Off-Grid Battery Selection
There is more to comparing batteries than just cost or amp-hour ratings. For example, based only on cost, the Store Brand, Trojan or Exide AGM's do not look so good. On the other hand you would not want to store a flooded acid battery in your computer room. There is no one best battery for all applications. If the batteries are in a remote communications site, low maintenance might be the most important feature. For the off-grid full time home, capacity. life, and long term cost would probably be the most important.
The "Best" battery for a particular system is not always the most expensive, but it is seldom the cheapest either. There are many things to consider.
How much storage, in amp-hours, do you need?
This will vary with the application and where you live. As a rough rule for home solar systems, the total battery capacity (in amp-hours) should be three to five times your daily usage. 3 if your system use is limited to summer use and weekend use. For the rest of the year 4 to 5 days standby is best. If you are in a good wind area and have a wind turbine as well, you can probably reduce this factor by a day of stand by time.
To calculate how many amp-hours storage you need:
Use your average daily usage in watts and divide by the battery voltage. For example, if you use 5 kwh (kilowatt-hours) per day, and have a 48 volt system, then dividing 5000 by 48 gives you 105 AH. Since you do not want to discharge the battery more than 50% in most cases, you would need 210 AH. If you want to keep running for 4 days of bad weather with no sun, multiply the previous result by 4, which brings you about a 850 AH total capacity.
How many Watt Hours in a battery?
Multiply battery voltage times amp-hours.
Batteries are dangerous. They contain sulfuric acid, emit explosive hydrogen gas when being charged and have the ability to release an enormous amount of amperage if shorted, far more than the power used to weld with. Use eye protection any time you are working with your batteries.
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