Use one of these lads, great bit of kit!
Can do any job for under 3 quid then 

Bloody hell thats good, what is it?
Its the Karcher window cleaning vac.
A fantastic piece of equipment.
We use them on all our internal work, at £45 its a bargain, however we find the battery is not so great so two options. Either buy about 40 of them and they will last all day with no trouble.
OR as we have done (sneaky I know) we have a set of jump leads 100 feet long connected straight to the van battery.