It's taken some time for the light to dawn: pumping water with 12 volt batteries is an expensive waste-of-time. I've been through several pumps and lugged several generations of heavy lead-acid batteries across the field and finally accepted the blindingly obvious: a mains powered pump is a much easier and more reliable solution to keeping the rainwater tanks (for polytunnel drip irrigation) full. Originally I'd intended to power the whole system by wind or photovoltaics but the expense and complications of doing this meant that I never did it.
So I bit the bullet last week and unrolled a long coil of cable - the run is about 80 metres - and now have an almost-silent submersible pump which pumps the rainwater collection ponds in about one tenth the time of the 12 volt pump (which was twice the price), its predecessor. It also automatically switches itself off with a floating switch. Now I can relax in front of my widescreen telly...
Moral: Avoiding using grid electricity for 'green' reasons can often be a much bigger waste of resources, not to mention a very big waste of money.
