In order to specify the size of solar panels required to supply enough energy for your yacht, caravan or motorhome, you will have to make a few basic calculations.


Purchase the largest leisure battery or install the biggest battry bank you can. This will help off-set periods of limited UV uptake, such as during cloudy conditions or at night-time, and maximise the storage capacity of the energy captured by the solar panels.

It’s also important to realise that a lead acid battery will output approximately 50% of its rated output, so a 100AH battery will give you around 50AH of usable power.

If you only have a small requirement and spent your time on 230V shore power/ hook up a large battery may not be necessary asuming youhave a battery charger.

AGM or Absorbed Glass Mat batteries are manufactured in such a way that makes them superior to ordinary lead acid batteries when used with Solar systems. They are up to 3 times more resilient when charging and discharging and provide better performance, due to very low internal resistance.  Lithium is even better The additional cost involved in purchasing these batteries is well worth it in the long run and we recommend them for use onboard yachts caravans and motorhomes.

Solar panel calculations to consider:

  1. The amount of energy the battery can store
  2. How much energy you are likely to consume on average each day
  3. How much energy a Solar panel can generate over the same period of time.

On average, you will need more power going into your leisure battery than being consumed, this is where a larger battery bank will help the system deficits.

If you are using an inverter to transform the 12v current from the Solar panel to 230V, don’t forget to factor in the energy required to power the inverter.

1. The amount of energy the leisure battery can store

To get the calculations under way it is necessary to convert your batteries Amp Hour capacity into Watts. In order to do this you will need to multiple the Amp Hour rating by the Voltage.

For example:

100 (Amp Hours) x 12 (Voltage) = 1200 (Watt Hours)

In the above example, a 1200 Watt Hours power supply would be available for 1 hour, or 120 Watt Hours would provide 10 hours. The more energy you take from the battery the faster the battery will become fully discharged.

Remember, lead acid batteries will provide approximately 50% of their rated power.

2. How much energy you are likely to consume on average each day

Check the power rating label on the appliances you would like your Solar Panels to run. They could be specified in Amps or Watts. If the power rating provided is in Amps you will need to convert it into Watts using the formula provided above.

To calculate the energy you will use you will need to decide how many hours a day you will be using each product. Simply multiply the power consumption by the hours you intend on using each item.

For example:

Appliance 1
24 (Watts per Hour) x 4 (Hours) = 96 (Watt Hours).

Appliance 2
10 (Watts per Hour) x 2 (Hours) = 20 (Watt Hours).

Appliance 3
16 (Watts per Hour) x 3 (Hours) = 48 (Watt Hours).

Total Watt Hours per day = 164

3. How much energy a Solar panel can generate over the same period of time.

Once you know how much power you require you should be able to size your Solar Power System accordingly. Solar panel power ratings are provided in Watts also, so in order to work out how many Watts of power a solar panel can supply to the battery, you will need to apply the following formula:

Watts (Solar Panel Rating) x Hours (Daylight/Sunlight Exposure) = Total Watts per day

It’s important to bare in mind that during the average winter day you will likely only experience 1 hours worth of power generating sunlight. However, during an average UK summer day, you can expect the Solar Panel to receive approximately 6 hours of sunshine during the day.

For example:

A 150 Watt Solar Panel receiving 6 hours of sunlight, will supply a leisure battery with 750 Watts or power.