It's Time To Formulate An Off Grid Solar Energy System

Published: 21st November 2011
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Now that you have figured out that going off the power grid is what will work best for you, it's time to begin crunching some numbers. To begin with, tally up the entire kilowatt total that is used up during the course of a typical. The best way to come up with that figure (although the slowest) is to take all of your household appliances and gadgets that use electricity and look up what the watt rating is. You should be able to find that information somewhere on the model tag. Now, just add up your best estimate as to how many hours each product is in use during the day, and you will come up with a figure of so many watt hours demand per day. Remember that 1 kilowatt hour is equal to 1,000 watts/hour.

Alright.....let's say you added up all of your demand as 25kwh or 25000 watts per day. Next you have to find out how many "peak" hours of direct sunlight your location receives each day. You can find charts that can assist you with that right here. Then you use the correct peak sunlight and use that to divide into the daily wattage use. Let's figure you receive 5 hours of peak sun... 25000 watts / 5 peak hours = 5000 watts per hour. This would mean that you need to install a 5000 watt (5kw) off grid power inverter. Basing your need on only "peak" Sun/hours will allow for any possible inefficiencies that might be present, as obviously the Sun is shedding daylight for more than just 5 hours.

You will absolutely want to have some way to back up your stand alone solar system, so you can relax knowing you will have a continuous supply of power when the Sun goes down and on cloudy days. Batteries are certainly the most utilized for this purpose, in particular, "deep cycle" batteries. Regular car type batteries are just not well suited for this type of application. These identical types of batteries are used in the same manner in conjunction with wind/turbine power systems. There are calculators out there on the internet that can help you in sizing up your battery bank, but it's always nice to know the old fashioned way (with our brain!).

As an example we will base the battery needs to allow for emergency/nighttime energy of 48 hours, or 2 days. It's not really that hard to figure out; we have our daily requirement of (25kwh) 25,000 watts; now we just double that sum to (50kwh) 50,000 watts; divide this by a 96vdc set up (best for this application) and we come up with a total amp/hour (Ah) need of 520Ah; now we need to double this figure (to cover depth of charge at 50%) to be 1040Ah. Deep cycle batteries should only be discharged to 50% of its total rating to prolong its life. Most regularly, residential systems use 12v batteries, so we will use some zero maintenance Trojan Deep Cycle Batteries model# 6D-AGM 200Ah. We need a total battery bank size of 1040Ah divided by each battery size of 200Ah to get 5.2 batteries; round this figure up to (6) batteries (never round down). This battery bank will all equate to (8) 12v batteries wired in series to get the 96vdc needed by (6) batteries wired in parallel to get the 1042Ah needed. You would end up requiring a total of (48) 12v 200Ah solar batteries for this project. The correct terminology for this would be 6 strings of 8 batteries.

The solar modules needed are sized much like the batteries. Take the 5kw system we came up with and go from there. Find some top rated quality panels you can find and just take their wattage rating and divide that amount into the total required wattage of 5000 watts. The charge controller in this example would be one rated at 96vdc by 60amp (these can handle up to 5.7Kw).

As was mentioned earlier, there are calculators on the web to help you through this whole process; however, even programs can mess up on you. We always use both methods to make sure we have the right figures. Batteries can easily cost similar to what your solar array, so you want to make sure and get it right the first time!

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