How much energy did the control change save?
Alder Climate Team, a fictional project team, tests new cooling controls in a distribution building.
The question
The existing no-change model uses 300 kWh per open day plus 6 kWh per cooling degree-day. Degree-days measure the demand for cooling. The opening schedule shows 20 open days in each period. The pilot meter total is provisional.
| Previous period | Pilot period | |
|---|---|---|
| Days in period | 20 | 20 |
| Cooling degree-days | 100 | 60 |
| Metered electricity (kWh) | 6600 | 5760 |
Compare electricity use in kWh.
“Compare the meter readings and update the estimate when the records change.”
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A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: What does the meter show?
What would you do here? Open the move you’d make.
Credit the full 840 kWh to the new controls
Tempting, but it misses something. This includes any effect of the cooler pilot period.
Report the meter difference and ask for a comparable baseline
The strong move. The meter shows the total. The baseline estimates what would have happened under the same weather.
Compare the periods only by the number of days
Tempting, but it misses something. Both periods have 20 days. Cooling demand still differs.
See how a strong candidate says it
“The provisional pilot total is 840 kWh lower. The pilot period also needed less cooling, so the meter difference alone cannot tell us how much the controls saved.”
Why it works: The raw comparison uses periods with different cooling demands.
Step 2: What is the comparable baseline?
What would you do here? Open the move you’d make.
Keep the previous meter total as the no-change estimate
Tempting, but it misses something. That uses 100 degree-days, while the pilot had 60.
Use the pilot weather and opening schedule in the model
The strong move. The baseline uses the conditions in which the pilot ran.
Reduce all 6,600 kWh in proportion to degree-days
Tempting, but it misses something. The weather term changes with degree-days. The daily load has its own term.
See how a strong candidate says it
“Using 20 open days and 60 degree-days gives 6,360 kWh without the change. The provisional meter total is 5,760 kWh, so the estimated saving is 600 kWh.”
Why it works: Using pilot weather on both sides removes the difference in weather built into the raw comparison.
Step 3: What changes after the schedule check?
The building was closed on 2 of the 20 days for a planned event unrelated to the project. Each closed day has 100 kWh of modeled load and no cooling term. All 60 degree-days came from the 18 open days.
What would you do here? Open the move you’d make.
Keep 20 open days in the baseline
Tempting, but it misses something. This attributes some of the lower load on closed days to the controls.
Remove the closed days and their load entirely
Tempting, but it misses something. The building still used electricity on both closed days.
Use the closed-day load and keep all 60 degree-days
The strong move. That reflects the actual schedule and its weather term.
See how a strong candidate says it
“I would use 18 open days at 300 kWh and two closed days at 100, plus 360 kWh for cooling. The baseline becomes 5,960 kWh. The provisional saving falls to 200 kWh.”
Why it works: The closure would have occurred without the controls project. Include it in the comparison.
Step 4: Does the corrected meter support savings?
A meter for a load included in the baseline was missing from the pilot total. It adds 240 kWh. Use a planning band of ±120 kWh around the revised baseline.
What would you do here? Open the move you’d make.
Report a confirmed 200 kWh saving
Tempting, but it misses something. This uses the pilot total before the missing meter was added.
Conclude the project definitely raised use by 40 kWh
Tempting, but it misses something. The planning range crosses zero. The point estimate cannot establish the direction.
Update the total and repeat the comparable measurement
The strong move. The baseline and meter have the same boundary, and a repeat can test the uncertain direction.
See how a strong candidate says it
“The complete pilot meter total is 6,000 kWh. Against 5,960 kWh, the estimated saving is −40 kWh. The planning band gives a range from −160 to 80 kWh, so I would leave the direction unresolved.”
Why it works: The meter now covers the same loads as the baseline. The range includes both a saving and an increase.
The answer, in short
- Decision
- Compare the full meter total with the baseline for the same conditions.
- Because
- Weather, opening days and the measurement boundary each change the result.
- What would change it
- A repeated measurement with comparable weather and service could resolve the direction.
Common traps
- Keep fixed daily load separate from cooling demand.
- Use the same meters on both sides.
Check your answer
- I stated the raw difference.
- I adjusted for weather.
- I included the closure.
- I stated the uncertainty around the corrected estimate.
Last reviewed Nov 11, 2026