Battery Bank Sizing

How to choose the right size battery bank for your solar system.

What is Solar Battery Sizing

Solar battery sizing (otherwise known as battery bank sizing) is one of the most important considerations when choosing the specifics of your solar electric system.

The main objective when sizing a battery bank is to get one that can handle the load coming from your PV panel array and provide enough stored power for your needs without having to regularly discharge to an unhealthy point.

By wiring multiple batteries together in different wiring arrangements you can design a battery bank that's right for your solar power system and thus correctly perform solar battery sizing.

Factors Affecting Battery Bank Sizing

The number of batteries you use in your solar system depends on the following factors:

  • The amount of money you have to spend on this solar project. Part of solar battery sizing is insuring you can buy enough solar batteries to handle your power storage needs.
  • You must also take into account the number of days you want to be able to go before needing to recharge your batteries. If you need to be able to power certain appliances for a specific number of days at a time without interruption, you'll need more batteries to carry a bigger load. This is determined by the number of batteries you use and how you wire them to affect your battery bank's total amp hours (storage capacity).
  • Another factor that affects solar battery sizing is the amount of power you will be needing for all of your appliances. If your appliances require many watts (power), you'll need enough batteries to store the power so you can use those appliances.
  • Another factor that affects the size of your battery bank is the amount of volts your solar system produces. If your system produces 48 volts, then you're going to want to have enough batteries in your battery bank to store 48 volts. Actually a little less is better - like a 36 volt system with a 24 volt battery bank, just to be sure your system can charge the battery bank even in the case of a sudden voltage drop. When sizing a battery bank, always size your solar panels bigger than your battery bank to be able to compensate for factors such as voltage drop, power fluctuations and energy loss due to wear on the system.
  • To charge a battery, a generating device must apply a higher voltage than already exists within the battery. That's why most PV modules are made for 16-18V peak power point. A voltage drop greater than 5% will reduce this necessary voltage difference, and can reduce charge current to the battery by a much greater percentage. Our general recommendation here is to size for a 2-3% voltage drop. So for a 12 volt battery bank, a 16-18V solar panel should be used to allow for unexpected voltage drop.
  • Another important consideration when sizing a battery bank is the storage capacity you will need your battery bank to have. If your area gets less hours of sunlight in the day, you're going to want more batteries so you can store more "amp hours" of power in your reservoir and last out the long night's stretch. When sizing a battery bank, the more amp hours you have the longer your total power reserve would take to deplete.
  • When doing solar battery sizing, you must also take into account the rate of discharge you want to have. Remember, the slower your batteries can discharge the more hours you'll get out of them. You can find out a battery's rate of discharge by looking at it and finding the value marked: (C-?). If you see (C-10) then this means the battery takes 10 hours to discharge fully, if it's (C-5) then the battery takes 5 hours to discharge fully.
  • Lastly, when sizing a battery bank, you must consider the depth of discharge you want to go to before recharging. (This is decided by your specific power needs / capacity and affects the battery's lifespan).

Basically, the bigger your batteries are and the more batteries you have, the more convenient it is for you and the better it is for your batteries' health. This is due to the fact that with more batteries / storage capacity you will have more power available, plus you will be discharging your battery bank in smaller (more shallow) cycles and thus increasing it's overall lifespan.

Therefore, as a general rule in solar battery sizing, it's always better to have more batteries in your battery bank and only discharge them 30-50% of the way down - than to have less batteries and discharge them more. Use a battery bank sizer calculator that can help automate the process for you.

Determining a Battery's Storage Capacity

An important part of solar battery sizing is determining the storage capacity, so you know how long you can use it for.

Sizing a Battery Bank - Watt Hours

Let's say you go out and buy a battery for your solar system that is 12 volts (push) and 105 amp hours (storage capacity).

You could find out approximately how much energy this battery will store / provide by calculating the watt hours. To do this, just multiply the volts (V) x the amp hours (AH) and divide by 100.

Volts x Amp Hours / 100 = Watt Hours

12V x 105AH = 1260 / 100 = 12.6 Watt Hours

What this means is that you can power a 100 watt appliance for 12.6 hours on a fully charged battery.

Make sure you find out what the specs on your batteries are before buying them. By knowing what to look for and what each spec means, you can insure your solar project's battery bank operates smoothly, efficiently and free of costly "battery bank sizing" mistakes.

Battery Life Expectancy

One thing you want to pay close attention to when solar battery sizing is how long the batteries you buy will last. The life expectancy of a sealed lead acid battery is rated using the number of cycles that battery can perform.

The "number of cycles" refers to the number of times the battery can be charged and discharged before it's dead.

So if your battery is a 3000 cycle battery, this means it can be charged and discharged 3000 times before it dies, that is providing it is consistently charged correctly and not discharged past acceptable levels. Batteries are considered to be at the end of their lifespan when 20% of their original capacity is gone.

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