So, you’ve invested in solar panels to power your house or RV, but now you’ve been plunged into a confusing world of amps, volts, watts, and sunlight hours. It can be confusing, but with some basic knowledge of how solar panels work, it’s possible to estimate how many you’ll need.
The number of solar panels it takes to charge a 100Ah battery depends on many variables, including the battery’s voltage, solar panel power output, and hours of sunlight your panels receive. For example, two 300W panels will charge a 12V, 100Ah battery in two and a half hours of sunlight.
With so much variety, trying to calculate your solar power requirements can feel overwhelming. But don’t be intimidated. Read on to learn everything you need to know to calculate the number of solar panels you need and make the right choices for your solar power system.
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A Few Battery Basics
A comprehensive knowledge of how batteries and solar panels work requires more understanding of chemistry and physics than most of us have. But some basics will help you get the most out of your solar energy system.
Batteries are classified according to three major characteristics: their voltage, their chemistry, and their capacity (source). Each of these characteristics can be explained at great length, but we’ll keep it simple.
Voltage refers to the potential of a battery, and it is marked on the battery using the symbol V. It is sometimes described as the “pressure” that will push electrons through a circuit (source).
The voltage of a particular battery is determined by the battery’s chemistry and is, therefore, a fixed characteristic of the battery. In simple terms, more “power-hungry” appliances, such as microwave ovens or heavy-duty power tools, require a higher voltage to run.
The battery’s chemistry refers to the types of chemicals inside the battery that react to convert chemical energy to electrical energy.
The most common battery chemistry types are lead, nickel, and lithium. You are probably more familiar with lithium-ion batteries in electronic items like cell phones, whereas the battery in your car is almost certainly a lead-acid battery.
In terms of solar power, lead-acid batteries were the preferred option for a long time, as they are reliable and robust. However, recent developments have seen lithium-ion batteries gaining more favor (source).
The capacity of a battery is the maximum energy that the battery can provide. This is where we arrive at the “Ah” marking on your battery.
Ah stands for ampere-hours (or amp-hours), which indicates how long the battery can be expected to supply a particular current before it runs out of charge. It can be calculated by multiplying the current (in amps or A) by the time (in hours, or h) (source).
The time the battery can be expected to last depends on the current that will be drawn by the appliance it is connected to. So, a 100Ah battery can supply 100 amps for a maximum of one hour, 20 amps for a maximum of five hours, or 5 Amps for a maximum of 20 hours.
However, in real-life settings, the battery’s performance can vary widely from the theoretical capacity indicated by this number. The theoretical assumption is for a DC (direct current) load. The battery may perform differently under an AC (alternating current) load.
For more information about battery capacity and use, read “How Long Will a Deep-Cycle Battery Run a TV?”
Charging a Battery
The process of charging a battery is the opposite of the discharge process, which means we can apply a similar calculation to the one we used for capacity.
In other words, if our charging source can provide 100 amps, a 100Ah battery will be charged in one hour. If the charging source can provide 20 amps, it will take five hours to charge a 100Ah battery.
You can use this basic theory to work out how many solar panels it will take to charge a 100Ah battery. But first, consider the limitations of your system, especially your battery.
The C-rating of a battery refers to the manufacturer’s recommended charging time. Although, in theory, the higher the Amps of the charging source, the faster you can charge your battery, though this is not infinitely applicable. There are real-life limitations that come from the battery itself.
Most batteries are rated at 1C, which means the battery can be fully discharged or fully charged in one hour. Attempting to charge the battery faster than this will result in degradation of the components and will shorten its life. This means the maximum power input is limited.
Calculating Your Solar Requirements
Armed with a basic understanding of the battery characteristics that will determine your solar power requirements, we can move on toward the more complicated aspects of solar power calculations.
Solar Panel Power Rating
The power output of a solar panel is rated in watts (W) and is dependent on the size and composition of the solar panel.
If you’ve started doing your research, you already know that solar panels come in a wide range of sizes and power outputs. But today, most home systems are rated between 275W and 400W per panel. Systems sold for RVs are usually smaller to fit onto the roof of a vehicle, and those may range from 100W to 160W per panel.
It’s important to remember that the power rating on your solar panel represents its performance under ideal conditions.
Since there is rarely such a thing as continually ideal conditions, it’s safer to assume that your panel’s true output is around 80% of this ideal rating. So, if your panel is rated at 100W, assume that it is providing 80W of power in a real-life setting.
Some Basic Assumptions
The bottom line is, a huge number of variables dictate how many panels you need to charge your 100Ah battery. These include how quickly you want to charge your battery, how large your solar panels are and their output, how many hours of sunlight you get, as well as where your panels are positioned…and the list goes on.
So, in order to calculate how many solar panels you will need to charge your 100Ah battery, let’s make some basic assumptions.
First, most batteries used in solar power systems are 12V batteries, so we’ll assume that your battery is the same.
Second, we’ll create two scenarios for your solar power system. For a home system, we’ll assume that your panels are rated at 300W. For a system mounted to your RV, we’ll assume that your panels are rated at 100W.
We will also assume that you get five full hours of sunlight a day and that your panels have an output of 80% of their power rating.
All the calculations we’ll do now can be adjusted for different scenarios — simply replace these assumptions with the numbers that apply to your situation.
Let’s Do the Math
First, let’s convert the battery voltage to watt-hours, which is a measurement of power usage or generation.
Watt-hours = voltage x amp-hours = 12 x 100
So a 12V, 100Ah battery will require 1200 watt-hours to charge fully. Now let’s consider our solar panels.
Scenario 1: Home
Each panel is rated at 300W, which means that their true power output is 80% of 300W = 240W. Each panel receives a total of five hours of sunlight per day.
Watt hours = power x time = 240 x 5
So a 240W panel will generate 1200 watt-hours in a five-hour day.
Since your 100Ah, 12V battery requires 1200 watt-hours to be fully charged, this means a single solar panel rated at 300W will be able to charge your battery in a five-hour timeframe.
Scenario 2: RV
Each panel is rated at 100W, which means that their true power output is 80% of 100W = 80W. Each panel receives a total of five hours of sunlight per day.
Watt-hours = power x time = 80 x 5
So, an 80W panel will generate 400 watt-hours in five hours’ time.
Since your 100Ah, 12V battery requires 1200 watt-hours to be fully charged, this means a single solar panel rated at 100W will be able to charge your battery in three five-hour days.
A Question of Time
The two scenarios provided above give you an idea of how to calculate your solar power requirements. The next question is a matter of convenience and practicality.
It’s clearly unreasonable to wait three whole days to charge a single battery. In fact, you may not want to wait for even one whole day. This is where the number of solar panels comes in.
If your solar panels are connected in series, you can add up their power output, and the time that it will take to produce the required 1200 watt-hours will reduce.
So in Scenario 1, if we have two 300W solar panels with an effective output of 240W each:
240 + 240 = 480W total power output
Watt-hours = power x time = 480 x 5
So, two solar panels rated at 300W each will produce a total of 2400 watt-hours in a five-hour day. This means it will only take two and a half hours to charge your battery.
You can keep extrapolating this calculation, and you should soon realize that if you have 5 solar panels rated at 300W each, it will only take 1 hour to charge your battery. Remember that most batteries are rated at 1C, meaning they should not be charged any faster than that.
For more information about charging times, read “How Long Does it Take a 100W Solar Panel to Charge a 100Ah Battery?”
There are many other elements that influence the charging capabilities of your solar power system.
Don’t forget to consider variables such as the thickness of your cables. Faster charging requires higher currents, which means you will need thicker — and more expensive — cables.
Likewise, make sure you select an inverter that matches the power rating and requirements of your battery and appliances. Consider speaking to your supplier or installer about the specific requirements of the system you have chosen.
There are many variables that dictate how many solar panels you will require to charge your system’s batteries. All of the calculations we have discussed above will be affected by these factors, such as your panels’ efficiency and positioning, or where in the world you live.
For example, many countries receive far fewer hours of sunlight in winter than in summer, and cloudy weather will significantly reduce your panels’ output.
However, remember that the most important factors when making a basic calculation of your solar requirements are your battery’s voltage, the power rating of your panels, and how fast you want your battery to charge. Keep these in mind, and you will find yourself successfully catering to your off-grid lifestyle.