Basics of Solar Panels: Sizing Your Battery Bank

In my last article on sizing your solar array, I said that if you have a solar array you should use batteries to store your power. This will enable you to run your home, tools and appliances when your solar panels are unable to provide enough power by themselves.

Keep in mind that this series is only about the basics. This article assumes a 12 volt system, in order to make the math easier when using a 12 volt automotive inverter, which will work fine for an RV or a basic power bank to provide emergency power. This is not an in-depth "How to wire your off-grid cabin and run heavy equipment off of it" guide, nor instructions on how to run life support equipment from a battery bank. If you are in a situation where having all of your equipment functioning every moment is required to keep you alive, learn more about the subject than what is in this article.

As a quick reminder, batteries are measured in two important ways: voltage and amp hours (definitions here).

What are you using it for? 
If you are using the battery bank to run shop equipment, you will have completely different needs from running simple lighting, and in more ways than just the power draw.

The most important thing to remember is this: if you have appliances that draw a lot of power all at the same time, you will need a larger battery bank, even if they draw the same total power as a single device which draws less power. This is due to the internal chemistry of batteries, which makes them less efficient at producing a lot of power all at once. Multiple appliances may draw the same amount over time, but if they draw a bunch at once, it lowers the efficiency of extracting energy from the battery bank and you lose a lot of power to heat (blame thermodynamics for that). 

My rule of thumb:

• If I am drawing 50% or more of the maximum stored energy in my battery bank in an hour, I assume I will lose around 20% of it. 
• (This actually depends on the exact battery, and there are all sorts of fancy charts that tell you exact numbers, but for a rule of thumb, it works.)
• I also consider this rule to scale: If I am drawing 25%, I assume I will lose around 10%; if drawing 12.5% I will lose 5%, etc.

The easiest way to deal with this is just to over size your battery bank.

Heat will also heavily effect how efficient your batteries are at storing power. Just assume that if it is too cold, it will not let go of stored power, and if it is to hot, it will not store power correctly. Room temperature (40-90 degrees F) is the temperature at which they are designed to operate. 

How will you be charging, and how often?
This effects the size of the bank in two ways:

1. How much power you need between times that you charge. 
• If you use something for ten hours, more power is drained than would be if you used it for five.
2. How you charge it. 
• If you charge it with a generator, it takes less time to charge.
• If you have a gas generator, you can charge your batteries on a predictable basis.
• If you are relying on a solar array, recharging times can be less predictable.

Types of Batteries
For most people looking at an off-grid system, batteries come in two basic types: Lead Acid and Lithium Ion.

Lead acid is less expensive and easier to get hold of, but is physically heavier and holds less charge. Cars and heavy equipment use them, as well as most power storage systems for things like hospitals. They are a mature technology, simple to work with, and you can pick them up used for cheap from some mechanics. 

Lithium ion is more expensive, requires a more expensive charger, and can actually explode if things go very wrong. That said, it is much more power dense, and physically lighter; your laptop and cell phone run off of lithium ion batteries, and deliver a lot of power in a small light package.

For the average person, I recommend using Lead Acid. It is simpler, safer, and easier. That said, if you need a lot of power in a small, light package, I would recommend you do the addition research in order to safely use them.

Sourcing the Batteries
Batteries are expensive. A number of people I know use older batteries that they purchased used, which are less expensive but have less capacity left. These are often available for sale at specialty automotive electrical repair shops for a quarter of new price or less, with about 75% of the storage capacity of a new battery. You may even be able to find a heavy equipment repair facility that will have them for sale, allowing you to purchase much larger batteries. Used batteries will always have the disadvantage that you don’t know exactly how much capacity is left, how many charge cycles (times it can be recharged) it has left in it, and how abused it was in a way it might make it fail, before you get it. 

New batteries have the advantage that they often come with a warranty (with price tag to match), and will consistently be of a certain quality and capacity.

There are various other sources of used batteries, (occasionally hospitals, aircraft hangers, farm equipment repair places, yacht owners, etc will have them for sale,) but all of them will have the same difficulties. 

Math!
Make a list of the items you will be running on the battery bank. 

• Using the example from the article on sizing a solar array, let us assume that I have a fluorescent light fixture with four bulbs in it, and each bulb uses 17 watts of power. That gives us a power use of 17*4, or 68 watts of power.
• This means that every hour, it will use 68 watt hours of power. This is important. 
• 68 watt hours from a 12 volt battery means that it is using 5.6 amp hours every hour of use. 

Next, figure out how long you need to have that list of appliances running. 

•  Let’s say that I need to use this light over my desk so I can work overnight and meet my deadlines for my editor. I assume that I will work for 10 hours that night. 
• This means that to power that lighting fixture for 10 hours, you will want a battery rated at 56 amp hours at 12 volts (inefficiencies notwithstanding).
• If I only needed it to run for two hours, my battery bank requirements fall to 11.2 amp hours at 12 volts. If I factor in loss due to inefficiency, I want around 14 amp hours of battery time to allow me to run it for two hours.

Depth of Discharge
Most batteries are not designed to fully discharge all of their stored power. If you manage to do so, it will actually damage them slightly, and make them able to store less.

Car batteries are actually only designed to go down to about 50% of maximum stored capacity. If you only use them occasionally, it is not such a big deal, and it may not be worth it to purchase a larger battery bank. If, on the other hand, you are using this for off-grid living, you should purchase deep discharge batteries and do the additional research to make sure that you are doing this safely.

Inverters
Inverters (devices to convert power from battery voltage to outlet voltage) are never 100% efficient. Remember to look at the inverter that you plan to use, and find out how much power loss to expect.

Replacement
Remember that batteries will need occasional replacement, and the more they are used, the more often you will need to replace them. They will also need replacement if you let them sit for too long; five to ten years is a reasonable maximum expected lifespan for batteries.

A Note On Safety 
This is electricity. Don’t mix it with water, alcohol, or drugs. 

•  Make sure that all of your tools are insulated, and if you feel worried about something, take a minute to make sure everything is safe. It is much cheaper to take an extra hour to do something than to hurt yourself. 
• Make sure that all of your wires are insulated, and rated for the amperage running through them. 
• Always double check your work. 
• Batteries are heavy, and many of them are filled with acid. Dropping them can hurt whatever you drop them on (including you), and if they break open the acid can cause severe damage. 
• Red is positive, black is negative. Don’t get them mixed up. Mis-wiring can lead to things like electrical fires and broken appliances, not to mention broken batteries. 
• Be careful. 

Remember to practice, and don’t lick the wires.