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3. Understand Power and Energy: watts, BTU/hr, ton of refrigeration watthours, BTU, therm, decatherm
Quick Terms:
Terms in depth:
Power vs Energy  Energy is what is delivers, power is the rate at which it is delivered. A story – a rock climber and a hiker start for the mountain peak at the same time but the rock climber finishes faster. What is the difference? Answer: They have done the SAME amount of WORK but the difference is the amount of TIME it takes them to complete the climb. The same is true for the difference between power vs energy. Power is energy divided by time. Power = work/time Converting power to energy requires you estimate the time usage. Question  Find the electrical use of a furnace using 800 watts of power. Step 1  Convert power to energy by finding the TIME the furnace runs, which will be given on the test or in a home you ask the homeowner or make assumptions. Assume 4 hrs/day run time for a 90day heating season Step 2  800 watts x 4 hrs/day x 90 days = 288,000 watthrs, or 288 kWh 1. watt  energy is commonly reported as kilowatts, kW, and is equal to 1000 watts. Solar panels are expressed as a 5 kW system, which is a unit of power. If you want to know how much energy a 5 kW system will produce, you need to multiply it by the time that it is on and its efficiency. 2. BTU/hr  is a unit of power (power always has time attached to it). It's brother, a BTU, is a unit of energy.
3. Ton of refrigeration  a measure of the amount of heat removed. 1 ton of refrigeration = 12,000 BTU/hr = 3,517 watts.
4. Watthours  unit of energy, found by multiplying a watt by one hour (or 60 minutes/hr x 60 seconds/min if you want to convert it to hours).
5. BTU  a unit of energy and stands for British Thermal Unit. 1 BTU is equal to the energy needed to raise 1 lb of water 1 degree F. If sloped drainage is the basis of all infrastructure design, then a BTU is the mother of all the ways to express energy. It is the most common unit used to compare applestoapples for fuel sources such as therms of gas, CCF of gas, kWh of electricity, gallons of gas or pounds of hydrogen.
Why should you care? Well you can calculate and tell a homeowner if it is cost effective to switch from a furnace to a heat pump. You can compare the cost savings of different electric, gas, solar or hybrid water heaters. 6. Therm  a unit of heat equal to 100,000 BTUs
7. Decatherm  a unit of heat equal to 1,000,000 BTUs
Next Section
1b. Principals of energy, air & moisture
1c. Combustion science
