Kilowatt-hour per Kilogram as a Monthly Measurement Metric

Many novice plastics processing sites use kWh/kg as a monthly measurement metric and wonder why it varies uncontrollably and by so much. They will generally be forced to use kWh/kg by the “head office” or by an accountant who does not understand the process.

Using kWh/kg as a monthly measurement metric to assess performance in plastics processing is a fatally flawed concept and only works when the production volume is constant each month. The moment production volume varies, then kWh/kg will vary with the production volume. As a metric it’s only suitable for use by people who think that everything can be measured by a single number (and can only hold one number in their head at a time). Real manufacturing is a little bit more complicated than that.

At most plastics processing sites with a reasonable base load, increasing the production volume will decrease the kWh/kg, and decreasing the production volume will increase the kWh/kg. This is the result of amortizing the fixed base load into an increasing or decreasing variable process load. It has nothing to do with increasing or decreasing energy efficiency. It’s simple mathematics.

Actions:

Never, ever use kWh/kg as a monthly measurement metric. It’s affected by production volume and will vary with production volume.
If you must use kWh/kg (and I strongly recommend that you don’t) then at least take a rolling average over a minimum of six months. This is still flawed, but less flawed than a monthly measure, because the effect of changing production volume will be reduced.
Try plotting kWh/kg on the same graph as production volume; in most cases you will find they are a mirror image of one another.

Note: Dr. Robin Kent is the author of Energy Management in Plastics Processing, published by Plastics Information Direct, and managing director of Tangram Technology Ltd., consulting engineers specializing in energy management in plastics processing. rkent@tangram.co.uk.