dock lights

How To Size Batteries For Solar So You Don’t Get Left In The Dark

The heart of a solar power system is the battery bank. The batteries store energy produced by solar panels so that it can be used to power lights and other electronic devices. You must size batteries for solar so that you store enough energy to run all of your devices for a given length of time. You must also have enough capacity so that your batteries never completely discharge. Actually, batteries must never be discharged more than 80%.

To figure out what size batteries for solar you need first think about what electronic devices will be run by your solar power system and how long you use them. To show how this works I will use the dock in the picture above as an example.

Size Batteries for Solar Power Dock

This dock has a DC powered boat lift, an underwater LED light, LED string lights, built-in marine stereo, and a portable refrigerator. The owners often use the dock at night for up to two hours. But on weekends they entertain guests for up to 5 hours.

In this case, the boat lift is not a problem. It is only used during the daytime when the solar panels are supplying power.

What the owners need to find out is how much power the lights, stereo, and fridge use. That will give them the maximum number of amps being discharged from the battery per hour. They can then put those numbers into a simple mathematical formula. This formula will tell them the length of time the battery will run all of their devices at that amperage. Don’t worry, I will walk you through how to do the math.

Battery Capacity vs. Discharge Rate

Battery manufacturers use C ratings to show a batteries capacity at a given rate of discharge. A battery may have several C rates listed. The one used most often for comparing deep cycle batteries is the C20 rate.

The C20 rate tells you the number of amp-hours the battery can produce when discharged over a 20 hour time period. Dividing the stated C rating by the number of hours tells you how many amps the battery can produce each hour for the 20 hours.
For a 100 amp-hr C20 battery: 100 / 20 = 5 amps
For a 90 amp-hr C20 battery: 90 / 20 = 4.5 amps

The batteries available capacity changes with its discharge rate. The discharge rate is the number of amps being taken from the battery at any given time. A 100Ah battery will have a greater capacity when discharged at 5 amps than it will when discharged at 10 amps and a greater capacity when discharged at 10 amps than it will when discharged at 20 amps.

Estimate Amperage Used

To figure out how much battery capacity you need you first have to figure out how many amps all of your electronics combined will use. Using the dock example above we list all of our electronic devices.

1 Apollo 40 underwater light
2 LED light strings
1 Marine stereo
1 Portable DC refrigerator

Where to Find the Amps or Watts Used

Now you can find out how much power each electronic device uses. There are three places you can look to get this information.

  1. On a nameplate on the back or bottom of the device.
  2. In the owner’s manual or specification sheet that came with the device.
  3. Online at the manufacturer’s website.

The power will be listed in either amps or watts. We will be calculating the amount of power being taken from our battery in amps. To convert watts into amps you divide the number of watts by the voltage the device uses. For example, an Apollo 40 underwater light uses 40 watts at 12 volts.
40 watts/12 volts= 3.33 amps.

Now you can estimate the maximum number of amps you will use.

Device How Many Total Amps
Apollo 40 1 3.3 amps
LED String Lights 2 1.5 amps each = 3 amps
Marine Stereo 1 8.3 amps @ 50% power
Portable Fridge 1 2.9 amps
Total 17.5 amps

How Long Will Your Battery Last?

We can find out how long a battery will last at any given amperage by using Peukert’s Law. Peukert was a German scientist who came up with a mathematical formula to tell how long a battery will last when discharged at any given amperage.

Peukert's formula

His formula uses an constant that tells us how efficient a type of battery is during discharge. A number close to 1 is very efficient. Higher numbers are less efficient. The chart below shows the Peukert constant for different types of deep cycle batteries. As you can see, AGM batteries are the most efficient types of deep cycle batteries. However, in the example below I use the Peukert constant for an average flooded battery. To find out how long a more efficient battery will last just change the Peukert exponent.

Peukert constant
AGM Gel Flooded
1.05 – 1.15 1.10 – 1.25 1.20 – 1.60

formula to size batteries for solar
Now we can figure out how long a 100Ah C20 battery will last with a 17.5 amp load applied.

Do The Math

Multiply the load in amps times the batteries specified discharge time in hours.
17.5 x 20= 350
Divide the batteries specified amp-hours by that number.
100 / 350= 0.29
Multiply that number by the specified discharge time.
20 x 0.29= 5.8
Divide that number by the Peukert constant.
5.8 / 1.4= 4.14 hours

We Need two Batteries

In this example, we can see that a 17.5 amp load will completely discharge our 100Ah C20 flooded battery in just over 4 hours. This is less time than the 5 hours our owners plan on using the dock. They need more battery capacity. They will need a second battery.

They can use a more efficient AGM battery. That will give them over 5 hours of time. But draining a battery more than 80% can damage the battery. By adding a second battery in Parallel they can double the storage capacity.

Types of Batteries

There are several different types of batteries used for storing solar energy. Each has its advantages and disadvantages. I encourage you to research the subject deeper before deciding what battery type is best for you and your needs.

Now you know how to size batteries for solar power. It isn’t hard if you follow these instructions. But if you have any questions please leave a comment and I will do my best to answer it.

I hope you found this post useful. If you did I encourage you to bookmark it for future reference and share it with your friends on your favorite social media site.

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