Powering 5V remote instrumentation

I'm looking for a solar-powered power supply for some instrumentation. Needs to provide 2.5W at 5v continuously (ie 24/7/365 and including midwinter) initially at sites in Southern England. Something like 99.0-99.5% (ie but <100% is acceptable) availability would be the target. We're looking for the most cost-effective solution to this requirement. Thanks, John.

In order to size your system to provide enough power in the middle of winter even on overcast days, it's best to work on a rule of thumb of a panel array of about 20x the continuous power draw of the instruments. So, for 2.5W continuous load, I would recommend a solar panel of about 50W. Obviously in the summer that will be giving you a power output way in excess of what you need - but you do unfortunately need that extra power to make the most of short winter days when the panel output will be much lower than rated.

Although you require only 5V, solar panels are almost invariably designed to charge 12V lead acid batteries, and your best bet is definately to use a conventional solar panel - regulator - lead acid battery set-up, and use a DC-DC converter to convert the 12V back down to 5V for your instrument.

The most cost effective solution would be to use our 50W solar panel kit in conjunction with a lead acid battery and DC-DC converter. You would be using about 5Ah per day, but in practice you want a battery with considerably bigger capacity as it's bad practice to discharge a battery heavily, and to ensure enough capacity for say a week of overcast conditions. I would suggest perhaps 40-50Ah as a suitable size of battery - a maintenance-free sealed lead acid type is probably best for remote instrumentation.