i reckon i save around 30 mins a day using hot water....obviously it cuts through the snail trails/bee/pollen spots and baked on bird muck easier esp in this weather(my water is at 50-60c!)....
add on jobs like fascias/roofs etc then you save even more time compared to cold water....
thats not the main reason i invested in a grippatank hydroheat diesel heater though...
the main reason is i dont have to worry about anything freezing during the cold winter months as my system has " frost stat" as standard(brilliant!) and i prefer to work with hot water all year round.its more comfortable and hoses are warm and easier to manage as well as the extra cleaning benefits(easier scrubbing/rinsing/quicker drying, etc)....
ive used gas heaters on and off for the last few years but i had a gas leak(loud bang!) which scared me to death!also they easily break in cold weather if you forget to drain the water out of them on a cold night(and forget to bung a heater in the back!)which i found out one cold december day....i also got sick of changing gas bottles...and i always seemed to run out of gas on a filthy add on job OR id be cleaning one on a windy day and the flame would keep going out(very annoying!)......
their not cheap and i wouldnt get one installed in a second hand van thats say 5 or 6 years old but a brand new one,yes (that your planning on keeping for 10+years).....
diesel heaters are much better(safer and more reliable heat) than cheap gas showers but their also a lot more expensive.....but i dont regret getting mine.....