percentages beware: we do math


Recently I was talking to a colleague about the pervasive use of the “percentage”.  You will have seen the advertising offering up to 30% energy savings ,  and no doubt you will have seen NGO’s giving awards for 25% percent energy savings!

It’s a great marketing, but not science.

  1. When a system or building is performing poorly,  it’s a simple matter to find one time savings, try to find another 30% saving in the second year!
  2. Good data is critical, don’t compare apples with pears.  For example, if building operation in year one 24/7, and in year two it is only 9-5. Just comparing the operating annual cost, magically you have made tremendous energy savings. Quoting these raw cost savings, without a good  metric or benchmark is quick, easy and misleading.  For systems influenced by the weather  i.e.  air conditioning and winter heating,  adjusting the data using degree days takes account of weather variations.
  3. Benchmark  – comparing your system, or annual operation cost to an industry standard is a meaningful, however finding a benchmark can be difficult indeed.

Some advisers that can’t do the math to quantifying the energy and cost savings,  so revert to percentage generalisations – Don’t try fool your client –  give data not generalisations.

Pitching your cost saving solution is not always easy, simply stating an energy saving is often dry, and frankly difficult for the majority of people to grasp.  Use an illustrative comparison that can be easily visualised.

For example, our proposal will save xxxx $ –  that’s the same as saving as driving 100,000 km per year.  Choose a comparison that can be readily identified, so your manager can see an image and the savings.

– John A. Herbert, consultant

About John A. Herbert, REA, energy consultant, BEAM Pro

John is director of energyLAB energy consultants firm, he has with more than 30 years engineering experience gained across three continents. He is Registered Energy Assessor, a member of HKAEE. Read his blog at
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