How to size your battery backup system

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Today I want to talk about how to choose the proper battery and solar collector size for your system.

In order to size the system you need to know a few things first. You need to know how long you want the fan to run on the battery (aka how long will it run after the sun sets) and how much power it takes to run the fan.

For example the battery backup system I installed at my parents house I wanted the fan to be able to run for 6 hours.

I called natural ligh (the fan’s manufacturer) to ask how much power the fan will draw… The service technician had no idea how much power the fan should use. I decided I could measure this value myself.. I waited until it was a bright, sunny day and I went up into the attic and measured the voltage across the fan motor’s terminals with a cheap voltmeter I bought at walmart. The voltage was 20 volts (DC of course). I then then users the voltmeter to measure the current across the fan leads, which measured to be ~ 1amp.

From knowledge of basic circuitry Power = volts * amps. So I now know my fan needs about ~20 watts of power to run.

Now we will calculate how big of a battery you will need to buy. How much power a battery has is measured in watt-hours. This is simply the power multiplied by the hours you want it to run. So if you want the fan to run 6 hours you would need 6*20 = 120 watt-hours. If your battery is labeled in amp-hours then simply divide the 120 watt – hours by the system voltage (20 volts), thus you need a battery 6 amp-hours of power. This would be true if you could discharge the entire battery each night. Most batteries have a minimum level that they can be discharged to. Usually this is about 50% of the batteries maximum power. Some batteries can be discharged further, so check the operating instructions or with the batteries’ manufacturer. I sized my battery to have two times the amount of power required ~ 240 watt-hours. If the price of the battery at 2x the power capacity is too expensive, then look for a cheaper battery with less capacity. I believe I spent about $35 for my battery in 2010.

One more note about the battery size/ how many to buy. Remember when I measured the battery’s voltage to be 20 volts in full sun? Well I did that so would know how much voltage the fan needs to spin at full speed. When the voltage at the fan drops (like when a cloud blocks out the sun) the fan slows down. So in order to make sure when the fan is being powered by the battery that it spins at full speed you need to have ~ 20 volts. Since most batteries come operate at 12 volts, you will need to wire two batteries in series so that you power the fan with 24 volts. The fan will in fact spin faster with the 24 volt battery system then when powered by the 20 volts from the sunlight. You do to use two batteries, in fact, I only used one battery on my parents system and it has been working fine for a few years now. If I were to build the system over again (which I will be doing in a few months to my own house) I would install two 12 volt batteries to increase the speed of the fan when run off the batteries.

I will be posting a complete parts list once I can find my project notes.

Now that you know you how much power you need and how big of a battery, you can determine how much power your solar panel has to produce in order to recharge the battery bank in one day.

Since you know you will need at least 120 watt hours of power day to replace the battery power used by the fan the previous night.

Because the sun doesn’t shine at full strength all day long you will need to build a solar panel that can supply about 1.5x the power required. So in this case you should build a panel that can supply 180 watt-hours of power per day.

A 60 watt solar panel should be able to recharge the battery bank in about 3 hours of full sunlight. This is the size of the panel that I decided to build/use.

If you can find used solar photo voltaic panels this will provide you with the best value for your money as new 60 watt panels can be over $200. A $200 solar panel makes the battery backup system too expensive.

I was able to build a 60 watt panel for $60 – $70 from parts I bought off eBay. Which ever path you choose keep in mind if you are low on money there is always options to make it work.. Go around and look for old panels on your neighbor’s roof. They probably aren’t using them any longer and may even give them to you for free in exchange for removing them. In the world of PIY when there is a will there is a way!

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