Small solar powered backup system

This is a small DIY system intended for backup lighting and chraging of small device such as phones/laptops etc.
The total rated power is 26W of solar, the inverter is built to handle 150W of power. The entire system cost around
  • Solar Panels £59.98 (Maplin)
  • Inverter £19.99 (Maplin)
  • Plug socket £3.99
  • Switches £3
  • Screw plugs £1.50
  • Everything else scavenged for free (timber, screws, wire etc)
 Solar Panels

The basis of the system is two 13W solar panels that I bought at Maplin electronics, they put out around 1.8Amps in full sun in the summer when I first bought them. This will probably drop to around 1.6A next year, as amorphous panels put out around 10% more than their rated power in the first year. The inverter is a bog standard 150W Square wave inverter

These two solar panels are in their winter position, at nearly vertical. The stands are designed so that I can lie them flat for a nearly horizontal position in the summer. These stands are made out of 3/4 plywood triangles screwed onto a length of 2x2 pine. they are weighted down with bricks to ensure that they do not get blown away in a storm. The two panels are connected in parallel for maximum current (about 1.4A at 13V). There is a screw connector in the inside of the plywood stands to connect the two solar panels.


 Control Box

The two panels are then connected to this "junction" box that contains the inverter and some fuses . The inverter has been modified so that the inverter can be controlled remotely. This is simply done by connected two wires in parallel with the inverter.

I decided to house the inverter and electronics in a separate enclosure to reduce the possiblity of a hydrogen gas explosion from the batteries, extremely unlikely with the ventilation I have but it pays to be safe. From this box we have the following connections:
  1. Solar Panel charging input.
  2. AC Mains out
  3. Control wires (4 core alarm cable)
  4. Battery power in
The inverter has a 7A fuse in-line with the power cable. This is plenty as the system is only designed to power a few CF Light bulbs.There is ventilation for the inverter in the side of the box, so it shouldn't get hot. Yes the cable grippers could have been a bit better! But I was only interested in getting the whole system working properly rather than making it perfect. The mains from the inverter runs up to the power socket in the house shown below.

 In house power socket

The system on and powering a light and phone charger. The left green switch is to turn the inverter on and off and the right little switch disables the power-loss auto on system. There is an orange neon light that indicates inverter power (240v) is available

This box contains a simple circuit to turn the inverter on when the power fails. It's
not ideal as it consumes power continously whilst there is mains, This is why it is very important to use an efficient SMPS supply rather than an iron cored adapter as those use a lot of power. The charger I used draws about 0.5W to power the relay. There are two switches in the supply box - one is to turn the inverter on and off and one is to disable/enable to emergency backup supply, so you can turn the inverter off when there is no mains power to save energy. Please make sure that the relay you use is rated for continuous use as some are only designed for intermittent use. Plugged in is a lamp which comes on in the event of power failure. In case you're wondering, the neon light was added after the left picture was taken.


  Yes, that battery box is absolutely horrible! It's suffered for around 2 years now in various systems, and is clearly in need of replacing which I will do soon. The battery is a car battery which was used but still has plenty of life left in it. You can see a MCB attached to the back board, this has since been removed as it is not needed (all the other parts are fused, and this switch poses a spark risk near the batteries). The wires are small as they only need to carry 1-2 Amps.

On the right is the complete battery box and control box(contains inverter). You can see the white control cable & the mains cable, they should be enclosed in a conduit which I will do eventually!


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