2004 May 19
RE 29846
e-mail 29846 reports that gas is billed in kWh in the UK.
This is bad news. In the US kWh is restricted to electricity only.
I quote from SI 10-2002 American National Standard for Use of the
International System of Units (SI).
3.4.5.1 Energy
The kilowatthour is widely used as a measure of
electrical energy, but this unit shall not be
introduced into any new fields.
The unit to use is kilojoule or sometimes megajoule.
Note that gas energy is billed for its higher heating value. That is
the energy delivered if all the water vapor is condensed to get out
the heat of vaporization. Residential furnaces do not condense. If
they did the metal parts would corrode.
The 2001 ASHRAE Handbook Fundamentals lists the energy of several
substances on page 18.3
methane higher heating value 55.533 MJ/kg
lower heating value 49.997 MJ/kg
so as you compare the cost to heat a house with gas or electricity
you get only 90% of the gas heat billed.
In Colorado gas is billed in therms. The PUC and the gas company
do a good job of measuring the energy. Both make audits of the
energy content, correcting for temperature and barometric pressure.
Both audits use a gas chromatograph to measure how much of each
substance is in the gas. The gas is measured each month giving a
variable "Therm Multiplier" listed on the bill. For April the
Therm Multiplier was 0.8446 which is multiplied by the number of
cubic feet shown by the meter.
This was started 7 years ago because a mountain resident said he
was being cheated because gas was delivered at low pressure. He
was right so the PUC said to correct for density.
The billing is technically correct but they do not use joule.
The FERC writes all its rules in BTU so Colorado uses BTU (from
which therm is derived).
Robert Bushnell PhD PE
