Measuring Energy and Power

It is important that the plumber has a sound knowledge of the of power and energy to understand the relative sizes of the gains and losses in solar systems and the auxiliray systems to which they are connected.  We have already drawn the disinction between the power of the sun at any instant and the energy that it can provide over a period of time, either day or year.  The calculations below will help you to calculate the amount of energy required to heat a cylinder, the heat losses from pipe work and the equivalent amount of fossil fuel required to heat domestic hot water.  The concepts will help somebody understand domestic hot water systems and will be invaluable in the assessment of the solar contribution to the space heating requirement of a house.

Units of Energy

In 1840 the English scientist James Prescott Joule discovered that heat is a type of energy, he determined the numerical relation between heat and mechanical energy, or the mechanical equivalent of heat by measuring the heat developed through the friction of water stirred by means of a paddle wheel attached to a falling weight.  He found that 772 lbs raised by one foot would raise the temperature of one pound (1 lb) of water a single degree Fahrenheit.  

A Joule is defined as the whe work done by a force of one newton traveling through a distance of one meter;  or roughly 0.1 kg dropping (or being raised) a distance of 1 meter. 

But energy can also be measured in other units;

• kWhrs (units of gas or electricity)
• BOE (Barrels of Oil Equivalent)
• kCal (Calories)

Energy itself exists in many forms.

• Chemical (e.g. Fuel Oil)
• Electrical
• Mechanical

Units of Power

The watt (symbol: W) is equal to one joule of energy per second. It measures a rate of energy conversion. So 1 watt equates to lifting a 0.1 kg weight at 1 meter per second, (or a 100g apple being dropped from 1 meter)   

Power and Energy

Power and energy are frequently confused. Power is the rate at which energy is used (or generated). Power is measured in a unit called a watt which is defined as one joule of energy per second. For example, if a 100 watt light bulb is turned on for one hour, the energy used is 100 watt-hours (W·h) or 0.1 kilowatt-hour.

In other words, 100 watts is the power of the lightbulb, but if left on of one hour it uses 0.1 kilowatt-hours. So kilowatt-hours (kWh) is the measure of energy used. Energy depends on power as well as time.

This same quantity of energy would light a 40-watt bulb for 2.5 hours.
Energy used is the amount of power acting multiplied by the amount of time.

Energy = Power x Time (in seconds)

Using this formula
1000 Watts or 1 kW is the equivalent of converting 1000 Joules every second. 
1 kWh is defined as 1000 Watts acting for 1 hour (3600 seconds), or in other words
1 kWh = 1000 W x 3600 seconds = 3,600,000 Joules