# 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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• ### Game Design: Student Design Challenge

Hello, I request clarifications on battery sizing, when calculating the required amp hours, do I divide the total energy used by battery voltage or Inverter output voltage (120v).

I have 270w solar panel. Maximum PV 37V CURRENT 9A. I Have 60A controller. It can supports upto 90v. Now I am using 12v battery and 12v inverter. I noted maximum power I can get is 12x9= 108w and I think I am wasting solar energy because of incorrect selection of battery and the inverter.Am I correct???

The basic answer is no, you are not losing as much power as you think. The MPPT inverter will track the input voltage and current for the amount of sunlight available in order to extract the maximum available power from the panel. Let us say you are getting 240W from the panel, then the output at 12v will be 20A if we ignore any losses. (A = 240W/12V) It is in effect a DC to DC converter

I use solar controller 60A. What ever the situation it won't deliver the current more than the rating of solar panel (more than 10A)

Solar panel maximum output current is 9A. In that case I think, if I use 12v battery i will get 12x9w, if battery is 24v , output can be 24 x 9A. Please check it.

please help my total daily energy is 15329.2 watt hours how many solar panels do i need

I have read the article and understood .I would like to take this special time to thank the writer of this article it had really help me a lot.i used to be confused in sizing this four component of solar system 1-solar panel
2-charge controller.
3-batterry bank.
4-power inverter .
now I have understood part of it but still remained one which has is missing in this article.how to select exact solar panel to charge the battery you needed .

I've designed a battery bank based on what I've learned from this article. Please review (see diagram) and inform me if the layout is correct/makes sense. Thanks in advance.

Min. AH Capacity: 1100AH
System Voltage: 48V
Battery Specs: 12V 250AH

Using 6 Batteries (3 wired in Series and 3 in Parallel) I'm able to meet both the voltage (4 x 12V) and amp (4 x 250AH) requirements of my solar system.

This article was very helpful as I am currently designing a Grid-Tied system with battery and generator backup for my house. I plan to 'Go Solar' as much as my finances will allow, only using the grid as backup (and to recharge the battery bank) since it is so unreliable.

I was having difficulty selecting the appropriate inverter and MPPT Charge Controller because I didn't know how to size them. And as for the batteries, I had an idea the number of batteries I needed--with regard to voltage--prior to reading the article, however, I didn't know how to size the batteries using amps-hours. But thanks to this article that is no longer an issue. Now I know I have to connect the batteries using a combination of series and parallel wiring to fully realize my solar goals. Thanks.

Regarding Battery sizing. You have multiplied (Daily emergy use times Days of autonomy times 2) why is the 2, what does it stand for?
Cheers

The author multiplied by 2 because the answer before that step accounted for 50% (or 1/2 battery storage capacity) of the battery's capacity. Therefore to get full capacity, multiply by 2.

I have 2tv,1 microwave, 15bulbs 6watt each,2freezers and ac pump.how many watt Parnell I need and how many batteries for ,6hrs

For 8kw solar roof top system i will use 2volt vrla get type battery with having 6hour back up and the battery capacity should be 7.2Vah. So please suggest me that how could i calculate the battery "Ah" and nos of batteries require for 8kw system??????????

I have designed excel sheet for calculation of Load calculation,Off-grid system and rooftop on-grid plant.if anyone want please send me mail or whatsapp.
Krishna
7415324645
krishna.23apr@gmail.com

I am interested but your number has no country code!

You've got Gmail, so why don't you just make it available, thorough your Gdrive?

Hello. I am interested in receiving this spreadsheet as I am trying to design a solar farm.

Please send it to me at daposulu@hotmail.com if possible.

Many thanks.
Sid

An exam question I came across recently: A small factory that will need between 30 and 80kw/hour energy consumption and 380 - 415v 3 phase supply, which is due to work for 12 hours a day. How do I calculate the number of solar panels required?

I have a 2018 Chrysler Pacifica hybrid . I would like to charge it via solar array and batteries . The battery pack is lithium ion and has 16 kwh storage . What size array and battery bank would I need . I will likely be using 6 volt lead acid battery banks @24 v to inverter .

I have 10 232AH 6v golf cart batteries @12vdc. I have 6 315 watt panels, Outback 80 CC, using Midnight Solar Combiner box. Is this too much power for this system?