How to power computers from solar panels?

The garden office has got one entirely unused feature — a large roof! What better place to mount solar panels for an off-grid power supply …

These Sharp PV modules (ND-R245-A6) [PDF datasheet] are manufactured in Wrexham and (although pricing is very opaque) appear to cost around £250 per module. I would need three or four. Each module produces a peak of 30V, 8A (= 240W) although in reality I’d guess the voltage would vary wildly from 0-30V.

Inverters, the part which normally converts the DC and highly variable power from the modules to mains AC voltage, are expensive, costing likely as much or more than the panels themselves. As this is an “off-grid” installation, this is useless. It would seem to be better to use the power to charge car batteries.

The car batteries provide the smoothed 12 V supply which would be used to run servers and lighting.

So there seem to be two problems in the way: (1) How to charge the batteries. I think I need something like this? (2) How to power servers directly off 12V …

Update

As always, it becomes much clearer once you have the key search term, and the word in this case is caravan. Apparently people who own caravans, static caravans, narrow boats and so on are in precisely this situation, and there are special purpose charge controllers used for this (made by Steca, amongst others).

Here and here [both PDF] are two excellent starter guides for the self-builder.

Now all I need to do is to size my requirements and work out how to power a server off a 12 or 24V supply.

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6 Comments

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6 responses to “How to power computers from solar panels?

  1. Phil

    Whatever you do, read the panel datasheet. There is a lot more to it than just picking panels, batteries and inverters.

    RS have a good general purpose solar panel data sheet which gives considerations which should be taken into account. You first need a detailed look at your power requirements, then cost your installation and how long it will take to recover the investment. Have a look at Eric Sandeen’s (I think) blog which goes into lots of specifics. (His is a grid tied system, so he feeds back into the grid, and that goes some way to offsetting costs)

    Other than that, you should look at least at leisure batteries, as they resist deep discharge alot better than standard lead acid batteries

    • rich

      I definitely need to size everything up. I haven’t yet even done back-of-the-envelope calculations to see if the entire plan is feasible. Is it possible to run a server on a solar panel? Look out for a forthcoming blog posting, I guess …

  2. About ten years ago we set up a couple of solar panels where I lived at the time (off the grid). We hooked them up to a deep cell battery (used for boats) and hooked an inverter up to the battery. The inverter fit into the electrical system for the trailer, so we had AC power for laptops, lights, etc. As I recall, the inverter wasn’t very expensive–certainly not as much as a panel. I think the panels were comparable in price to the units you’re looking at, if the exchange rate is around 2 USD/Lb.

    Last month my wife bought me a panel with iinternal battery for charging a phone or tablet, manufactured by Joos.

    • rich

      I’ve been told that you shouldn’t plug an inverter into a PV charging system (like the Steca ones I mention) because inverters can transiently draw hundreds of amps. The theory is that on the load side of the inverter, the load is only drawing a fraction of an amp (at 230+ volts), but that translates to a huge current on the 12V input side of the inverter.

      However I have no direct knowledge of whether this is true or not.

      The charging systems (and inverters) as you say are not very expensive.

  3. If the power demand of your server is below 200W, then you may want a Z3-ATX-200 unit made by PicoBOX. It takes anything between 16V and 24V DC as input and provides ATX-compatible power. They also have a cheaper Z2-ATX-200 model which accepts only 12V input, but it passes the 12V input directly to the motherboard and is thus unsuitable for powering a PC from car batteries (because they don’t give out 12V with the +/-0.6V tolerance defined by the ATX standard).

    Note: the 200W rating may be a lie, I have tested only the Z2 model and only up to 130W (it works as advertised within that limit). Also I was told by a representative of another PSU manufacturer that it is impossible to deliver 16A over a barrel connector because it will melt down. Hopefully it is fixable by replacing the connector.

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