Now that you have figured out that cutting the cord to your local utility grid is what you want to do, it’s time to begin crunching some numbers. First on your plate will be to tally up the entire kilowatt total that is used during the course of an average day. The best way to come up with that information (albeit the slowest) is to gather all of your household appliances and gadgets that use power and look up what the watt rating is. You should be able to find that information somewhere on the model number tag. Now, just add up your best estimate as to how many hours each product is in used during the day, and you will come up with a figure of so many watt hours demand. Remember that 1 kilowatt hour is equal to 1,000 watt/hours.
Alright…..let’s say you added up all of your demand as 25kwh or 25000 watts per day. Next you need to find out how many “peak” hours of direct sunlight your location receives each day. There are charts available that can help you with that step; one of those are located right here. Then you use the correct peak sunlight and use that to divide into the daily wattage use. Let’s figure you get 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 requirement on only peak Sun/hours will allow for any possible inefficiencies that could be present, because obviously the Sun is shedding sunlight for more than just 5 hours.
You will definitely want to have some way to back up your off grid solar system, so you can be at ease knowing you will have a constant supply of power when the Sun goes down and on cloudy days. Batteries are certainly the most utilized for this purpose, particularly, deep cycle batteries. Regular automotive type batteries are just not well suited for this type of application. These identical types of batteries are utilized in the same way in conjunction with wind/turbine power systems. There are calculators out there on the web that can help you in sizing up your battery bank, but it’s always good to know how to use 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 work 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) requirement of 520Ah. We will now have to double this figure (to cover depth of charge at 50%) to 1040Ah. Solar batteries should only be discharged to 50% of its total rating to prolong its useful life. By and large, most residential systems use 12v batteries, so let’s 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 deep cycle batteries for this project. The correct terminology for this would be 6 strings of 8 batteries.
The solar modules needed are sized pretty much like the batteries. Take the 5kw system we came up with and go from there. Find the best quality solar modules you can find and take their wattage rating and divide that figure into the total required wattage of 5000 watts. The charge controller in this example would be one rated for 96vdc by 60amp (these can handle up to 5.7Kw).
As was written earlier, there are calculators on the internet to help you handle this whole process; but even computer programs can make mistakes. We always use both methods to make sure we have the correct figures. Batteries can easily cost equal to what your solar module array does, so you want to make sure and get it right the first time.
For more information on Solar Power Systems you can visit: www.solarvoltpower.net