How Many Solar Panels Do I Need To Charge My Battery Bank?
One of the first questions that beginners ask as they plan out their solar setup is: “How many solar panels should I buy?”
While it may seem like a simple question, the answer depends on a huge number of factors.
These factors include:
(1) Your daily drain and how much sunlight you expect to receive
(2) The kind of batteries that you buy
(3) Your charge controller
(5) The capacity of your batteries.
Drain From Appliances and Amount of Sunlight
Perhaps the single most important bit of information you need to know to buy the correct number of solar panels is what you’ll be using them for.
Different users have wildly different builds and appliances.
This makes guessing at your number of solar panels very difficult.
You could be hoping to install solar panels on the roof of your RV to help power a TV or recharge a laptop.
You could be looking for a small solar panel to bring with you on a backpacking trip to recharge your phone.
Alternatively, you could be planning a roof installation with the hope of powering your entire home with solar to become entirely energy independent.
Different parts of the country vary in how much sunlight they receive.
This is influenced by your latitude and how cloudy your region is, amongst other factors.
It’s also important to consider that you’ll be receiving far more sunlight in summer than the winter.
Because generalizing with this is impossible, we’ve created a calculator that can guess at your average drain and sunlight hours based on your zip code.
Different Types of Batteries
There are several different kinds of batteries you can use for your solar setup.
A common and cheap solution is lead deep cycle batteries.
A more expensive but higher quality battery would be a lithium battery.
Many RV’s find that a few lead deep-cycle batteries more than suffice for their application.
On the other hand, Tesla solar setups use a massive lithium battery to store energy.
Lead deep-cycle batteries are cheap, but should always be held above half charge and can require maintenance.
They also typically have a shorter shelf life than lithium batteries.
Lithium batteries have several advantages over lead deep-cycle batteries, but they come with an added cost– sometimes ten times as much.
When we talk about recharging batteries, two parts of the electric current are important: amps, and voltage.
If our battery bank runs at 12 volts, we want to recharge it with a current at or a little above that 12 volt rating.
Once we’ve reached our desired voltage in our charging current, additional amps result in a faster charge time.
EX: Our battery bank has a 400 amp/hour capacity and runs at 12 volts
A 12 volt 40 amp current will recharge our battery in around 10 hours.
A 12 volt 10 amp current will recharge our battery in around 40 hours.
You may look at this and think that maximizing your amperage is the way to go.
However, that has its own problems.
High amperage currents charge our batteries faster, but they also require much thicker wiring and specialized connectors.
As electricians will tell you, high amps means high heat.
We’d have to make sure our cables were low gauge(thick) to handle the increased heat.
High amperage current also doesn’t “play-nice.”
Expensive and bulky connections are necessary to deal with high amperage current.
Additionally, if our solar panels are far away from our battery bank, we can also run into problems of voltage loss, which occurs in low voltage current over long distances.
A 14 volt 40 amp current could drop to below usable levels if the current needs to travel over long distances.
So how do we reconcile these two facts?
If we try to charge our batteries with a high voltage current it will damage them.
High amps charge our batteries faster but make everything more complicated.
That’s where our Solar Charge Controllers can help.
Solar Charge Controllers limit the amount of voltage that reaches our battery bank.
They also limit the amount of energy lost during the night time by cutting the current off to our solar panels.
There are two kinds of solar charge controllers, MPPT and PNW.
PNW charge controllers limit the voltage coming into your battery by cutting off the voltage at the useful point.
If you have a 50 volt and 10 amp current coming from your solar panels but only want 15 volts, the PNW charge controller will send a 15 volt 10 amp current into your battery.
This is less than ideal due to the wasted energy.
On the other hand, MPPT charge controllers will convert the excess voltage into extra amperage to charge our batteries faster.
This is ideal because it allows us to wire our solar panels with the easy-to-work-with high voltage/low amperage current without wasting energy.
MPPT charge controllers are more expensive, though many find them to be worth the extra investment.
The Capacity of Your Batteries
Another way to know how many panels you need is to look at your battery bank.
Batteries come labeled with an amp/hour rating.
This rating tells you how much energy each one can store when it’s fully charged.
You can use our solar calculator to know how much energy your applications will drain each day.
You’ll want to make sure your total amp/hour storage in your battery bank can handle this load each day (double the load if you’re using lead deep-cycle batteries).
Your batteries amp/hour will tell you how much energy you have available to be used in powering your appliances.
You can increase your available amp/hours by buying multiple batteries and wiring them in parallel.
As we said before, the current from your solar panels has two elements:
Voltage and Current.
These are measured by volts and amps.
If your battery bank consists of 12 volt batteries wired in parallel, your recharging current will have a voltage at or a little above 12 volts.
Once this voltage is achieved, we can calculate our recharge rate by looking at the amps of our recharging current.
If our batteries have 400 amp/hours of energy when fully charged, it would take a 40 amp current around 10 hours to recharge them fully.
It would take a 5 amp current 80 hours to recharge our battery bank.
So How Many Panels Should I Buy Then?
As we discussed in this article, there is not a one-size-fits-all answer.
To find a solution that works for you, you need to first get some idea of:
(1) What you’re hoping to power with your solar setup.
Solar energy can be used in a huge variety of applications, from home power to RV power to emergency situations.
You can use SanTan Solar’s Calculator to get some idea of how much energy you’ll need for your specific situation.
(2) The kind of battery bank you’re hoping to work with.
Lithium batteries have several advantages, but you’ll pay a premium for them.
Lead deep-cycle batteries are the cheaper solution in more ways than just price.
(Remember to keep those lead deep-cycle batteries above half charge!)
(3) Your Solar Charge Controller will help you convert current into usable voltages and amperages.
MPPT charge controllers give you more flexibility in wiring your solar panels but cost more than PNW charge controllers.
(4) Finally, we want to make sure our battery bank has the storage needed for each daily use.
Remember, we don’t want to damage lead deep-cycle batteries by allowing them to go below half charge, so be sure that your battery bank has more than double the capacity of your daily drain.