Solar power for Straw Bale Emergency Shelter.

Today I read the post Straw Bale Emergency Shelter.  I got it from my subscription to Earthbag Building Blog. I saw myself so much in the first lines of the post, that I have decided to collaborate by adding the plan for a small solar installation, which would allow the new jackrabbits’ neighbour to have some lighting to read a book at night (i.e.The one straw revolution), listen to a bit of radio, and power a laptop with its unlimited uses.

I am not a professional solar installer. I have gathered the entire info through searching the internet and reading books. This plan could just be something to compare with or just a starting point for further planning.

This is the plan:

And here’s the explanation and budget:

40 Watts solar panel: I believe this panel would give us enough power and, at the same time, it is small enough to hide inside the house if we are going to be away for a couple of days or put it into the car if we are going to be away for longer ( That’s if we have a car!) . Let’s see:

Small Hi-Fi system=20 Watts/hour   x 2 hours per day= 40 Watts/day

2x 3W Bulbs= 6W x 4h = 24W/d

Laptop = 30W/h x 3h= 90W/d

That makes a total of about 160 watts per day. If our solar panel were to be under direct sun for 4 hours, that would be enough. But there are more things to considered, like the efficiency of the system, shadows over the panel…So I would consider doing the maths with an extra hour.  I mean:  if 160 watts is what we are going to need, with a 40 watts solar panel, we should have 5 or 6 hours of sun per day, otherwise we would need bigger solar panels or alternative/back up power.

Charge controller:  This little device is the brain of our system. It puts the power from the solar panel into the battery, and takes it from the battery into our radio, laptop or lights. The charge controller seems extremely important in order to keep the battery in good condition (not overcharge or discharge) and also avoids power going from the battery into the solar panel at night.  The one I chose has three outputs with individual switches, so no need for switches in the wall for lights. We could use one output for each light and another for the inverter. Also the individual switchers could save us some of energy (keep in mind that some devices still use power when switched off if they are left plugged into the mains).

3 Watt 12V DC Bulbs: I have never used one of these, but if they do what they say, they could be fantastic. They run with 12V DC, so they can be powered from the battery (through the charge controller), so saving us energy (having an inverter is always a waste of energy. Changing 12V DC produced by the solar panel into 220V AC used by normal devices, such a radio, consumes power). So everything we could power as 12V DC directly, avoiding the inverter, is a save.

300W Inverter:  The one we would use transform 12V DC power produced by the  solar panel and stored in the battery, into 220V AC power. After what I said above, you are going to see the inverter as an enemy! Well sometimes you need to compromise, I guess. You could try to power your laptop with a 12V DC to 19V DC inverter, and find a radio that works with 12V (like the ones for cars and bikes that use the vehicle’s battery), but…nearly everything is designed for 220V mains nowadays, so I think is worth having an inverter. With this 300W inverter you will be able to do just that, plug a radio, a laptop, maybe a TV (if you really need to use one!!! stay away from the stupid box!!!), but forget about plugging high power suckers like toasters, kettle, microwave, or hair dryer!? The one I chose also has a USB so we can charge our mobile phone…sweet!

110 Ah deep cycle Battery: This is where we store the energy produced by our solar panel.  You could spend your life reading about batteries, and still learn things! This is the part that confuses me more. I would go for the 110 Ah. Ah means amperes/hour. It works like you could use 110 Amperes in one hour, 55 in two hours, 36 in three hours, and so on. So if you were to run your system from the battery, you would need a total of 13.3 amperes for one day (160W/12V=13.3A). I know! You are going to need to read a bit about watts, Amperes and Volts. Basically, a Watt is a measure of power (like the power that a device needs to work. i.e. 20W for a radio; or the power that a solar panel produce i.e. our 40W solar panel), and is the result of multiplying Volts, that measures the difference of potential, by Amperes, which measures the intensity of a current. Too much info!! Sorry!!

So we need 13.3 Amperes for one day, and the battery holds 110. But, it is not good for a battery to get discharged over its half capacity. So we should consider having a 55Ah instead of 110Ah, if we want to keep the battery for a long time (we all know about batteries losing their charging capacity. How long last your laptop’s battery now compared with when it was new?). Also, not every day is going to be sunny! So I think 110Ah is a good choice.

Deep cycle means that the battery can be recharge more times than normal batteries. Also, these batteries are sealed and need no maintenance, which is quite nice.

And finally it amounts to ( prices taken from ebay UK, and comes in pounds!). Click in the devises for links to ebay ( just examples) :

40WSolar panel=79 £

Charge controller=26£

300W Inverter=16£

110Ah Deep Cycle Battery=98£

12V 3W bulb= 12£

If you add a few more money more cable, bulb holders…it comes to about 250£ (300Euros, 380Dollars).

Remember I am not a professional on this, so you will need to make your own calculations. And please, be careful with electric stuff!!!

Any advice/corrections most welcome.



About Quemao Viejo

A project to scape the Rat Race with a micro-camping site and a veterinary practice using local medicinal plants.
This entry was posted in eco-building, energy, sustainability and tagged , , . Bookmark the permalink.

One Response to Solar power for Straw Bale Emergency Shelter.

  1. Hey, details, details! This is a great guide, don’t ache your self over thinking you didn’t do a good job on it or its too little. It gets the entire point across without sounding TOO geeky. Ciao, and great guide for rookies.

    -Sharone Tal

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