Calculating Your Solar Power Requirements

Calculating Your Solar Power Requirements

There are three main things to consider in order to choose a Solar panel or create a Solar system.

1 : How much energy can your battery store?

2 : How much energy will your appliance(s) use over a period of time?

3 : How much energy can a Solar panel generate over a period of time?

Firstly you need to know how much energy your battery can store and then select a Solar panel that can replenish your ‘stock’ of energy in the battery in line with your pattern of use.

1 : How much energy can your battery store?

Battery capacity is measured in Amp Hours (e.g. 17AH). You need to convert this to Watt Hours by multiplying the AH figure by the battery voltage (e.g. 12V).  this is just the simple calculation below

  •  X (Battery size in AH) x Y (Battery Voltage) = Z (Power available in watt hours

  • For a 20AH, 12V battery the Watt Hours figure is 20(X) x 12(Y) = 240 WH (Z)

This means the battery could supply 240W for 1 hour, 120W for 2 hours or even 2w for 120 hours i.e. the more energy you take, the faster the battery discharges. 

However you are never really able to take all the power from a battery as once the voltage drops below your equipment’s requirements it will no longer be able to power it. There is a simple rule of thumb for this but please check your battery’s specifications to make sure.

  •  Lead acid battery’s will give you around 50% of their rated power. (i.e. a 10Ah battery has 5Ah of usable power)

  • Li-ion battery’s will give you around 80% of their rated power. (i.e. a 10Ah battery has 8Ah of usable power)

                A common question that people ask regarding the battery’s is,

Q. Are car battery’s just as good for solar as leisure battery’s?

A. The answer to this is no they are not. The reason is because a leisure battery has been designed to be discharged and recharge, a Car battery is designed to provide a lot of power quickly but it’s not able to cope with a low internal charge and recover fully.

We provide a wide range of sealed led acid batteries that are ideal for solar applications here

 

2 : How much energy will your appliance(s) use over a period of time?

The power consumption of appliances is generally given in Watts (e.g. A small portable TV is around 20W this information can be found on the data sticker that most electrical items have). To calculate the energy you will use over time, just multiply the power consumption by the hours of intended use.

The 20W TV in this example, on for 2 hours, will take 20 x 2 = 40WH from the battery.

Repeat this for all the appliances you wish to use, then add the results to establish total consumption like below.

TV 20w on for 2 hours per day                              = 40w per day

Radio 10w on for 5 hours per day                          = 50w per day

Water pump (20w) on for 20mins per day               = 6.66w per day

Main Light 30w on for 3h per day                           = 90w per day

Spot lights 10w on for 1h per day                           = 10w per day

                                                                                    Total      = 196w per day

An easy way to lower your power usage is to swap out halogen lights for LED lights. LED lights generally use 80% less energy for a similar light level.  We have a range of 12V LED lights here

3 : How much energy can a Solar panel generate over a period of time?

The final part to sizing your solar system is the solar panels. The power generation rating of a Solar panel is also given in Watts (e.g. our part number STP010, is a 10W solar panel). In Theory, to calculate the energy it can supply to the battery, you multiply Watts (of the solar panel) by the hours exposed to sunshine.

In practice it’s not a great way to calculate the output from a solar panel so we work to a few simple rules.

·         We would generally advise that an average UK winters day will only give you 1 hours sunshine

·         An average UK summers days will give you 6 hours of sunshine.

So in winter a 10w panel will provide 10w worth of energy back into your battery. (10w x 1 = 10w)

In Summer a 10w panel will provide 60w worth of energy back into your battery. (10w x 6 = 60w)

Using the above calculation takes into consideration any losses in the system from the regulator, cables and battery you may be using.

We can supply you a range of solar panels form a small 5w unit all the way to 150w units, for more information on this look at our web site here

4 : Putting it all together to size your system.

Knowing your power requirements and the time of year you want to use the system is vital to this step.  We will use the example above with a power requirement of  196w per day.

If this is the requirements for late spring to early Autumn use we could use this equation

Watts required / time of year sunshine hours = panel size                    196 / 6 = 32.6W panel

As we don’t make a 32.6w solar panel we would recommend looking at a 30w or 40w solar panel for this application.

However if this was an all year requirement i.e. also needed in the UK’s winter the numbers would change

Watts required / time of year sunshine hours = panel size                   196 / 1 = 196W panel

As we don’t make a 196w solar panel we would recommend looking at two 100w solar panels for this application.

The final Piece 

The final Piece to complete your solar system is the Charge Controller or Voltage Regulator. Its basically the same thing just a different name. This essential piece of your solar system controls the Charge put into your battery, stops overcharging and prevents the solar panel pulling power from the battery at night. 

We hope this Guide has helped you understand the sizing of a simple solar panel setup. If you have any other questions regarding sizing a system for your requirements please give us a call on 0800 7747755 (from inside the UK) or +44 1684 774000 from any other place.

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