A lot of the time, the idling you may hear next to a truck that's parked up on break may actually be a refrigeration unit. Most fleets have telematics that would give a driver a red flag if they idled excessively, a lot of trucks even turn themselves off (although it's easy to bypass).

On the market, there's battery electric APU's that drive AC units, like: https://voltaair.com/product/iq-no-idle-ac-unit-for-trucks/#...

But unless they are mandated, an idling truck uses <1 litre per hour, assuming AC over summer months only (as they will have a webasto style diesel heater for the winter), results in about $1,000 per year in idle fuel. Assuming a fleet has their vehicles on a 3 year rotation, the APU needs to be $3k or less, with zero failures for it to be economically worthwhile. (additionally, the charge for those batteries needs to come from somewhere, you can do retarder/brake enabled alternator charging which would help though).

So unless mandated by carbon emissions targets, the lower risk option is just to burn diesel.

However, at around 1k litres per year, that's around 2.5tonnes of CO2, so if cost isn't a lever that can be pulled to change this behaviour, CO2 may be.