CES common mistakes
CES · Understand the traps and how to avoid them.
Treating energy units as interchangeable - converting kWh to kBtu or therms by grabbing a rough factor from memory (e.g., using 3,412 Btu/kWh as if it were 3,600, or confusing kW demand with kWh consumption) and then carrying the error through an entire savings or payback calculation.
The exam mixes utility billing units (kWh, kW, therms, MMBtu, CCF) inside single scenarios, and candidates who memorized one conversion early on tend to reuse it without checking whether the question is asking about energy (kWh) or power (kW). Time pressure pushes them toward mental shortcuts.
What to do instead
Before computing, write down the unit of the answer requested and the unit of every input. Keep a small reference card of exact factors (1 kWh = 3,412 Btu; 1 therm = 100,000 Btu; 1 ton = 12,000 Btu/h) and do a dimensional check on the final number - if the units don't cancel to the requested unit, the arithmetic is wrong regardless of how plausible the number looks.
Building an energy baseline by averaging all twelve months of utility data, including months with unusual occupancy, a plant shutdown, or a one-time meter issue, instead of normalizing for weather and operating conditions.
Averaging feels objective and is fast, and candidates under exam time pressure default to the simplest statistic. They also underestimate how much a single anomalous month skews a twelve-point average and therefore the projected savings.
What to do instead
Practice separating weather-dependent from weather-independent load (e.g., regress monthly consumption against heating or cooling degree days) and excluding documented anomalies with justification. On the exam, look for the answer choice that references normalization or degree-day adjustment rather than a raw average; if a scenario gives a shutdown month, treat it as a flag to normalize, not to average.
Confusing energy efficiency measures with load-shifting or demand-response measures when estimating savings - claiming kWh savings for a measure that only reduces peak kW, or double-counting savings from two measures that interact (e.g., lighting retrofit plus a HVAC schedule change).
Candidates learn measure categories in isolation and then apply them mechanically. Interactive effects are the hardest part of measure stacking, and it is easy to sum individual savings without recognizing that reduced lighting heat lowers cooling load and raises heating load.
What to do instead
For every measure, state explicitly whether it affects kWh, kW, therms, or several, and whether it is weather-dependent. When measures stack, apply an interaction or net-to-gross adjustment and choose the answer that reflects non-additive savings. Study the logic of lighting-to-HVAC interaction so you can estimate direction and rough magnitude rather than guessing.
Misreading a commissioning sequence - accepting a functional performance test result when the system was tested outside design conditions, or signing off on a project where the OPR (owner's project requirements) and BOD (basis of design) were never reconciled.
Candidates memorize commissioning phases (pre-design, design, construction, acceptance, post-acceptance) as a list but do not internalize that each phase has a document deliverable and a verification gate. Exam items describing a plausible-sounding but incomplete process are easy to accept at face value.
What to do instead
Anchor every commissioning question to the document trail: OPR drives BOD, BOD drives design review, design review drives commissioning plan, and functional tests verify against the OPR, not against the contractor's intent. When a scenario omits a document or tests at non-design conditions, the correct answer is usually the one that flags the gap or requires retesting.
Treating measurement and verification (M&V) as an afterthought - selecting an M&V option (A, B, C, or D) without matching it to the measure's risk, cost, and whether savings are weather-dependent or involve complex interactive equipment.
The four IPMVP options are easy to memorize by letter but hard to apply. Candidates pick the most rigorous option reflexively ('D is best') or the cheapest ('A is fine'), ignoring that the correct choice depends on the measure type, stipulated vs. measured savings, and the value of the savings at stake.
What to do instead
Build a decision rule: continuous measurement of whole-facility data suits multi-measure retrofits (Option C); retrofit isolation with continuous measurements suits major equipment where savings are large and weather-dependent (Option B); stipulation is acceptable only where savings are well understood and low-risk (Option A); calibrated simulation is for complex measures with no easy baseline (Option D). Justify the choice in one sentence before answering.
Confusing energy service company (ESCO) contract structures - mixing up shared savings, guaranteed savings, and chauffage/design-build arrangements, or failing to recognize who bears performance risk in each.
The contract types sound similar and candidates focus on the financing language rather than on risk allocation. Exam items often hinge on a single clause about who pays if savings fall short.
What to do instead
Learn each structure by its risk owner: in a guaranteed savings contract the ESCO guarantees a savings level and covers shortfalls; in shared savings the ESCO is paid a percentage of realized savings with less downside protection; in a chauffage arrangement the ESCO takes on operating responsibility and often fuel-price risk. For any scenario, first ask 'who bears the shortfall risk?' and let that drive the answer.
Applying a single sustainability metric (e.g., total kWh reduction or a simple payback) as if it captures environmental performance, ignoring source energy, emissions factors, and scope boundaries.
Candidates default to site energy because it is what the meter shows, and they treat emissions factors as constants rather than grid- and time-dependent values. Scope 1/2/3 boundaries are frequently blurred in exam scenarios.
What to do instead
Distinguish site vs. source energy and know that source energy and emissions depend on the regional grid mix and, for electricity, on when the load occurs. When a question asks about environmental impact, check whether it specifies scope and boundary; if scope 2 (purchased electricity) is intended, use the appropriate grid emissions factor rather than a generic one, and be wary of answers that equate kWh savings with a fixed tonnage of CO2 without stating the factor.
Treating a simple payback period as a complete financial decision metric - rejecting a measure with a long simple payback that has a strong life-cycle cost or net present value, or comparing paybacks across measures with different lifetimes.
Simple payback is the easiest metric to compute and is heavily used in practice, so candidates over-rely on it. They also forget to discount future cash flows or to account for measure life, escalation, and financing cost.
What to do instead
Know the limits of each metric: simple payback ignores time value and measure life; NPV and life-cycle cost capture both; IRR can mislead when comparing projects of different scale. On the exam, when a scenario gives a discount rate, escalation, or measure life, the intended answer almost always uses a discounted method. Compute NPV or LCC when those inputs are present, and reserve simple payback for screening only.
Misreading utility rate structures - applying a flat blended $/kWh rate to a load that actually falls under a demand charge, time-of-use schedule, or ratchet clause, thereby badly over- or under-estimating bill savings.
Candidates see a single 'average rate' in a scenario and use it everywhere, not recognizing that demand charges and ratchets can dominate the bill for load-factor-sensitive facilities. TOU periods and power factor penalties are also commonly overlooked.
What to do instead
Before computing savings, identify which bill components the measure actually affects: energy (kWh by TOU period), demand (kW, including any ratchet that holds the peak for 11–12 months), power factor, and fixed charges. Practice reading a sample bill structure and mapping each measure to the components it shifts. If a measure reduces peak demand, savings must include demand-charge impact, not just energy.
Assuming a rate schedule, tariff rider, or demand-response program eligibility rule applies uniformly, without checking the customer class, load threshold, or enrollment conditions stated in the scenario.
Candidates generalize from one familiar program (e.g., a commercial TOU rate or a demand-response incentive) and apply it to a scenario involving a different customer class or a load below the program threshold. Utility program rules are detailed and easy to skim past.
What to do instead
Read the scenario for customer class, peak demand, and program conditions before selecting an answer, and treat any stated threshold as a hard gate. Study the typical structure of demand-response programs (event limits, baseline method, penalty for non-performance) so you can spot when a scenario's facts disqualify the customer or change the economics.