Can anyone lend me a charger?

 "We will make electricity so cheap that only the rich will burn candles." - Thomas Edison

Derrick's advice to get a Raspberry Pi was inspired. Or, depending on your level of technical knowledge, utterly obvious. I could attach the Pi's output pins to some sort of valve, set the thing up as a web client or server, and put it outside in a waterproof box.

To start off with I needed power. As I can't run any cables from the house to the garden, I expect it will have to run on batteries, or some sort of battery/solar combination.

I did some back-of-the-envelope calculations. The most efficient Raspberry Pi, the model A+, uses 1 watt. Let's assume a wi-fi dongle uses 200mW (I have no idea what it actually uses). So the whole setup would draw 1.2W. That's 28.8Wh per day.

• 4 x AA NiMH (2000 mAH) batteries = 10Wh
  Life: 8h 20 minutes
• 4 x D NiMH (10,000 mAh) batteries = 50Wh
  Life: 1 day 17h 40 minutes
• 1 x lead acid golf battery (12V / 22Ah) = 264Wh
  Life: 9 days 4 hours
• Massive lead acid battery (12V / 220Ah) = 2640Wh
  Life: 3 months 1 day 16 hours
• Solar power (summer's day = 4h sunshine equivalent average). Need at least a 7.2W solar panel, ideally much more. This costs £30-£70. I'd also need a large amount of battery storage for nights and cloudy days.

Conclusion: a Raspberry Pi uses too much power. Even more than an iPhone, which hitherto I had believed impossible.

How, then, can I use less? This article describes a system called the “sleepy pi”. It uses a second device, an Arduino, to periodically wake up the Pi to perform a task, and then put it back to sleep. The Arduino uses less power to begin with, and unlike the Pi, has its own sleep mode.

So the Pi could wake up, poll a web server for instructions, and then decide whether to turn on the hose, and for how long. Perhaps it only needs to wake up every hour. You don't need hair-trigger response times on a hosepipe, except to spray an enemy cat.