Building Services Engineering 5th Edition Handbook

Energy economics 39
EXAMPLE 2.4
A gas-fired central heating and hot-water system is to be installed in a residential property.
The gas tariff is 1.8 p/kWh plus a standing charge of 10 p per day. The estimated annual
heat energy that will be used by the occupants is 165 000 kWh. The annual maintenance
works cost £125.Find the total energy bill and the average cost per kWh, for the year.
From Table 2.2 the overall efficiency of the heating system, η, is 70%.
energy usefully consumed = 165 000 kWh/year
energy paid for at the gas meter = 165 000 × 100
70 kWh/year
= 235 714 kWh/year
The nearest whole number of kWh is the significant number. Do not waste time with too many
decimal places. Fractions of a millimetre, Watt, Pascal or kWh are of little or no significance to
the reality of the calculation.
total cost of energy = 1.8
= £4243
p
kWh
× 235 714
kWh year × £1
100 p
annual standing charge = 365 days
year ×10 p
day × £1
100 p
= £37
total annual energy bill = £4243 + £36 + £125
= £4404
£4404
overall average cost of energy =
235 714 kWh × 100 p
£1
= 1.87 p/kWh
The cost in use, UC, for other fuel or energy sources is calculated from the basic price. These
are pence per litre for oils, £ per tonne for solid fuels and £ per kg refill for liquefied petroleum
gas. The specific gravity of liquid fuel, SG, is used to convert volume to mass measurements.
When the specific gravity of an oil is 0.83, 1 l of the oil weighs 0.83 kg. One litre of water at 4 ◦ C
weighs 1 kg. Oil and paraffin is sold by the litre but its heat content, gross calorific value GCV, is
usually listed as around 45 MJ/kg. For oil
UC = C p 1 × 1l
SG kg × kg
GCV MJ × 100
η × 1kJ
1 kWs × 3600 s
1h
C × 100 × 3600
=
SG × GCV × η × 10 3 p/kWh