Annual Energy Saving (£s)
4 Step Calculation
- All values are based on a 12 month period
- Cost for 1kW of Electricity = £0.215
- 1kW = 1000watts (60W = 0.06kW)
- (60W lamp running for 1hour uses 0.06kW of power)
- Average yearly run time = 2500 hours
- Labour cost to change a lamp = £0.00
Step 1:
Total cost to run a lamp = [wattage(kW) x average yearly run time(hrs) x cost for 1kW (£)]
Example 60watt lamp
Cost (£) = (0.06 x 2500 x 0.215)
Cost (£) = 10.80
Step 2:
Total cost of lamp replacements = (number of lamps used x cost of lamp)
Number of lamps used = [average yearly run time(hrs) / lifetime of bulb(hrs)]
Example 1000hr £0.15 lamp
Cost of Lamps used = [(2500/1000) x 0.15]
Cost of Lamps used = £0.51
Step 3:
Total cost of labour = (labour cost to replace x no. of bulbs used)
Example 3 lamps used
Cost of labour = (0 x 3)
Cost of labour = £6.00
Step 4:
Total cost of Lamp for 1 year = [(Total cost to run) + (Total cost replacements) + (Total cost labour)]
Accountants View
Example 1
Cost of replacement lamp = £3.64
Lifetime of lamp = 12000 hrs
Average yearly run time = 2500 hrs
| Year |
Cost £’s (equation) |
Cost £’s (actual) |
| 1 |
0.91 |
3.64 |
| 2 |
0.91 |
0.0 |
| 3 |
0.91 |
0.0 |
| 4 |
0.91 |
0.0 |
In step 2 we worked out that;
Number of lamps used = [average yearly run time(hrs) / lifetime of bulb(hrs)]
For the 12000 hr lamp; (2500/12000) = 0.25
For that year the total spent on replacement lamps, will be ¼ of
the full price.
This allows the lifetime of the bulb to be incorporated into the cost
equation.
Accountants would say the money going out in year 1 is £3.64 and
£0.00 for the next 3 years. The expenditure for year 1 is therefore
higher than years 2, 3 and 4.
If the calculation for energy saving is done
from the accountants view, then over a 12 month period (where a new lamp
was installed in month 1) the saving is less.
Carbon Footprint - (emission of CO2 gas)
1kW (electricity produced) = 0.43kg CO2 (Defra 2007)
CO2 emissions must be in metric tonnes
1 metric tonne = 1000 kg
Total output of CO2 for 1 year = (annual kilowatt hours x 0.00043)
Annual kilowatt hours = wattage (kW) x average yearly run time
e.g. 60watt Lamp (0.06 x 2500 = 180kW)
(180 x 0.00043 = 0.0774 tonnes / year)
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