Sizing Solar Energy Systems

Once you've decided that you'd like to install a solar energy system, the most important thing is to now decide what size system you need to meet you energy requirements. This needs to be based on the amount and type of electrical devices that you need to power and also needs to take into account the seasonal variations in available sunlight. Our solar calculators will do these calculations for you.

Step 1: How much power do I need each day?

At this stage, you need to make a list of all of the electrical devices you will be running from your solar energy system. You then need to know the power consumption of each in WATTS (W) and the length of time each day you'll be using them (in hours). These two numbers are then multiplied together to get a daily power consumption figure in WATT-HOURS (Wh). If you don't know the power consumption in watts, it can be easily calculated by multiplying the current rating (in AMPS) by the operating voltage of the device (in VOLTS).

Example (12V devices at 12V):
Two low-power light bulbs (10W each) x 4 hours a day = 80Wh
Small refridgeration unit (50W) x 12 hours = 600Wh
TV (40W) x 3 hours = 120Wh
Kettle (600W) 5 cycles of 12 minutes = 650Wh
TOTAL system requiement = 1400Wh

Step 2: What size solar panel or solar panel array should I buy?

The basic aim is to replace the amount of power taken out of the battery storage each and every day. So, based on the example above, we need to try and replace 1550Wh every day; i.e. our solar panels need to generate this power. One of the most difficult things is knowing exactly how much energy will be produced by a solar panel on any given day as this is, obviously, affected by the amount and intensity of sunlight available. For the UK, we use a calculation based on a solar panel producing 70% of its maximum output for 5-hours a day, which ensures that we can produce enough electricicty all year round. For installations in different countries or where only summer use is expected, we can be more generous in our calculations.

1400Wh divided by 5* hours of sun a day = 280W per hour
280W at 70% efficiency (280 / 0.70) = 400W per hour
Therefore, we need a solar panel system of about 400W (such as 4 x 100W professional monocrystalline panels)
*based on a typical winter's day

Step 3: Which charge controller do I need?

The charge controller is a vital part of your new solar energy system and it needs to be large enough to manage the currents flowing from your solar panels. It is essential that the total current rating (in amps) of your solar panels does not exceed the rating of the charge controller, otherwise the electronic protection software will constantly switch off the system. To calculate the current rating of your system, multiply the 'short circuit current' (Isc) by the number of panels. You may also wish to oversize the controller to allow additional panels to be used later.

4 x 100W professional monocrystalline panels (Isc = 5.92A)

Minimum charge controller size = 23.7A

Charge controllers come in a wide range of different shapes and sizes, with different types of display (e.g. LED or LCD) and different operating modes. Wherever possible, try to use the largest, most functional charge controller that you can afford - these devices will look after and protect your expensive batteries or battery bank, so should be of a good quality.

It is possible to use a solar panel without a charge controller, for trickle charging a battery (i.e. keeping it topped up when not in use). However, it is very important that the short circuit current of the solar panel NEVER exceeds 10% of the capacity (in Ah) of the battery. To be 110% certain that there will be no potential damage to the battery, we strongly suggest that the solar panel current never be more than 2% of battery capacity.

Step 4: What batteries can I use?

As with many things, there is a wide choice of different batteries on the market today. Since the batteries in a solar system will be constantly charged and discharged, you MUST NOT use standard car batteries as they are designed only to give a very high current for a very short time. Standard car batteries will die very quickly and you need to use only deep cycle or leisure batteries.

It is important that the batteries that you choose have enough capacity to store the energy that you produce.