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Off-Grid Battery Selection

There is more to comparing batteries than just cost or amp-hour ratings. For example, based only on cost, Surrette/Rolls, East Penn (Deka) or Crown batteries do not look good at all based on cost but build excellent batteries!

Horses for courses; you would not want to store a flooded acid battery inside your computer room or cottage unless they were in a sealed or in a  ventilated compartment as flooded lead acid batteries gas off Hydrogen and Oxygen an explosive combination.  Batteries that start vehicles need instantaneous high amperage requiring lots of battery surface area but the electrical draw while heave is extremely short.  Off-grid batteries rarely need to cope with high amperage loads but endure long periods of discharge and deeper cycles drawing more from the batteries  There is no one best battery for all applications. If the batteries are in a remote communications site, low/no maintenance might be the most important feature. For the off-grid full time home, capacity, life, and long term cost effectiveness 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. Three times 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 power 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 (the 20 hour)amp-hours rating.
For example a 12 volt 240 AH battery can supply (under perfect conditions and to 100% discharge) 12v x 240 Ahr = 2880 Watt hours.  For a result in kilowatts, divide watts by 1000.  The above example would give you a result of 2.88kW.hr

Caution

Batteries are dangerous.  They contain sulfuric acid, emit explosive hydrogen & oxygen gases 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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Last modified: January 12, 2023