Just refitted a large consumer unit with Wylex miniature RCBOs.  Looks like a decent compact product.  When energised on no load the metal CU casing ran about 5 degrees C above immediate surrounding ambient (in a very large cupboard).  Wylex spec says full load losses are 1.5 to 2.6W per pole and these are two pole devices.  I think this would break down between no load loss and a loss that varied with current (along the lines of copper and iron losses with a t/f).

Given the temperature rise I observed (which so far as I can tell with my cheap IR thermometer) is uniform across 20 devices the standing no loss dissipation is likely around 0.5W plus each.  Whilst this will keep consumer units warm it will also add to bills.  A 20 device CU with 0.5W per device will use 87kWh costing about £13 pa.  

I haven't got the kit to measure the losses accurately but this is certainly something to think about when specifying larger installations.

Looking across other manufacturers I see Hager quoting 3W for a single pole RCBO at full load so the Wylex unit does well by comparison.

Has anyone measured the standing losses on these and other 'electronic' devices? 

I expect we will see a new section in the Regs on losses before long given the increasing focus on energy efficiency.