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How to Calculate the Length of Time It Takes a Battery to Charge from a 12v Solar Panel

The time taken for a solar panel to charge a battery depends on the current of the panel. As battery capacity is measured in Ah (amp-hours), you need to know the solar panel current to estimate, how quickly it will charge your battery.

For example,

A 200W solar panel normally produces 8 Ampere per hour on a sunny day. If you have a 100A/h battery, this will take approximately 200/8 = 13 hours of sunshine to charge from 0% to 100%.

The current produced by a12 volt solar panels or charging kits is proportionate to their wattage. Therefore a 100W solar panel would take twice as long to charge the same battery compared to a larger 200W solar panel.

Deciding Capacity of Your Batteries

The capacity of your batteries is very important for the size of your solar panel. It does need to be taken into consideration for two reasons:

    Batteries have a limit on the initial charging current – This is important for more expensive batteries such as gel or AGM batteries. If you have a small battery like a bike or a motorcycle, you can’t really use a very big solar panel or a charging system unless you are absolutely sure that the battery can accept this current. The tall tubular battery is the best battery used for solar power charging. This battery are called C10 rating Batteries. This is helpful in Deep discharge and quick charging with more than 1000 repeated cycles of charging and discharge.
    Typically batteries have a limit on charging current equal to 10% to 20% of their capacity. Therefore a 100A/h battery cannot be charged by a current higher than  20%*100A = 20A (this would be the equivalent of a 320W solar system current).
    This will not normally be a concern unless the battery you use is really small. However it is wise to the specifications of your battery to see whether there are any restrictions on the charging current.
    If the battery cannot store the charge, the energy is wasted. When you have invested in a solar panel of a higher value capacity for your boat or home, it would be of no use if the energy generated go to waste simply due to the limited capacity of your battery.
    For example, if your 12v solar panel starts charging the battery in the morning and completes the job in 4 hours, the rest of the energy generated throughout the day is wasted. This is a complete waste, especially if you need the energy in the evening or at night for lighting etc. So it is always advisable to have a higher capacity battery for the selected solar panel so that energy is also available during night and non solar days.

Using Solar Power Inverters to Convert 12V DC to 240V AC

A solar power inverter is used to convert 12V DC obtained from solar panels to 240V AC to run appliances. This should be taken into consideration when calculating our usage.

Solar Inverters typically work at around 85-90%. Peak efficiency meaning that they draw 10-15% more energy from your battery than the devices actually requires. This means the daily power consumption used to calculate the size of a home solar panel should be increased by approximately 10-15%. Large inverters used to power small loads will have even lower efficiency.

Calculating the Power Required for an Electric Fridge

Please note, an electric fridge typically have very high power consumption. Such a unit will often require a 200W solar panel alone. This is in contrast to a gas fridge which requires much less energy to operate.

It is therefore important to estimate the power consumption of your fridge correctly and incorporate this into your calculations when choosing the right solar panel for appliance.

Calculating the Solar Panel Size

The average daily power generated by a home solar panel should be equal or more than the average daily power consumption of your appliances.

For example, let’s assume that on a typical day we can expect 10 hours of sunshine. If we have a 100W solar panel on the roof of our home, it will generate approximately 100W*10 = 1000W/h (watt-hours) – this is almost in line with the average daily power consumption 1000W/h.

In reality there will be some losses of energy generated by the solar panel. mainly due to following reasons (1) Solar charge controller (2) Cables and (3) Battery self-discharge. These are typically kept within 20% of total output. It is therefore advisable to choose a solar panel for your home that offers a daily power rate of approximately 20% higher than the daily power consumption of appliances requirement.

Sometimes it is really difficult to estimate the daily power consumption, especially when the power rating of individual appliances is not available. In this case we recommend the following approach:
– If you simply need to trickle charge your battery to compensate for self-discharge in your caravan, a small 5W-10W solar panel or charging kit should be sufficient
– In the case of small loads such as energy saving lighting, a stereo, mobile phone charging and low usage of a computer (5-6 hours a day), we would recommend a 100W solar panel or charging kit for your home
– For larger loads or heavier use including non-energy saving light bulbs, large TVs, frequent use of water pumps or other equipment, we would recommend a larger 200W charging kit or solar panel.

Calculation of Average Daily Power Consumption

In order to choose a solar panel, you need to know how much energy your appliances use each day in a 24 hour period. The average daily power consumption can be estimated individually for each device (type of load) and added together to create an overall consumption requirement.

For example:
– 4 x 10W 12V energy saving LED lights, used 4 hours a day
– 1 x 40W 12V LCD TV, used 3 hours a day
– 1 x 20W 12V water pump, used not more than 60 minutes a day

Based on the above, the average daily power consumption will be 4*10W*4 + 40W*3 + 1*20W*1 = 300W/h (watt-hours)

Once this estimate is available, it’s very easy to decide on the size of 12 volt solar panel required for your caravan or home.
How to Choose the Right 12V Solar Panel for Your Caravan, home or Boat

Our customers always ask us about the right size of solar panel to charge 12V batteries in vehicles and boats. Unfortunately, based on our experience, there’s no single right answer to this question. Solar panels for caravans, homes and boats can all be very different.

As a rule of thumb, there are two factors which need to be considered when deciding on the solar panel size you require:

    Your average daily power consumption
    Capacity of your batteries

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