How to calculate your solar power requirements.
There are three things to consider in order to choose a Solar panel or create a Solar system.
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 according to your pattern of use.
How much energy can your battery store?
Battery capacity is measured in Amp Hours (e.g. 7AH). You need to convert this to Watt Hours by multiplying the AH figure by the battery voltage (e.g. 12V).
For a 7AH, 12V battery the Watt Hours figure is 7 x 12 = 84WH
This means the battery could supply 84W for 1 hour, or 41W for 2 hours (the more energy you take, the faster the battery discharges).
How much energy will your appliance(s) use over a period of time?
The power consumption of lamps / appliances is given in Watts (e.g. 9W CF lamp). To calculate the energy you will use over time, just multiply the power consumption by the hours of use.
The 9W CF lamp, on for 2 hours, will take 9 x 2 = 18Wh from the battery.
Repeat this for all the lamps / appliances that you wish to use, then add the results to establish total consumption.
How much energy can a Solar panel generate over a period of time?
The power generation rating of a Solar panel is given in Watts (e.g. 10W or 10Wp) and Peak Output current (Amps).
To calculate the energy that the Solar panel can supply to the battery, multiply the Solar panel’s Watts (W or Wp) rating by the hours exposed to sunshine, then multiply the result by 0.85 (this factor allows for natural system losses).
For the 10W Solar panel in 4 hours of sunshine, 10 x 4 x 0.85 = 34 Wh energy output. This is the amount of energy the Solar panel can supply to the battery during 4 hours of full sunshine.
An alternative calculation that is generally considered to be more representative is
Peak Output current (Amps) x peak sun hours = expected output in Ah.
This expected output in Ah x battery voltage = energy output Wh
For the 10W panel in the above example the calculation for expected output would be 0.66A x 4 hours = 2.64 Ah. The energy output is thus 2.64 x 12V = 31.68 Wh
So, how long can I consume energy from the battery
You cannot use all of the battery capacity to run your lamps / appliances as it is not good practice to run your battery completely flat. If you have correctly ‘sized’ your installation you should not run your battery flat. Averaged over a week or month, your Solar panel output should balance your energy consumption.
If you start with a fully charged battery it should remain charged over a period of time. If you continually run your battery flat you should consider reducing your energy consumption or increasing the size of your Solar panel.
The ‘rule’ should be energy consumption in 1month = energy output from solar panel in 1 month.
As a rule of thumb, we work on an energy output of 1/8th of peak sun during the winter (average 3 months). So, you need to recognise that typically energy consumption is highest when the energy generation is least. If availability of power is critical during winter months you should size your Solar panel according to your requirements more carefully. There is a very useful tool to help you determine sunlight data throughout the year. Please click here to link you to the sunbird website.
Calculating the permissible lamp operating time.
To calculate the lamp operating time, divide the battery Ah by the lamp current and multiply 0.4 -> Max. Lamp operating time in hours. The factor 0.4 (40%) is the amount of energy generally considered to be available without discharging the battery too much. Some people work on a higher factor of 60% ~ 70% but we recommend that you work to 40% to be prudent.
Example: 7 Ah battery with 9WCFL12V lamp: 7 Ah / 0.75 x 0.4 = 3.7hrs.
NB lamp current is calculated by dividing the lamp Watts by the battery voltage (normally 12V for lighting systems). In the example above, 9W / 12V = 0.75 A
However, you will find that we generally provide the data that you require for the solar panel to help you decide whether the solar system will meet your particular requirements. As an example, click here and scroll down the page until you reach the diagram / table to see what we mean.
For the technically minded.
The power output of a PV module depends on the number of cells in the module, the type of cells, and the total surface area of the cells. All manufacturers rate their modules in terms of their peak power (Wp) under standard test conditions: i.e. 1000W/m² of sunlight (‘peak sun’); 25 ºC; and air mass of 1.5. Modules nearly always produce less than their rated peak power in real-life conditions.
How much energy can a Solar panel generate over a year?
1 sq metre of PV module ~ 115Wp and 1 kWp of well sited PV array in the UK will produce 700-800 kWh of electricity per year.
A useful tool for estimating power output in kWh/month using sunlight data for anywhere in Europe or further afield is available by clicking here.
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