Calculate a store that holds 65kW of heat
As you only rate me as 5/10 could I suggest you review your statement above?
It seems to demonstrate a lack of understanding of physics!
Tony
It is actually 65kW of energy, but heat is better for plumbers to understand. Will you give us the calcs to work it out, and cylinder size?
I'll help you along. Some basic for you, as you get confused with energy and power. Understand these basics first.
Power
The watt (W) is a unit of Power.
The kilowatt (kW) is simply 1,000 watts. A one-bar 1 kilowatt electric fire or ten 100 watt light bulbs will consume one kilowatt.
BTU/hr is a unit of Power
Energy
Energy is Power x Time.
You pay for energy not power. What you have to pay for is the product of power and time. This is obvious - the electric fire operating for three hours is going to cost three times a much as for one hour. Therefore the chargeable electricity 'unit' is the:
kilowatt-hour (kWh)
This is by tradition in the world of electricity metering just called a
'unit'. What you are paying for is
energy, rather than
power.
kWh is energy
Wh is energy
The BTU is a unit of Energy
The BTU is
not power. Hence the division by time to get back to power. People often speak of say, a "60,000 BTU boiler" - when what they really mean is 60,000 BTU/hr.
One kWh (
energy) is equivalent to 3,412 BTU (energy)
Note: One figure has a time factor and one does not.
A 60,000 BTU/hr (
power) boiler is rated at approx 17.6 kW (
power).
Note: The time factor figures are reversed for power.
Boilers and Car Engines
Although some people think of the watt (a unit of
power) as an electrical unit, it's not restricted to electricity. Boilers, whether powered by natural gas, LPG or oil, and heat emitters (radiators) have
power outputs quoted in watts or kilowatts. So do car engines nowadays.
In days gone by, boilers etc. were rated in British thermal units (BTU or formerly BThU) per hour (BThU/hr), which is
power.
For the engine, horsepower was used, and:
One HP is 746W.
So a 75 kW engine is equivalent to near enough 100 HP.
Gas
That just leaves gas, which nowadays in the UK is charged in kWh (
energy), just like electricity. There is a difference though in that the electricity meter measures kWh directly, whereas the gas meter records the volume of gas used in multiples of 100 cubic feet (or in cubic metres on newer ones). The calculation to get from volume to energy in kWh (
energy) is shown on the gas bill.
The conversion factor is not constant since it involves the calorific value of the fuel, which varies from region to region.
Therm
Again, in the past, gas was charged for by yet another energy unit, the Therm. One therm is simply 100,000 BTU (
energy), equivalent therefore to 29.31 kWh (
energy).
