I wanted to take a moment to talk about batteries. Over the years I have had a chance to try out several different batteries and have yet to try some others. If you are going to have an off grid solar system, batteries are absolutely necessary to store the energy that your solar panels are gathering from the sun. You must have enough battery capacity to store enough energy to supply your needs while the sun is not shining. Usually you need to have a small generator backup for those times when the sun doesn’t shine for days or you may need to run some power tools or cloth irons for short times – but we’ll talk about generators in another post.
There are several different concepts to work with. The most common and least expensive (up front cost) is the lead – acid battery. This is the type most cars use. The Deep Cycle style battery is usually the one you would use with a solar system since they have more storage capacity and can stand up to deeper discharge repeatedly without damage. These batteries must be checked frequently to make sure the fluid levels are adequate so that you do not “fry” the battery and ruin it. Distilled water is used for maintaining the fluid levels.
Several popular varieties are available. The L-16 style is built with a deeper plate design so that the sulfating problem common to batteries is minimized. This makes the battery taller than the average battery so you must consider this when designing the space to put them in. These batteries are currently running about $300 each, depending on where you buy them. Another style is the “golf cart” style that are currently running about $85 each at Costco. These are 6 volt batteries so you need at least two of them to make a 12 volt system. The difference in price represents the difference in Amp/Hours storage capacity. The L-16 has a 400 amp/hour capacity and the golf cart style has about 120 so you can see the justification for the price.
There are Gel – cell batteries that are basically maintenance free because they are sealed and you do not have to check the fluids. They are more expensive, but also less care required to keep them going. Many people find the cost prohibitive for using in a solar array, but they do have the plus side of less worry and usually longer life. They usually have a longer time warranty as well, so it is a calculation as to whether they are more cost effective in the long run.
Another battery that is making the headlines lately is the Lithium Battery. Most famous to date with headlines splashed all over the various press outlets is the Tesla Battery. There are two sizes that will be offered by the end of September this year (2015) The 10K (Kilowatt) model is $3500 and the 7K is $3000. While this is a heavy upfront cost, my understanding is that they come with a 7 year warranty and they are expected to last at least 10 years.
This style battery is what is being used in the electric cars and has a good charging capacity as well as very low maintenance. It also has built in a charge controller and voltage converter so that it can be added into a variety of applications. I believe that there will be competing manufacturers in the near future and the cost will come down – good ole’ market driven free enterprise!
One of the keys in determining how many batteries you will want to add to your system is the amount of amps you will be using each day. There are many solar books out there that will help you calculate that. The main thing is that you have enough batteries to hold that amount of amperage or amp/hours. The 10K batteries by Tesla would be 833 amp/hours capacity. (10,000 watts % 12 Volts = 833 amps) It would take 6 L-16 batteries to give you that much capacity (some extra too) so that calculates out to about $1800 for the lead acid batteries versus $3500 for the Lithium. Time has yet to show us if the increased costs of the Tesla batteries are justified.
For our solar array, we have 6- 210 watt panels up for a total of 1260 watts. This is sufficient to run our small refrigerator, lights, computers, sewing machine and other light appliances. If you take 1260 watts multiplied by 6 hours of solar charging per day, you have 7560 watts per day that you want to be able to use. Much of it goes to the refrigerator which we have on a timer to just run during the daylight hours when we are opening the door. If we wanted to store 1/2 of the energy to the batteries, we would need to store 315 amp/hours of battery capacity. The golf cart style battery is a 6 volt so you would need 6 of them to give you enough storage capacity for the system we have on our roof. That is a cost of about $500.
I hope this has given you some useful information on batteries and how to size your system. As you can see, if you keep your electricity demands simple, you can keep your installation costs down. It’s true that many appliances and gadgets are nice and convenient, but with a little sacrifice and planning, you can really keep costs down and still be comfortable.