FORMULAS

FORMULAS

1. ENERGY EFFICIENCY = useful output energy/total input energy

   • Both measured in Joules (J) Does not have any units Always a number between 0 and 1
   • Example ... The efficiency of the Lamp. Energy in =10 J Useful energy out= 1 J So, you do.... 1/ 10=0.1 and that is your answer. REMEMBER it does NOT have any UNITS
   • The energy is turned into a THERMAL energy that SPREADS out

2. SPECIFIC HEAT CAPACITY = Energy/Mass x Temp.rise

3. ENERGY TRANSFERRED (J) = MASS (kg) x SPECIFIC HEAT CAPACITY (c) x TEMP. RISE (0)

   • Example... how much energy is needed to heat 2kg of water from 10 degree celsius to 100? ENERGY NEEDED = 2 x 4200 x 90 = 756,000 J

4. PAYBACK TIME = initial cost/annual saving

   • Example: adding sliver reflectors behind radiators costs £25 and saves £50 per year. Payback Time= 25/ 550 = 0.5 months or 6 months
   • The LOWER the U-value, the BETTER the material is an INSULATOR

5. ENERGY = POWER x TIME

   • Example: A 2200w kettle used for 30 minutes - how much does it cost? Electricity costs 8p per Unit. 2220/1000 = 2.2 kw ---0.5 hrs --- 2.2 kw x 0.5 hrs = 1.1 kwh --- 1.1 kwh = 1.1 units --- 1.1 x 8p = 8.8p
   • STEPS TO DO IT 1) change w to kw 2) change time to hours 3) energy = power x time 4) # of kwh = # of units 5) # of units x cost per unit