How Many Batteries For a 500 Watt Solar System?

A 500 watt solar system can power a lot of appliances and devices, perfect for RVs, camping and even small homes. In many instances you will need batteries, but how many? And what type should use?

A 500 watt solar panel can charge a120ah deep cycle battery with 5 hours of sunlight. This charging time is possible if the solar panel produces 25 to 27 amps an hour.

How Many Batteries Does a 500 Watt Solar System Need?

A battery is paired with a solar panel to store energy. As solar panels produce electricity, it is stored in the battery so it can be used later when there is no sunlight available.

The calculation formula is:

Solar power watts / volts = amp hours

Amp hour x sun hours = battery size that can be charged

Let us say you have a 12V 500 watt solar array. 12 volts is the nominal charge, but it actually goes up to 18 volts when charging. So that means:

500 watts / 18 volts = 27.7 amps

Due to weather variables and other factors, the number might range from 25 to 28 amps. But for simplicity let us use 27.

So your 500 watt solar panel produces 27 amps an hour. Multiply it by the number of sun hours available. Example, 6 hours:

27 x 6 = 162

A 12V 500 watt solar panel can produce 162 amps with 6 hours of sunlight, enough to charge a 150ah battery.

This formula applies to any solar panel size. If you had a 1000 watt solar array, the system can produce 324 amps. That is good enough for two 150ah batteries or the Ampere Time 300ah LiFePO4 battery.

How Sunlight Affects Solar Panel Battery Charge Time

The other key is the number of sun hours available. In our examples we used 6 hours, but it can be 4, 5, 8 or 10 depending on your location.

If we took the same solar panel but had 10 hours of sun, the output will be:

500 / 18 = 27

27 x 10 = 270

With that much sunlight you can charge a 250ah battery.

We are only talking of one day. Assuming the same number of sun hours for tomorrow, you can charge even more batteries.

So while a 500 watt solar system can charge a 150ah battery with 6 hours of sun, it can recharge a 300ah battery over two days. This assumes the same number of sunlight hours available for the next day, which is a pretty safe bet during summer.

That brings up another point: the output of any solar panel, even the large ones, will ultimately depend on how much sun it receives and it is able to convert. The conversion rate is determined by its efficiency, 21%, 23% etc.

How Long Does a 500 Watt Solar System Take to Charge Batteries?

The charging time depends on two factors, the sun hours and if the battery is empty or not. Here are some examples.

A 500W solar system can charge a 200ah battery with 7 hours of sun. If the battery is only 50% discharged, it should take 3 and half to four hours.

FLA, SLA, AGM and gel batteries should never be fully discharged, 50% is ideal and 70% is the maximum. With lithium you can do a complete discharge or up to 90% depending on the model. Combine this with the sun hours available and you can see how varied the charge time can be.

The calculation steps are still the same as before. The only thing you have to change is the number of sun hours and the battery capacity, i.e. if it is fully or partially discharged.

Suppose you have a 500 watt PV array charging a 100ah battery. Remember that 12V systems reach 18 volts during charge, so we are using 18V here.

With 5 sun hours it will look like this:

500 / 18 = 27.7

27.7 x 5 = 138.5

The solar system can recharge the battery in 5 hours or less.

If the battery is 50% discharged, it only needs 50ah and should be recharged in 2 hours or so.

Using the same steps, you can calculate the charge time for any battery size. Replace the sun hours with your own figures and you should have a good runtime estimate.

Solar Panel and Battery Voltages Explained

We have been focused on 12V solar panels since they are standard for RVs. But for home use, 24V is more commonly used. Check this guide to know more about 12V and 24V solar panels.

Battery voltages also vary from 12V, 24V to 48V. But the calculations we provided above will still work. You just replace 12 volts with 24 volts.

The difference is 24V systems produce lower amps per hour. If you recharge large battery banks this is something to consider. But for RV electrical systems, 12V solar panels and batteries are still the standard.

If you want to use a 24V system, you can either get a 24V battery or connect two 12V batteries in a series to increase its voltage.

You can do the same with solar panels. If you have a couple of 12V 500W PV modules, hook them up in a series and you get a 24V solar array.

A series connection combines the voltage of the solar panels or batteries, but not the amperage. If you want to increase the amps, connect them in parallel.

You can also use a series parallel combination. For instance, you can connect three batteries in a series and then connect two of them in parallel, just make sure to have charge controllers installed. You can do the same with solar panels.

With a 500W solar panel, it may not be necessary to set up a large battery bank. If you are going to charge several batteries, you should increase the solar array capacity.

Which Batteries Should I Use For 500W Solar Systems?

There are a lot of options, but basically it comes down to flooded lead acid batteries, AGM and lithium. FLA batteries have a 50% discharge rate. For AGM batteries it is 60% to 70% and lithium 75% to 100%.

Each battery has it own pros and cons, and you can find more details here . The most efficient performance wise is lithium ion, and it is also the most expensive. FLAs are the most affordable but require the most maintenance.

AGM does not last as long as lithium but has a good life cycle, and it requires less maintenance than FLA. So it is the most frequently used by solar power users. However the battery to use depends on your needs.

If you want to charge the battery as fast as possible, do not fully discharge it. Not only will that speed up the charging but will also prolong its life cycle.

With a 500 watt solar system, an AGM battery should be sufficient. A lithium battery will work, but there is nothing you won’t be able to power than an AGM cannot. If you are going to expand the solar array to 1000 watts and higher, then you may consider lithium.

Tips For Using Solar Panels with Batteries

• You cannot buy a single 500 watt solar panel. You have to combine 5 x 100W, 2 x 250W etc.
• Check the efficiency rating of the solar panel. 21% and higher is ideal. The higher the rating, the more sunlight will be converted to current.
• Position the solar panels so they face south for maximum sunlight exposure.
• Decide what type of battery you will buy.
• The charging time will vary depending on the season.
• Perform maintenance on the batteries if required. That is necessary for good performance.
• Use the right wire sizes. The cables must also be the right distance for the batteries and solar panels, i.e. as short as possible.
• If you are going to run AC powered appliances, get an inverter. The inverter needs to be large enough to handle the battery capacity.
• Use a quality charge controller for the battery. This is necessary to prevent overloading and overcharging.

Conclusion

With so many options and possibilities, it is important that you know exactly what you want from the solar panels and batteries. Knowing how much power is required as well as the sun hours available is going to make things easier.