Financial Times: High quality global journalism requires investment. Please share this article with others using the link below, do not cut & paste the article. See our Ts&Cs and Copyright Policy for more detail. Email ftsales.support@ft.com to buy additional rights. http://www.ft.com/cms/s/0/d66381fc-05dd-11e5-b676-00144feabdc0.html#ixzz3bheclh6l For the wealthy of India, the annual heatwave before the monsoon is inconvenient, especially when water stored in black tanks on the roof becomes too hot to use for a...