i have been watching this invention for over a year now, no batterys, or hand pumping, with interest, 4 gallons tanks, and i must say they are getting bigger and stronger, believe it or not, this trolley can be used for domestic window cleaning as it is now, i predict that in 2 years from now, this trolley will be in use with wfp, the price i can buy it now is roughly £40, not bad for portable domestic work