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So, you've decided that you want to install a solar energy system. Here are some easy steps to help you work out what size system you need.
Step 1: What electrical devices are you going to use and how much power do they need?
To do this, make a list of the electrical devices that you want to use, their power requirements in WATTS (W) and the number of hours each day you think you will use them. Multiply the WATTS by the HOURS and this will give you a system requirement in WATT-HOURS (Wh).
Example:
Two-way radio (50W) x 8 hours a day = 400Wh
Fish Finder (30W) x 4 hours a day = 120Wh
Electric low-power light bulb (10W) x 4 hours a day = 40Wh
TOTAL system requiement = 560Wh
Step 2: What size solar panel do I need?
The system above shows that we need 560Wh of electricity per day to make sure our devices will operate. The solar panel(s) need to be able to replace AT LEAST this amount each and every day. A 100W panel is able to produce 100W each hour at 100% efficiency. As we know, the efficiency of solar panels depend on the light conditions, so a good rule of thumb is to assume that they will only operate at an average of 70%.
Example:
560Wh divided by 5* hours of sun a day = 112W per hour
112W at 70% efficiency (152 / 0.70) = 160W per hour
Therefore, we need a solar panel system of at least 160W (such as two 85W Sunpower SPR-85 panels)
*based on a typical autumn / winter's day
Step 3: Which charge controller do I need?
Your charge controller needs to be able to cope with the maximum amount of current that will flow through it. To estimate the current flow of your system, divide the wattage of the solar panel(s) by the voltage (typically 12V). You should also add 20% to account for peak outputs.
Example:
Two Sunpower SPR-85 panels on a 12V system
170W / 12V = 14.2A + 20% = minimum 17.0A
If you wish to calculate more accurately the maximum current and voltage for your chosen solar panel array, you should look at the 'open circuit voltage' (Voc) and 'short circuit current' (Isc) data. Each solar charge controller has a maximum input voltage and current specification that you must not exceed.
Typically, solar panels are connected 'in parallel', so the voltage stays the same and the current increases in multiples of the number of panels. It is, however, possible to connect 'in series', so that the voltage increases, but the current stays the same. In some cases, a combination might be suitable:
Example:
4 x 135W Kyocera panels in parallel = 22.1V + 33.6A
4 x 135W Kyocera panels in series = 88.4V + 8.4A
4 x 135W Kyocera panels (2 in series + 2 in parallel) = 44.2V + 16.8A
There are a number of different features available on charge controllers, such as LCD displays and weatherproof casings, which you need to choose based on your specific requirements and budget.
For trickle charging a battery (i.e. keeping the charge topped up when the battery is not being used), it is sometimes possible to do it without using a charge controller. However, the maximum current (in amps) from the solar panel must NEVER exceed 10% of the charge capacity (in Ah) of the battery. To avoid any risk of damage to the battery, we recommend that the maximum current never exceeds 1% of the charge capacity.
Step 4: What battery do I need?
As part of a solar energy system, the battery is constantly being discharged and recharged. This means that standard 12V batteries as found in your car are not suitable since their health will rapidly deteriorate. For a solar energy system, you need to use a 'deep cycle' battery, which is specifically designed for constant charging. We recommend that you use a battery with a minimum rating of 110 Amp-hours (Ah), i.e. 110A for 1-hour, 55A for 2-hours, etc. This is the equivalent of 1320Wh and so is suitable for most recreational solar energy installations. For higher power systems, a battery bank of two or more batteries should be used or a higher power battery, e.g. 220Ah.
For more detailed calculations, please refer to our advice section: Battery Bank Size Calculations.
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