nigel your waters hotter because the weathers warmer!your starting temperature in your tank before heating will be higher.....
i always charge my batteries up every night,i only do around 10-15 miles a day though,i also have my van running at lunchtime to top up the batteries a bit for the afternoons work....
hot water systems are great in the warmer months,esp for the sticky pollen,baked on birdmuck,insect spots(little yellow spots) that seem to cover some windows(i clean a lot of windows in the leafy suburbs and some semi rural jobs surrounded by farmers fields)......
my waters around 50-60c at brush end which is plenty hot enough...