Designing a home solar energy system is certainly an exciting process, but it can become rather confusing too. When designing your system, there are two vital components to consider first; solar panels and batteries.
Once you’ve decided what your energy needs are, you’ll need to decide how many batteries you need and then what size panels are required to charge your battery bank.
However, this is easier said than done, and just what is the correct solar panel to battery ratio?
In general, you’ll need a panel that can charge your batteries at a steady rate so that you can have them fully, or almost fully charged at the end of the day. This charge rate depends on a variety of factors, but there are some formulas to help you choose the perfect panel/battery ratio.
In this article we’ll be covering the following:
- Choosing the right panel/battery combination
- Calculating panel to battery ratios
- Calculating your panel and battery power needs
If you’ve just invested in a new battery for your solar system and want to know what panel you need to properly run it, you’ve come to the right place!
How to choose a battery for a solar panel?
Choosing the right panel and battery combination depends on a variety of factors, including:
- Your energy consumption. How much power are you currently using, every day?
- Your location. Do you live close to the equator? How much sun do you get every day, and how much overcast weather is there in your area?
- Your energy needs. What are the energy needs of your solar setup? Are just using it for backup or are you completely off-grid?
- Your budget. While solar panels and other solar products have certainly gotten more affordable in the past decade, it is still not cheap to convert to 100% solar power. Your budget may dictate how quickly you can make the transition.
When matching your panel and battery, carefully consider the above points before making a decision, as this will help you make the best choices for your system in the long run. Let’s look at how to choose the battery for a solar panel.
For most applications, a good general rule of thumb is a 1:1 ratio of batteries and watts, or slightly more if you live near the poles.
For example, if you have a 100-watt panel producing about 6 amps per hour, or 30aH per day, coupled with a 200aH battery, your battery will not be getting enough amps for it to charge fully within a day or two, unless you are using less than 30ah per day.
This could work if you are only drawing minimal power for charging phones or something similar, but it will put unnecessary strain on your solar setup in the end.
Alternatively, a 300-watt panel charging a 100aH battery results in a lot of wastage.
Even if you were using the battery to its maximum potential, every day, you would only need 50aH of incoming charge per day — maximum — to fully charge the battery. A 300-watt panel can easily provide double that, and so is not being used to its full potential.
That said, as we mentioned earlier, in areas where you are not getting very much sun or if you live closer to the poles, a bigger panel to battery ratio is preferable.
Ideally, no matter your application, the 1:1 ratio is a good rule to follow, especially for small solar setups under a kilowatt. A 100-watt panel and 100aH battery is an ideal small setup, and you can expand it from there.
Matching solar panel to battery size
Let’s take a look at the general rule of thumb mentioned earlier: a 1:1 ratio of batteries and watts. A 200-watt panel and 200aH battery is a great combination, to begin with.
If you’re using a 200-watt solar panel you can estimate roughly 15 amps of incoming power per hour — in perfect conditions.
This will equate to roughly 7 hours of charge time, or 100aH per day, depending on where you live and how much sun is reaching your panel.
It’s important to note that a 200aH battery will only have 100aH (50%) of usable power, as going any lower than this will potentially ruin your battery, or dramatically shorten its life in the best-case scenario.
With a 200aH battery and a 200-watt panel, you should be able to fully charge your battery — or at least get very close — in a single day.
With this formula in mind, you’ll need to calculate your energy needs, and then from there, you can estimate what battery storage you need, and then what panel you’ll need to sufficiently charge the batteries.
How do you calculate a battery for a solar panel?
There is a simple formula for deducing what panel size you need for your battery, but this depends on how many hours of sunlight(roughly) you’re getting per day, which, for most cases, we can average out at around six. This simple formula is:
Battery amp hours multiplied by voltage and then divided by daylight hours.
For example, a 100ah, 12v battery would be 100×12=1,200. This divided by 6 hours equals 200. So, if you want to charge a 100ah battery from flat to full in a day, a 200-watt panel in ideal conditions would do it.
Now that we’ve got a better idea of what to consider when matching a solar panel and batteries, let’s take a look at the best panel size for particular battery setups.
Ideally, you’ll want slightly more power coming from your panel than you need from your batteries, as this will reduce the chance of running out of power and help during overcast days.
Remember, these are rough estimates, like your location and your energy needs will also play a part in the battery and panel size you need. Here are the best panel sizes — in general — for most common battery specifications.
A 12v battery needs at least 13.6 volts coming into it to charge efficiently. However, a 12v battery can be as small as 50aH or as big as 200aH, so it’s the amp hour rating of your battery that’s most important.
With that said, you’ll need a panel that is delivering between 13.6 and 17 volts, and depending on your battery’s ah rating and your power needs, we recommend a panel of at least 100 watts.
Panels made for charging 12v batteries can be as small 10-watts and as large as 200-watts, but panels for 24v batteries begin at around 300-watts, minimum.
So, depending on your needs, you’ll need to get a 24v panel of at least 300-watts.
When charging 48v batteries, you’re going to need a ton of power. These batteries hold roughly 5700-watt hours of power, and depending on your power usage you’ll need a lot of panel power to recharge the battery every day.
Ideally, you’ll need at least two kilowatts(2kWp) of panel power. This could come from eight 250-watt panels wired in series or five to six 350-watt panels.
A 100-watt panel is the best bet for a 50ah battery. You’ll be getting around 6 amps per hour (maximum), which will easily charge your battery in a day or less.
According to the above-mentioned formula, you’d need a panel of between 120 and 140-watts to charge an 80ah battery in a day.
While you could get away with a 100-watt panel as mentioned in our 1:1 ratio example, if you have a lot of power needs a 180-200 watt panel is best.
For best results, a panel of around 210-watts is ideal for a 120ah battery but anything above 120 watts will do the trick too.
Ideally, you’ll need around 250-watts to charge a 140ah battery in a day, but 150-watts and above is fine more most applications.
200ah is a lot of power, and your panel will be working hard to provide it. A panel of 350-watts is ideal, but 200-watts and above will manage too.
Solar panel battery sizes:
100-watt solar panel
Maximum 80-100ah, but ideally a 50ah battery.
200-watt solar panel
Ideally, a battery of 100-120ah but could work for a 150ah battery too.
300-watt solar panel
Best for 24v setups, and you’ll need a battery of at least 100ah to draw 1,000 watts or more, but a 200ah battery is ideal.
400-watt solar panel
Around 250ah of power, ideally a 200ah battery, or 2x120ah batteries.
500-watt solar panel
A 500-watt panel setup(2x 250-watt panels) can easily charge a 200ah battery in a day, so you could have 2x200ah batteries charging if you are not running them flat every day.
1000 watt solar panel
With 1,000 watts of panel power (4×250-watt panels, 3x 330-watt panels), you could easily get enough power to charge 2x200ah batteries, and probably three or even four if your energy usage is moderate.
Solar panel battery ratios:
When we start getting into large solar setups — 1kw and larger — you’ll need to start looking at 24v and 48v systems, which are able to handle the higher voltage loads.
For these large systems, 12v panels and inverters will not be sufficient. 12v, 24v panels, and 12v batteries can be used, but they’ll just need to be wired properly in series to bump up the voltage.
1kw solar system
This will depend on the individual panel sizes you opt for. As an example, you’ll need four 250-watt panels or three 330-watt panels.
The biggest single panel currently available is 615-watts, which you’ll need two of for a 1kw system.
Again, you’ll need to look at the load you need to run. If you’re running a 1kw continuous load, a 200ah battery will run for an hour, maximum.
Ideally, a battery bank of four 200ah batteries with 1kw of panels is best, or around 600ah of battery power.
2kw solar system
2kw of panels(8x 250-watt panels, 6x 330 panels, 3x 615-watt panels), and up to ten 200ah batteries.
4kw solar system
4kw of panels(12x 330-watt panels, 6x 615-watt panels), and 2,400ah of battery storage. Once you start getting into systems as large as 4kw, it’s best to go for lithium-ion batteries for power storage.
8kw solar system
8kw of panels (12x 615-watt panels), and 5,000ah of lithium-ion battery storage.
10kw solar system
10kw of panels (15x 615-watt panels), and 7,500ah of lithium-ion battery storage.
12kw solar system
12kw of panels (18x 615-watt panels), and 10,000ah of lithium-ion battery storage.
14kw solar system
14kw of panels (21x 615-watt panels), and 12,500ah of lithium-ion battery storage.
Choosing the right size
Choosing the right solar panel for your batteries depends on several factors, from your location to your energy needs, but there are some basic formulas you can follow.
Hopefully, we have helped remove some of the mystery and helped you work out the perfect panel and battery combination for your power needs!