Energy Facts & Figures


Assumptions, Definitions, and Limitations was conceived and devised as a website to advance awareness and knowledge of the economic and environmental impact of our inefficient houses. As such, it has been developed as a source of free, unbiased information for Homeowners wishing to make their houses healthier and more efficient. It is intended to be used as a guide and for interest only.


Rather than attempt to include a large array of building types and sizes that accurately represents the UK housing stock, or rely on people to perform the rather laborious task of measuring their own houses, it was decided to base the calculations on three typical house types in the UK, of three typical ages and average sizes, and typical energy usage.
We therefore settled on the following:
Type (UK%) Terraced House (30%) Semi-Detached (28%) Detached (20%)
Age (UK%) 1900-1920 (25%) 1930-50 (35%) 1970-80 (20%)
Size 75m2 95m2 135m2
EPC E Rating D Rating C Rating
kWh/m2 300 200 100
kWh/year 22,500 19,000 13,500
It was further assumed that the houses were of the typical construction of their time, which informed u-values (see below).
The number of occupants does not affect how much energy is required for space heating, but does affect how much is required for hot water. Hot water usage is based on UK average volumes for showers, baths, washing up, and general washing, (40, 80, 10, 10 litres respectively – DEFRA/Energy Savings Trust) and an assumption of weekly usage per person. 
All typical UK fuel types are available to be chosen; the option sets the respective CO2 and £/kWh rates – see conversion figures below.
Boiler age determines the boiler efficiency, which in turn has a great impact on overall energy use. The following efficiency rates are used, generalised from the Sedbuk boiler efficiency database:
< 5 years 10 years 15 years 20 years 25 years 30 years 35 years  > 40 years
90% 85% 80% 75% 70% 65% 60% 55%

Electricity Consumption

The calculator does not investigate electricity consumption since its usage is almost entirely down to personal choice and lifestyle, and any attempt at an average would be essentially meaningless. 
Electricity usage is simple to ascertain for oneself however – simply walk around your home and jot down every single electrical device, appliance, and light that you use regularly, make a note of its power rating in kilowatts (so 50 watts would be 0.05 etc), and estimate on average each day/week/month how long each device is used for, and then convert to yearly usage. Then simply multiply the power rating by the yearly hours and tot it all up – you should have a figure in kilowatt hours (kWh) close to your yearly electricity bill.
This will then enable you to see which are the really power hungry devices in your home and therefore which to turn off or replace!

Conversion Figures

Every fuel type has a different ‘footprint’ - that is to say it is more or less likely to add to climate change - and this is measured in kg of carbon dioxide (or equivalent) per kilowatt hour (kgCO2e/kWh). It may also be necessary to measure pounds per kilowatt hour (£/kWh), kilowatt hours per kilogram of fuel (kWh/kg) or kilowatt hours per meter cubed of fuel (kWh/m3).
Such conversion factors for every type of typical fuel source are revised yearly and given by DEFRA at their Greenhouse Gas Conversion Factor Repository, here:
Note: ‘equivalent carbon dioxide’ (CO2e), includes the global warming effects of other gases in the production and/or combustion of the fuel type, such as methane and nitrous oxide.

Heat Loss Calculations

In order to compute the bulk of energy used by the houses, heat loss calculations are necessary. These use average external temperatures in the form of Heating Degree Days (HDD) and u-values for all of the building’s elements in order to work out how much energy is required to keep the house at the required temperature throughout the year, taking into account the efficiency of the heating system and ventilation losses.
Heating Degree Days are calculated using local weather data to give the amount of days heating is required over a given temperature in each month. Although there is bound to be a big difference in HDDs between the North of England and Cornwall, we have kept the calculator simple by using a UK ‘average’ set of data from the East Midlands weather station (ID EGNX) and have averaged data over 3 years, from 2009-11.
Base desired internal temperature is set at 15.5 (HDDs are calculated for those times cooler than this) – this assumes that the desired temperature is about 20° and that the difference is met by solar gains, body heat, and heat from cooking etc.
The weather data was downloaded from
The u-values (w/m2K) are either given by Building Regs Part L Appendix A, or calculated using published thermal conductivity figures (k values - w/mK). Some u-values are aggregates/averages since there is no definitive u-value for a Victorian brick or slate tile. Similarly, insulation materials can vary widely, and we have kept the Calculator simple by using an average k-value of 0.035.
U-values for windows have similarly been aggregated using examples taken from a range of sources including Building Regulations, English Heritage, the Building Research Establishment (BRE), and the Energy Savings Trust.
Ventilation Losses have been worked out using a simplified version of the Standard Assessment Procedure (SAP) calculation method, utilised by Building Regulations Part L.


As with the rest of the website, the figures supplied for renewables are based on averages at best, and it is always prudent to get a specific feasibility study carried out by a professional on your property before going ahead with the purchase of renewables – unless of course you’re making them yourself!
Biomass boilers may be fed by log, chip, or pellet – typical calorific values were gained from the Biomass Energy Centre. Conversion figures were gathered from the DEFRA website as above.
The wood stoves ratings used vary depending upon the size of house – 5kw for terraced, 7.5kw for semi, 10kw for detached.
Solar thermal output figures are based on a UK industry averaged 1.3kWh per m2 per day. In reality, location, direction, and panel type could dramatically change this figure. 
Similarly, Solar PV figures are based on an averaged 0.34kWh per m2 per day. The potential income of PV does not include the savings made to your electricity bill.
Since Micro CHP is in its infancy and currently only 1 boiler is passed for FiTs, we have used the printed figures for Baxi Ecogen for our calculations.


All costs are to be used for budget purposes only – they are based on rounded-up, rough averages that include an element for supply and fitting.
Insulation costs allow for other associated costs such as scaffolding and moving drainpipes for example. Renewables costs also allow for associated costs, such as chimney lining in the case of wood stoves.
Before instructing anyone to start work on your property, ideally make sure you have 3 quotes and that you have seen examples of their work.

Payments & Funding

Both Feed-in Tariffs (FiTs) and Renewable Heat Incentive rates (RHI) are based on figures published by DECC. Figures for typical domestic installations have been used:
Wood Stove 0.122
Solar HR 0.192
PV 0.134
Micro CHP 0.129
Eligibility to apply for Energy Company Obligation (ECO) grants are as set out by the Department of Energy & Climate Change. The actual grant totals for each item are calculated using formula which incorporates the ins and outs of the ECO scheme, based on potential savings. In-Use Factors and Lifetimes are published by DECC.
Potential Green Deal Loan figures are based on maximum first year repayments, which are in turn the total financial savings made by introducing the Further Improvements. Green Deal Loan calculations are based upon the actual equations.



The Information Pages and Factsheets have been custom written for Where there have been knowledge gaps, or information has required verification, the following sources have been consulted:
The main source of knowledge, both from within the author's brain and for clarification and revision purposes, comes from lectures and notes as attended/received as part of  the Centre for Alternative Technology’s Graduate School of the Environment courses, both MSc Architecture: Advanced Environmental and Energy Studies, and Professional Diploma in Architecture: Advanced Environmental and Energy Studies. 
Otherwise, we have used a wide range of websites - too many to list in full. Below are those we found most useful:
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