I agree with your comparison with a road digger. These 12V units are a good idea but, in practice their use is fairly limited. Years ago I bought a s/h diaphragm compressor, ie, it has no piston but an eccentric operated diaphragm which pumps up to around 40psi. It has enough volume to operate my small spray gun and has enough pressure & volume to inflate a newly installed tubeless tyre. The 12 V units have nowhere enough volume to do that. I see, on Ebay, there are currently two secondhand diaphragm units being offered at around £30/40. Also, there are many small piston types which are more than adequate for home use and some of these are less than £75