How to Size Your Off-Grid Solar Batteries

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Introduction: How to Size Your Off-Grid Solar Batteries

If you are designing a solar electricity system and don't have access to the grid, you are going to have to deal with batteries. When you start looking at solar batteries you are going to encounter a little math. Fortunately,  SolarTown is here to guide you through the calculations!

The general goal when designing an off-grid battery bank is to get a system that is big enough to supply all your needs for a few cloudy days, but is also small enough to be charged by your panels. Before you get started here you may want to check out this article. It serves as a good introduction to the solar batteries I'll be talking about:

http://www.solartown.com/learning/solar-panels/solar-battery-storage-systems-if-you-cant-tell-your-agm-from-your-gel

Alright let's get started! Here are the steps to sizing your off-grid system.

Step 1: Calculating Your Amp-hour Needs

1.  Inverter size
To determine the inverter size we must find the peak load or maximum wattage of your home. This is found by adding up the wattage of the appliances and devices that could be run at the same time. Include everything from microwaves and lights to computers and clocks. The sum will tell you which inverter size you need.

Example: A room has two 60 watt light bulb and a 300 watt desktop computer. The inverter size is 60 x 2 + 300 = 420 watts

2.  Daily energy use
Next find the energy used in a day. Figure out how long each electronic device will be run in hours during a day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a very rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases.

Example: Light bulbs run for 5 hours a day. Computer runs for 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours

3.  Days of autonomy
Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

4.  Battery bank capacity
Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.

Example:  You want the battery bank to last three days without recharging and that you use 1.8 kwh per day. As 1.8 x 3 x 2 = 10.8kwh, this is the energy we need from the batteries. Converting this to AH we have to divide by the voltage of your system. This can be 12, 24 or 48 for commercial application. If we choose to use 48V, the minimum AH capacity is then 10 800/48 = 225 AH. Now if you divide by your battery's rating you find the number of batteries you must use.

Step 2: Don't Overcharge Your Batteries!

Once you have sized your battery bank and solar panel array, determining which charge controller to use is comparatively straight forward. All we have to do is find the current through the controller by using power = voltage x current. Take the power produced by the solar panels and divide by the voltage of the batteries.

Example: A solar array is producing 1 kw and charging a battery bank of 24V. The controller size is then 1000/24 = 41.67 amps. Now introduce a safety factor. Multiply the value you have found by 1.25 to account for variable power outputs: 41.67 x 1.25 = 52.09 amps

In our example we would need at least a  52 amp controller. The Flex Max 60 MPPT Charge Controller would fit our specifications.

Step 3:

Battery Wiring – Putting it all together

Before buying your batteries you need to figure out how many you need. Wiring is going to play a major role in determining this number. The goal is to find a configuration that produces target AH and voltage. There are two methods of wiring components in a circuit: parallel and series. In a series configuration the battery voltages add up while in parallel, current adds up.
Series and parallel connections can be combined to produce the voltage and AH that you require. Just remember:

      Series    →  voltage adds, current  does not
      Parallel  → current  adds, voltage does not

Previously we claimed that you could find the number of batteries you would need by dividing the AH capacity of your system by the AH rating of your batteries. This actually depends on how you wire together your system. Also remember that if a used battery is connected in parallel to a new one, it will degrade the fresher battery decreasing the lifespan of the whole system. Some people say that ideally you should just use a long line of batteries connected in series for your battery bank. Unfortunately this is not always possible due to voltage and AH requirements.

Step 4: You're Done!

You've now gone through all the steps necessary to size your off-grid battery bank system. If you are looking for more information on solar panel policy and technology, please visit SolarTown!

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72 Comments

hii.. I have 6 PV panels (280 W , 30V monocrys. multibusbar for each ) and have 2400 W offgrid solar charger MPPT . I ran 2 refrigerators on it ( about 1.5 A for each at 220V mains here in Iraq ) ..but used 4 batteries in series to give 48 V with only 17AH / battery .. what is the minimum AH of each battery should I use please ?? ..in summer it can runs on thoselittle capacity batteries but now it cannot and charge drop rapidly so fast though the batteries are new and fresh .. please advice . best regrds ..

Hay I have 300watt solar panel and a battery of 200ah. Can it power 8bulbs of 26w and a TV of 75w 12volt.

also i have 2 100 ah battries i thinking of using would that ok

hi can help me i want solar power my greenhouse which runs a pump of 50w for 24 hours =1200 also i grow light of 600 watts for 8 hours 4,5000 watts so i be looking at inverter of 6000 or more also i cant quite work out what size charge controler i have 50a one on hand would this be suitable .

in your examples, how many solar pannels would I need?

If you multiply the batteries capacity by 2 ( 50% DOD ) and 3 days autonomy, isn't you might oversize the amps needed. I believe 70-80 % DOD is good enough for most VRLA Gel type batteries. We should increase the load used by 15% due to losses incurred converting AC to DC. Correct me if I am wrong.

The number 3 which indicate 50% depth of discharge is come from where ?

Because we shouldn't 100% discharge our batteries.

And as per studies, 50% discharge is the safest level w/o shortening your battery life. Take note that this mostly applies on lead acid types, try to google other %discharge of different batteries.

The 3 does not indicate the 50% depth of discharge as you say.
It represents your estimated number of days of power use, without the system being recharged by the sun. This example is over a 3 day period of cloudy weather, you may wish to change that to 4 days or 5 days etc. The number 2 is what you would multiply with to represent the 50% discharge. Hope this helps.

I am in the process of designing my small off grid solar system. Help me out plz. Can a 500 watt pure sine wave inverter run a 0.5hp submersible water pump.