This can be easily calculated and uses information that should be readily available on the data sheet for your driver.

PFC (power factor correction) x efficiency gives a number which if you then divide the output power (in this example we will use a 90w light source) by the PFC x eff gives the output VA (watts in principle)

e.g. based on our PLEC-100 range at 230v would be as follows

pfc @ .96 and efficiency of 92% @ 350mA gives a sum of 0.96 x 0.92 = 0.88

Then 90 divided by 0.88 gives 102.27VA (Watts in principle) meaning the power supply/driver has a gear loss of 12.27VA (Watts) by deducting the 90W from the total.

Now consider other products in the marketplace with a pfc of .7 (some are even lower!) and efficiency of 75% giving 0.7 x 0.75 = 0.525

Then the 90W divided by 0.525 gives 171.4VA(watts in principle) meaning the power supply/driver has a gear loss of 81.4VA (Watts) by deducting as above

The difference between only one 100W driver in this instance is 81.4W. When you consider that if a nationwide supermarket installed just one of the latter products in each store (with 2482 stores) for 16 hrs a day the additional energy used by the less efficient competitor product would equate to 202kWh. At 10p per kWh this would be a staggering 3232kWh or £323.25 per day so it is easy to see why this is an important factor when choosing a product.