Building Energy calculator


Using your own data

To accurately calculate how much energy in the home you are using annually, use actual fuel bills. The Excel form BuildingEnergyCalculator-2015 allows you to input your own data, taken from your own fuel bills. Click on this form and fill in the six empty boxes, A, B, C, D, E and F with your data. If you want to see a completed form for my house, click on Copy of BuildingEnergyUse-L-Pringle-2014.

Box A: – Input the year that the energy use figures relate to and your contact email, for reference, if you intend to submit your calculation to add to our data base.

Box B: – Input your daily standing charges for electricity, gas and LPG here, in £/day.

Box C: – Input the area of building here. This is the floor area, the width of the building multiplied by its breadth, times the number of floors that have living space.

Box D: – Input your annual use of electricity and fuels, and their cost per unit in pounds Stirling (e.g. 1kWh electricity is 14 p, so cost per unit is £0.14). Natural gas use in bills from energy companies are normally quoted in kWh, but if you are using meter readings, multiply cubic metres (m3) by 11.03 to convert energy use to kWh.

If you use logs, woodchips, etc., where you do not know how much you have used, weigh what you use over a week or two and use these figures for the period that you operate the stove. Technically, wood products are neutral polluters, in that they absorb CO2 as they grow, which is then released when they are burnt. If you just want to estimate your carbon footprint, wood used can be ignored. This will ignore any fuel used in the transport of the wood.

If you have a tank of heating oil, next time you get a fill, fix a wooden stick to the sight glass, with its bottom end level with the oil level. After the fill, mark the new level of the oil on the stick. The height of the mark up the stick is equivalent to the size of the fill. The stick can then be calibrated, with the bottom end of the stick being moved to a position level with the lowest level the oil is allowed to fall to. This stick can then be used to measure heating oil use in future.

Boxes E & F: – Input the details of your building here. This is only required if you are sending a copy of your calculation back to me This will help me compile a list of actual energy use in buildings, which will let people see how their own energy use per m2 compares with others.

The energy performance certificate (EPC) rating is only applicable if you have had your building assessed using the rdSAP system of assessment. As the assessor will only give credit for insulation that they can see, no credit is given for insulation hidden behind plasterboard. This ridiculous situation means that EPCs for refurbished buildings are virtually worthless, but are insisted on by government where buildings are to be rented out or require this assessment for some other reason.

The air leakage, measured in airchanges per hour, is the figure from a pressure test using a fan mounted in a doorway, to see how leaky buildings are to wind ingress.

Unprotecting the form

When you have downloaded the form, you can unprotect all the calculations by going to the “review tab” and clicking the “unprotect” icon if you want to alter anything. This is risky in that you can mess up the calculations if you do not know what you are doing. If you do mess things up, just download a new form.

Comparing my “Primary Energy Indicator” with others

The official rdSAP EPC “primary energy indicator (PEI)” for my house  EPC MyHouse-15, with a floor area of 143 m2, is 321 kWh/m2/year, while the actual PEI, based  on my own meter readings and solid fuel weighings for 2014 was 185 kWh/m2/year. The official EPC is therefore 73% too high!

The leakage rate for my house is 7 airchanges per hour. This relatively high rate is considerably  better than when we moved into the house in 2006 and was achieved by a huge effort to seal windows, chimneys, skirting boards, holes behind kitchen units and cupboards and service inlets.

Using Home Energy Scotland (Energy Savings Trust)

By phoning 0808 808 2282, the Scottish Government’s Home Energy helpline, you can give them details of your home – insulation, heating system, zoning, etc- and they will put this data into their computer program and come out with what they predict will be your spending in energy and what improvements you can make. They will also give your house an estimated energy rating A-G. What is good about this service is that if you say you have put 75 mm of insulation behind your plasterboard, unlike a survey for an EPC, they will believe you and estimate your energy bills based on this information. The phone call takes half an hour, gives you an immediate predicted energy use and cost and is followed up with a letter summarising the data and recommendations. It allows you to know if you are as efficient as you can be or whether you should modify your building to improve its efficiency. For my house their estimated fuel bill was £1367 per annum, while my actual bill was £1204. They worked out the house was a C compared to the EPC being a D. Sadly, if we ever sell our house, the EPC is the rating the potential buyers will see.