Solar powered laptops - how much power do they need?

Hi Andy,
I want to run 2 possibly 3 laptops off solar instead of using the power chargers all the time. 2 of them are on about 18 hours a day and all the time off  powerpack. If I've used your calc pages properly for one laptop I would need a 1700 w panel to give 138 amphrs for a 12 v system. Surely this isn't right is it? The chargers / power packs are 19v and 4.74 amps and the laptop is on 18 hrs a day. Would it be better to buy an inverter?

P.S. Just wait, it won't be long before the term "Alternative energy" applies to coal, gas and oil....

Hi Alan,

I think the figure of 19V and about 4.74A (19 x 4.74 totals just under 100W power) is the maximum a charger would be drawing; you might well find that the average power a laptop needs is rather less than that. Though just how much depends on many factors! Presumably the laptops aren't actually being actively used for 18 hours a day - much of the time they will be idle, so if you change the power settings to switch off the screen and power down the hard drive, or go into standby while they are not being used, the consumption will go down dramatically. You could get one of those little electricity meters from Maplin if you wanted to measure their actual consumption.

I do have one customer who has set up a solar powered webserver using a laptop - I think he reckoned on nearer 30W rather than 100W. See the gallery page or his web page.

Having said that, you are right that you would still need a large array if you want to be able to power the system all through the winter with solar panels alone. The fact is that you get very much less power in winter than summer with solar, so to size the system such that you get enough even in the depths of winter means that you do need a big system. I reckon on solar panels rated at 20x the average power consumption. So if you have a 30W load on for 18 hours a day, thats 75% of the time, so an average of 22.5W - to power that fully in winter would need approximately 450W of solar panels. In summer you would get away with not much more than 100W of solar panels per laptop.

Hope that helps! All the best,

Andy

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.