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What size solar panel do you need to charge a 12v battery?

First things first…

What size solar panel do you need to charge a 12v battery? Well, that’s a tricky question to answer without knowing how much power is required, and how big the 12v battery you need to charge, is.

Generally speaking, the size of the 12v battery is less important than the size of the solar panel. Ideally, the amount of power a solar panel generates needs to be the same as (or more than) the amount of power required per day.

If on an average day, there is say 4 hours of sun, and you are using a 100W solar panel, then the panel will produce around 400 watt-hours of electricity. However, some of this power will be lost through the solar charge controller, cables, and battery. Charging a small capacity 12v battery using a large 100W solar panel for example, is not an efficient use of the suns rays. A small 12v battery will charge quicker than a larger one, but once the battery is fully charged any additional power produced by the solar panel will be wasted rather than stored in the battery for use later on.

So, as a rule, it’s not a good idea to use a large solar panel to charge a small battery. Another variable to consider is the batteries charging current limit. You need to check that your battery can receive the output current of the solar panel you intend to use.

What size solar panel do you need to charge a 12v battery?

So what size solar panel do you need to charge a 12v battery?

Reading through the rest of this article should give you a good understanding of solar energy basics. However, you can skip this information if you prefer, and just go straight to the section that best describes your needs. All you have to do is choose one of the scenarios below – then skip to that section. There you’ll find the perfect solar panel solution for you, and that particular scenario.


Scenario 1:

I want a solar panel to prevent my 12v battery from going flat when it’s not being used. I just want to keep the battery topped up using a trickle charge.

Yep, that’s me – Skip to this section.

Scenario 2:

I will be using a small amount of power for approximately 2 to 3 hours a day. e.g using energy efficient led lights, charging a mobile phone, and occasional laptop use.

Yep, that’s me – Skip to this section.

Scenario 3:

I need plenty of power for running my lighting, water pump, charging my mobiles phones, frequent use of my laptop, watching TV and powering my other devices.

Yep, that’s me – Skip to this section.

How much power do you actually use per day?

To work out how much power you use in a day. You first need to know the wattage of your devices and appliances. And also how many hours a day on average, they are being used. For example, if a 10W device is used for 4 hours, then you multiply 10 x 4 which is 40 watts. Use this method to calculate the total amount of energy consumption per day for all your devices. Then add them together, to get the total amount of energy used per day.


  • 1 x 15W water pump – used for 2 hours per day (30 watts).
  • 1 x 10W stereo – used for 1 hours per day (10 watts).
  • 3 x 10W lights – used for 3 hours per day (90 watts).
  • 1 x 40W TV – used for 2 hours per day (80 watts).
  • 1 x 25W laptop – used for 2 hours per day (50 watts).
  • 2 x 3W mobile phone charging – used for 1 hour per day (6 watts).

The total amount used per day = 266 WH (watt-hours)

The total amount of watt-hours used per day is the amount of power that is required to replenish the 12v battery.


How many watt-hours does my battery have?

To work out how many watt-hours a battery produces, you need to multiply the amount of AH (amp-hours) by the battery’s voltage. For example: for a 100AH 12v battery, you would multiply 100 by 12 which is 1200 watt-hours or 60WH for 20 hours. (Information source for this conversion.)

To supply the demands of Scenario 3 above, (frequent daily use of lights, water pump, laptop, mobile phone charging and watching TV, etc.) will require the use of a 100W solar panel paired with a good sized leisure battery (100AH plus). This will ensure that you’ll have plenty of stored power when you need it most.

How long does it take to charge a 12v battery with a solar panel?

This depends on a lot of factors, such as the efficiency of the solar panel, how much power is already in the battery, and how much sunlight the solar panel receives. As a general guide. On a sunny day, a 100W solar panel will produce approximately 4-5 amps per hour in full sun. This means that the solar panel would take around 18-25 hours to charge a fully discharged 100AH 12v battery. A solar panel half the size (50w) would take approximately double the amount of time to charge the same size battery.

Can you run a fridge with a solar panel?

Yes, you can. However, fridges are power-hungry appliances. If you want to use solar energy to run a fridge, then it would need a solar panel of its own: typically around 100W to 150W plus. You would also need to connect the solar panel to its own 12v battery via a solar charge controller. A compressor type fridge can work well using solar energy. However, if you have an absorption type 3-way fridge, running it on gas will usually be more efficient.

Recommended Solar Panels

We have selected three popular solar panel kits that are available on Amazon. All three kits are supplied by the UK based solar company, Photonic Universe, which have been in business since 2009. Photonic Universe only uses the highest quality solar cells in their solar panels. These cells are produced by one of the leading German solar manufacturers, SolarWorld. Their solar cells give excellent performance both on sunny days and in low light conditions. Not all solar panels are created equal so beware of cheap solar panels. As they say – buy cheap, buy twice!


10W solar panel kit for trickle charging a 12v battery:

Solar panel dimensions: 340 x 240 x 20 mm

If you’re looking for a way to keep your 12v battery charged when it’s not being used (such as the leisure battery in a campervan, caravan, boat, etc.). Then the 10W 12V Photonic Universe solar power kit with 5A charge controller is the ideal solution. The easy to install kit will supply a trickle charge to the 12v battery to prevent it from going flat. This will also prolong the life of the battery.

This 10W kit is suitable for any sealed, gel, or wet lead-acid 12V battery larger than 8Ah. The highly efficient 10W monocrystalline solar panel is designed for outdoor use and is totally waterproof. The supplied 5A solar charge controller (not waterproof) has a built-in overcharge protection function. So there is no danger of your battery being damaged due to overcharging.

The kit comes with 3m of battery cable which incorporates an inline fuse and two crocodile clips (positive & negative) to connect straight to your battery.

50W solar panel for small loads & light use:

Solar panel dimensions: 590 x 505 x 25 mm

If you use a small amount of power for approximately 2 to 3 hours a day, (e.g using energy efficient led lights, charging a mobile phone and occasional laptop use) then this 50W 12V Photonic Universe solar charging kit with 10A controller is for you.

The 50W Photonic Universe solar panel consists of 5 busbar monocrystalline high-efficiency top quality German made solar cells. These cells are able to produce more power than most other solar panels of the same size. Designed for permanent outdoor use, the solar panel has a durable sealed aluminium frame and is totally waterproof, giving many years of free electricity.

The kit includes a 10A solar charge controller which is compatible with sealed, gel, and wet lead-acid 12V batteries. The solar controller has an automatic cut-off function to prevent your battery from being overcharged. The solar charge controller will also prevent reverse current discharge from the battery at night.

The kit comes with a generous 5m of special solar cable. All in all this an excellent solar panel kit. It is an ideal choice for campervans, caravans, or boats, as well as many other applications where reliable solar energy is required.

100W solar panel for heavy loads & frequent use:

Solar panel dimensions: 1200 x 540 x 35 mm

For large loads and frequent use, then this 100W 12V Photonic Universe monocrystalline solar charging kit with 10A automatic solar charge controller is what you need. The 100W Photonic Universe monocrystalline solar panel will generate approximately 400 watt-hours of power with 4 hours of sunlight.

It will provide plenty of power for running lighting, regular use of a water pump, laptop, charging of mobile phones, watching TV and powering other devices. As with the Photonic Universe 50W solar panel above, this 100W panel also uses the same high-efficiency German made solar cells, which are housed in a tough aluminium waterproof frame.

The kit comes with a 10A Photonic Universe solar charge controller which incorporates an automatic cut-off feature that will protect your battery from overcharging. The controller uses Pulse Width Modulation technology to increase charge acceptance. PWM also prolongs the life of your 12v battery. The kit comes with 5m of special solar cable which has been designed to keep power loss down to a minimum.

This 100W Photonic Universe solar charging kit is the perfect choice for all off-grid applications such as campervans, caravans, and boats. It’s simple to fit, and will provide free electricity for many years to come. A great investment.

How to use a solar panel to charge a 12v battery:

To avoid overcharging the 12v battery most solar panels over 30W will need to be connected to a solar charge controller. This sounds complicated, but it isn’t. As you’ll see by watching the video below, wiring up a solar panel to a solar charge controller is really very simple.


What size solar panel do you need to charge a 12v battery?

Well, hopefully after reading this article you now have much more of an idea of the answer to that question! I’m sure you’ll agree that solar energy is an exciting technology and is only going to get better. It offers many affordable solutions to sustainable free energy and can be used in for numerous applications.

So let’s have three cheers for Helios shall we! Hip, Hip…