useful of 20 years , however you may know that the boiler will be
replaced in 10 years, therefore you will update the 'Max Life, User Field' to 10. This will adjust the persistence factor ac cordingly.
If the measure is to improve the energy efficiency of a building/system that has a shorter residual life than the recommended technology, the savings should be capped at
the residual life of the building/system.
Inherent degradation reflects wear and tear which is not (or in the case of non-accessible items, cannot be) addressed by maintenance, or the inherent reduction of
performance through time. Inherent degradation is the degradation that occurs before any control inefficiencies are applied e .g. poor maintenance, sub-optimum controls,
poor management and operation etc, and therefore assumes that equipment will be maintained effectively or replaced.
The model assumes that inherent degradation of energy performance at the end of life will be between 0% degradation and 40% d egradation (i.e. 60% energy performance
at end of life) as described in the table below.
Operational degradation reflects the rectifiable deterioration in the energy performance of equipment as a result of use. The degradation rates reflect maintenance policy, the
skill required to rectify performance to approximately the previous level, and the timing of rectification (hence the probabl e period of lower performance prior to rectification).
It is assumed that if reduced efficiency or failure is noticed during maintenance, it is likely to be rectified at the same t ime.
Operational degradation depends on the maintenance regime in place. The model allows for two levels:
• Good practice maintenance – assumed to follow HVCA SFG20 (Standard Maintenance Specification for Building Services) guidance on maintenance schedules and tasks and
their frequency. Good practice should be maintained throughout the measure‟s lifetime.
• Basic maintenance - assumed that limited inspection and / or rectification is undertaken, resulting in errors and inefficienci es which are only corrected on an annual basis
(which is less effective at arresting operational degradation).
Carbon Trust will use basic maintenance assumptions for the purposes of our reporting of impact and effectiveness. This is a conservative approach as we often do not know
how robust an organisation‟s maintenance regime will be, or for how long it will be maintained. However, when the Carbon Trus t advises customers, the good practice
maintenance assumption will be used with the caveat that the lifetime value assumes that good practice maintenance will be ma intained throughout the measure‟s useful life,
highlighting to customers the benefits of good maintenance. Other users of the model may choose to adjust the model according to the maintenance regime in place.
The model assumes maintenance interval periods and annual rates of degradation before rectification as described in the table below:
Degradation Typical Applications
0% Where a minimal loss in energy efficiency is expected, despite degradation of appearance.
Mostly due to corrosion, wear and tear and fouling (e.g. heat transfer surfaces) which are not corrected by
maintenance, reflecting inaccessible or hidden equipment or building fabric where there is no clear feedback
5% mechanism to indicate reduction in efficiency and/or no practical method of arresting degradation
Applied to a larger group of categories, generally sensor drift or component deterioration, e.g. integrity of
Applied to glazing (deterioration due to loss of close fit and therefore heat loss by infiltration) and steam traps
20% (failure mode may be “open”).
40% Applied to swimming pool covers. Deterioration of joints and seals on covers.
degradation Basic Maintenance Good Practice
driver Factor Maintenance Factor Typical Application
Annual rate of Annual rate of Included are adjustments to the user interface and routine
degradation = 4%. degradation = 2%. maintenance, such as software control checks, set points,
Controls Degradation arrested Degradation arrested calibration, time schedules, cleaning of filters and heat transfer
maintenance annually. annually. surfaces, sensor drift and burner adjustments.
Annual rate of Annual rate of
degradation = 4%. degradation = 3%.
Inspection Degradation arrested Degradation arrested Refers to checking by unskilled staff, e.g. time clock settings,
Maintenance annually. quarterly. noisy bearings or worn belts.
Annual rate of Annual rate of Covers aspects of monitoring and targeting, controls and
Retention of degradation = 4%. degradation = 4%. operating systems requiring skilled staff where degradation may
skilled staff/ Degradation arrested 3 Degradation arrested 3 occur when the role is not filled or the incumbent is new to the
retraining yearly. yearly. job.
Annual rate of Annual rate of
Policy/Strategy/ degradation = 2%. degradation = 2%. Business support is required for these to remain effective,
Motivation & Degradation arrested Degradation arrested examples are awareness training and setting, monitoring and
Awareness annually. annually. reporting carbon targets.
Annual rate of Annual rate of
degradation = 0%. No degradation = 0%. No Refers to passive measures and installation of measures where
No deterioration deterioration. deterioration. maintenance has no impact.
Figure 1 illustrates the impact of inherent and operational degradation on the expected carbon savings achievable by a measure over its lifetime. In
Make selections from the white boxes
Technology Group Maintenance Type Good Practice Maintenance
Main Technology Max Life, User Field 60
Sub Technology Discount Rate 3.50%
INVALID SELECTION PLEASE CHECK
FACTOR 1 Inherent FACTOR 2 Operational
Examples of Carbon
Persistence Factor Years Breakdown
Financial PF Financial PF
Years PF Breakdown % Breakdown % Years Breakdown % Breakdown %
1 0.00% 0.00% 44 31 0.00% 0.00%
2 0.00% 0.00% 45 32 0.00% 0.00%
3 0.00% 0.00% 46 33 0.00% 0.00%
4 0.00% 0.00% 47 34 0.00% 0.00%
5 0.00% 0.00% 48 35 0.00% 0.00%
6 0.00% 0.00% 49 36 0.00% 0.00%
7 0.00% 0.00% 50 37 0.00% 0.00%
8 0.00% 0.00% 51 38 0.00% 0.00%
9 0.00% 0.00% 52 39 0.00% 0.00%
10 0.00% 0.00% 53 40 0.00% 0.00%
11 0.00% 0.00% 54 41 0.00% 0.00%
12 0.00% 0.00% 55 42 0.00% 0.00%
13 0.00% 0.00% 56 43 0.00% 0.00%
14 0.00% 0.00% 57 44 0.00% 0.00%
15 0.00% 0.00% 58 45 0.00% 0.00%
16 0.00% 0.00% 59 46 0.00% 0.00%
17 0.00% 0.00% 60 47 0.00% 0.00%
18 0.00% 0.00% 61 48 0.00% 0.00%
19 0.00% 0.00% 62 49 0.00% 0.00%
20 0.00% 0.00% 63 50 0.00% 0.00%
21 0.00% 0.00% 64 51 0.00% 0.00%
22 0.00% 0.00% 65 52 0.00% 0.00%
23 0.00% 0.00% 66 53 0.00% 0.00%
24 0.00% 0.00% 67 54 0.00% 0.00%
25 0.00% 0.00% 68 55 0.00% 0.00%
26 0.00% 0.00% 69 56 0.00% 0.00%
27 0.00% 0.00% 70 57 0.00% 0.00%
28 0.00% 0.00% 71 58 0.00% 0.00%
29 0.00% 0.00% 72 59 0.00% 0.00%
30 0.00% 0.00% 73 60 0.00% 0.00%
31 0.00% 0.00% Source: The Carbon Trust
32 0.00% 0.00%
33 0.00% 0.00%
34 0.00% 0.00%
35 0.00% 0.00%
36 0.00% 0.00%
37 0.00% 0.00%
38 0.00% 0.00%
39 0.00% 0.00%
40 0.00% 0.00%
41 0.00% 0.00%
42 0.00% 0.00%
43 0.00% 0.00%
44 0.00% 0.00%
Persistence Factor Graph
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Source: The Carbon Trust