To compare energy choices, state the criterion, use the same energy demand for every option, then calculate with the supplied values. Only then pick an option, and say what the choice does not cover.
This lesson is part of SPM Science green technology. It builds on classifying green-technology applications, because each option has a purpose.
What do I do before calculating?
Write the criterion and the assumptions first. Without them, the numbers can be used to support any choice.
- State the criterion, for example lowest emissions while running.
- State the assumptions, for example the same demand of 100 kWh a day for each option.
- Choose the columns that match the criterion.
- Calculate, then compare.
Worked example: a school power choice
The data below are invented for practice. A school needs 100 kWh of energy each day. Only emissions during use are counted, and the demand is the same for each option.
| Option | Emissions (kg CO₂ per kWh) | Running cost (RM per kWh) | Installation (RM) |
|---|---|---|---|
| Diesel generator | 0.80 | 0.90 | 8 000 |
| Grid electricity | 0.60 | 0.45 | 0 |
| Solar panels | 0.00 | 0.05 | 60 000 |
Emissions per day. Diesel: 100 × 0.80 = 80 kg. Grid: 100 × 0.60 = 60 kg. Solar: 100 × 0 = 0 kg.
Running cost per day. Diesel: 100 × 0.90 = RM90. Grid: 100 × 0.45 = RM45. Solar: 100 × 0.05 = RM5.
If the criterion is lowest emissions while running, solar is best, with 80 − 0 = 80 kg less per day than diesel, which is 80 × 30 = 2 400 kg over 30 days.
What about payback?
Payback compares the starting cost with the running saving over the current choice. Against grid electricity, solar saves RM45 − RM5 = RM40 a day.
Payback time is 60 000 ÷ 40 = 1 500 days, which is about 4.1 years. This assumes the same demand every day and that the panels supply all of it, which a real school would need to check, because solar output changes with the weather.
A careful conclusion names the limit: “Solar panels give the lowest emissions, but they need a high starting cost and may not supply power at night.”
The mistake that costs marks
The usual slip is to choose by whichever column looks most impressive, then ignore the criterion. A student writes “solar is best because it costs only RM5 a day to run”, which ignores the RM60 000 to install.
| Step | Wrong | Right |
|---|---|---|
| Choosing the column | The one with the smallest number | The one that matches the stated criterion |
| Assumptions | Left out | Same demand, running emissions only |
| Conclusion | Solar is best | Solar is best for running emissions, with limits |
The fix is to reread the question’s criterion after you calculate, and check that your answer responds to it.
Check yourself
Using the same table, a clinic needs 50 kWh a day. Find the running emissions per day for the diesel generator and for solar, and state how much is saved by using solar.
Answer
Diesel: 50 × 0.80 = 40 kg. Solar: 50 × 0 = 0 kg.
The saving is 40 − 0 = 40 kg of CO₂ a day. The assumption is that solar supplies all 50 kWh and only running emissions are counted.
What to study next
Next, see how conservation fits in: explaining resource conservation. To judge a claim with a criterion, use evaluating claims using a stated criterion.
The graph evidence and fair-comparison lab gives more data practice. If you want a teacher to go through your reasoning, see online one-to-one Science tuition.