In order to ensure that houses in the Energy Park are indeed energy efficient and the degree of their efficiency could be understood by both technical interests and the public the Development Corporation developed the Energy Cost Index (MKECI).

The MKECI enables a house design to be thoroughly and accurately assessed in term of overall energy performance, at the design stage.

The MKECI is assessed using a computer model which evaluates all the energy demands for the house under standard occupancy assumptions. The computer programme does not actually predict what energy will be used in the house since this will depend critically upon how it is eventually used; in exactly the same way mpg figures for cars are given for certain driving conditions (such as a constant 56 mph) which are never realised in practice. Nevertheless the test results, in both the house and the unrealistic driving test, are a useful indicator of performance.

In order to evaluate the house design using the Milton Keynes Energy Cost Index the designer has to specify the areas of the walls, floors and roof, the insulation built into each element, the areas of the windows and weather they are single, double or triple glazed, the house orientation and how overshaded it is, what steps have been taken to draughtstrip the house, what heating system is to be installed, including the controls, how hot water is provided, what provisions are made for cooking and ventilation, weather low energy lights or appliances are included, weather there is a conservatory or sunspace and if so what is its orientation and construction, what type of soil the house is built on, how many bedrooms it has and weather there are wind shelters. It takes a designer about an hour to put together all necessary details from his drawings into the microcomputer used in the evaluation. The calculation of the Index itself takes a few seconds.

The Energy Cost Index is a measure of the total annual fuel costs per square metre of floor area. The fuel costs include heating, water heating, cooking, lights and appliances - these are estimated on the basis of average national rates of consumption so the index reflects the total fuel costs for the house. The final number is an index - in that it is unaffected by general shifts in fuel prices; it is affected by changes in the relative prices of different fuels. By using a figure per square metre the index is independant of the size of the house - though of course the actual fuel bills will be larger the bigger the house.

The Energy Cost Index has been vetted and endorsed by the Building Research Establishment. Also, because every house on the Energy Park and Energy World has to go through this process, people buying houses here can be confident that the very aspect of the design has been checked in an expert fashion.

The Energy Cost Index has a different benifit as well since it is very easy for the designer to make changes to the design on the computer, and to find out weather the change improves the index or not. It takes a short time to evaluate the effect of trying a different type of heating sysytem, a larger conservatory or a differnet type of glass in the windows. This facility has been extensively used by the designers in Energy World. In some cases the process of just specifying all the items for evaluation has encouraged builders to think about aspects of the house that would otherwise be overlooked. But in most cases it has been the process of exploring different options that has led to better designs.

Having determined the nature of the energy performance standard and the way it is assessed, the level had to be determined at which the housde would be allowed to develop in the Energy Park. If the standard was set to high then the improvement in energy performance would not be very significant. If the standard was set to low then it wopuld be to expensive for builders to reach that standard. Somewhere inbetween was the level at which efficiency houses could be built with little or no increase in building costs. Using the experiance gained from the Pennyland and Linford projects it was found a good performance level corresponded to an index of 120, and that this was acheived by the Pennyland houses. A comparison was then made of houses built at the current building regulation levels of various countries assessed using the MKECI. From this analysis it was agreed that the 120 level would enable the Energy Park to stand as an international demonstration project.

Taken from: An International Exhibition of 50 Energy Efficient Houses