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A deep cycle battery is a special type of battery that discharge electrical energy gradually as we need it, and that can keep working up to 80% use of the charge if you can afford the Lithium-Ion battery. However, choosing the right deep cycle battery for your solar panel system can be intimidating because there are quite a few factors to consider, calculations to make, and more.
Simply put, all of the available options can make this process very difficult. Today we are here to help you to size battery bank for solar panel based on daily power usage, days of autonomy, depth of discharge, and system voltage.
Nowadays, deep cycle batteries are often used as battery banks to store the extra energy generated by a solar panel setup.
Low cost and maintenance free deep cycle AGM lead acid batteries are ideal for applications which require constant charging and discharging, such as solar panel battery banks in most off-grid PV systems.
4 Factors Determining the Size of Battery Bank
As stated above, there are 4 main factors which will determine the size of the battery bank you will need for your off-grid solar panel setup.
1. Daily Power Usage
The first factor that goes into determining the size of the battery bank is how much power you will be using on a daily basis. This means that you have to evaluate or measure the total power usage of all of your appliances. Knowing how long you will use each appliance each day is important to determine as well.
There are many different ways to get this power usage figure depending you are living off-grid or on-grid and how much effort you are willing to spend on this crucial task. (Trust me, flipping over each electric appliances to read their power labels in your house is hell of a job.) After hours of research, here is the list of possible ways to determine your off-grid and on-grid load in your household.
On-Grid Load Determination
Here is the list if your household is tied to an utility company:
Get the daily average from annual electric bill
Check with your utility provider (mine is PG & E) to see if you can get a record of your utility bill in the last 12 months. You can easily determine your average power usage by summing up all 12 months usage and divide by 365.
For instance, my total power usage between April/18 to April/19 was 2884KWh. The monthly average is 2884KWh / 12 = 240.3KWh. The daily average is 240.3KWh/30 = 8.0KWh.
Check power meter reading
If you have access to a smart power meter in your house, this method is great for kitchen (air fryer/oven) and laundry (washer/dryer) appliances that you can't read their power label easily. For instance, to find out how much your dryer's power consumption, you record the meter reading before and after you turn it on and take the difference between the two.
Use a Watt meter to measure power usage
For appliances that don't have wattage written on their product labels (See the LED TV label below) or device without product label, I use my Kuman Electricity Usage Monitor Plug Power Meter to measure their wattage. This handy Kuman watt meter can also measure voltage, ampere, and kilo-watt-hour (No more guessing needed ?).
Off-Grid Load Determination
Here is the list if your are living in a mobile home:
Read the power label of each appliance
Most appliances have a power rating label at the back or at the bottom of it. For small to medium kitchen appliances such as toaster and electric kettle, this is the most effortless approach to determine their power consumption. However, it will be harder to read the label at the back of your refrigerator. For large appliances, I suggest you can look up their power specification from their manufacturers online.
Look up consumption from appliance online specification
This is the recommended way for large appliances such as refrigerator that you can't easily get to their product label at the back of it or appliances you're considering. Most of the time the power/energy consumption will be given in kWh per year.
Calculate consumption with online load calculator
In case you don't know, many battery vendors do provide their own online load calculators, here the ones I recommended:
Estimate consumption from an appliance lookup table
Rice Cooker (1.8L)
Toaster (2 Slice)
Waffle Maker (4 Slice)
Jacuzzi (No heater)
TV (42 Inch LCD)
TV (42 Inch Plasma)
TV Cable Box
Video Game Console
Ethernet Switch (8 Ports)
LED Monitor (22 inch)
2. Days of Autonomy
Days of autonomy means how many hours or days your battery pack should be able to provide energy for without being charged. In other words, if it has been extremely cloudy out, your solar panels will not have charged the battery much or even at all, but you will still need to draw power to run your appliances.
Days of autonomy specifically refers to the number of consecutive days where you may not be able to charge the system. A data logging anemometer combined with local weather stats should help you determine how long that battery needs to last for before it requires recharging. This is crucial to keep in mind, especially if you live somewhere that may not get a lot of consecutive sunny days.
3. Depth of Discharge
The next factor to consider when sizing your battery bank is the depth of discharge. A battery cycle takes any battery from a fully charged state to discharge (use), and then back to full power by charging with the solar panels. The depth of discharge refers to the amount of energy which can be safely discharged from a battery before it requires recharging.
The important thing to keep in mind is that most deep cycle batteries should never be discharged to less than 50% capacity before being recharged, with some exceptional models being able to go down to 25% before they require charging.
That said, certain battery types, such as lithium-ion batteries, can only be safely discharged to 80% before the require charging. In other words, you cannot actually use 100% of the energy stored in a battery without causing potential damage.
4. System Voltage
The other factor which you need to be aware of in order to properly size your battery bank is what voltage level your system has. Typically, your system will feature either 12V, 24V, or 48V.
What Battery Size Do You Need?
An important thing to be aware of is that the battery size is measured in units of amp hour or (Ah). Therefore, you need to know the amp hour in order to determine what size of battery you require.
What is Amp-Hour (Ah)?
An amp hour is a measure which tells you the amount of energy charge in a battery which will allow one amp (ampere) to flow for one hour. Be aware that utilities will charge for electrical energy based on kilowatt-hours, and as you remember from above, 1 kilowatt-hour is equal to 1,000 watt-hours.
Something you need to know here is how to convert Wh to Ah and back, and you will need to know how many volts you are working with to do these calculations.
The formula to calculate Wh from Ah is as follows:
(Ah)*(V) = (Wh)
To convert Ah from Wh, the formula is as follows:
Calculating Battery Size in Ah – A Practical Tutorial
Now that you have all of the important information and factors at your disposal, let’s go over a quick example of how to make this calculation, so you can actually figure out what size battery you need.
Step 1. Determine Daily Total Power Usage
First, you have to figure out the total power use of your system. For this, look at your appliances and see how many watts they use. This is measured in watts per hour. Therefore, for a 40 watt appliance that runs 5 hours per day, the energy usage would equal 200 watt-hours. Now, add together all of the appliances. For the sake of this tutorial, let’s just stick with 200 watt-hours.
Step 2. Determine the Days of Autonomy
Now you need to determine the days of autonomy, i.e., how many consecutive days you expect to be cloudy where the system will not charge. Let’s assume that the days of autonomy is 5.
Step 3. Determine the Depth of Discharge
The depth of discharge refers to how much energy can be discharged from the battery before it needs to be recharged. This will be determined by your specific battery type. Let’s assume that you have a lithium-ion battery with 80% discharge.
Step 4. Determine System Voltage
Now you simply need to determine what voltage your system has. Let’s assume this is 24V.
Step 5. Determine the Correct Size of Battery Bank
The first four steps were all about getting the information you need. Now that you have all the relevant information, you can make the calculation. Here is the formula to calculate the required battery size.
Battery Bank (Ah) = ((daily power usage (Wh) X days of autonomy) / Depth of Discharge) / System Voltage
Using the above information, insert the values into the formula. In this case, it would look like this:
Battery Bank = ((200 x 5/.8)/24) = 8.9 Ah. Keep in mind that the numbers we have used are very small, as we did a calculation for a single appliance. However, your final result is bound to be much higher.
Calculating the sizing for your deep cycle bank of solar batteries does require some patience in adding things up and a bit of arithmetic, but does not require an advanced degree in science or math. Additionally, sizing calculators on various websites can be a big help.
As a quick recap, to size battery bank for solar panel, start by determining total power usage, then the days of autonomy, then the depth of discharge, and finally the total voltage of your system.
Take all of those values and then plug them into the formula which we laid out in the tutorial above. This will tell you the size of the battery you will need for your solar PV system.
Something that can’t be stressed enough is that you always need to keep safety in mind. Always follow all safety guidelines for working with solar and electrical systems.
Do some research first and be sure to consult a pro if needed. You don’t want to end up damaging your system or causing serious injury because you don’t know what you’re doing. The bottom line is that if you are uncomfortable or confused, it’s probably best to consult an expert.
ABOUT THE AUTHOR
Peter has been installing PV systems since 2016 as a volunteer participating in over 30 installations events in the Bay Area, California. He has a mechanical engineering background and he has been involved in the solar industry since 2015. In his spare time, he likes to explore cool solar gears that make his camping trip more enjoyable and fun.