Watt is that? How to size your solar power generator?

Watt is that?

Although it is not necessary to know the terms commonly use in a solar generator setup, a basic understanding of the terms can help you to estimate and size your solar energy system to meet your energy needs.

Watt is the unit used for the measurement of the amount of power. It is the product of Voltage and Current, that is when you multiply voltage (V) by current (I), you get Power in watt. (P=VI)

Voltage (V) is the potential difference between 2 points, say point A and point B. When point A is higher than point B, there is a potential difference which is measured by Volt. The bigger the difference, the higher the Voltage.

When there is potential difference between 2 points, electrical energy will flow from one point to another just like water flowing from a higher ground to a lower ground. The amount of electrical energy flowing through is known as Current and is measured in Ampere (I).

As for Resistance (R), I see it as a slope between point A and point B, the higher the Resistance, the gentler the slope and it will take more time for the same amount of electrical energy to pass through.


How to size solar power generator?

First, we need to know the appliances we want to power by the solar energy. The electrical appliances in our house are usually rated in Power or Watt. For example, an electric fan is rated about 50W. Second, we need to know how many hours we want to run the appliance. Let's say we want to run it for 8 hours per day. Then the energy we need to power the fan for 8 hours is 8 x 50W = 400Wh (Watt hour).

So your solar panels would need to generate at least this amount of energy. If you are staying in landed property, you can probably get 8 hours of sunlight. But you don't always get the full intensity of the sunlight due to cloudy sky and the angle of the sun beam, so you would have to discount that by say 1/3. If you are powering the fan through inverter, you have to factor in another 10% to 20% loss. So I would say it's better to over estimate and get a bigger capacity panel. In this case, I would suggest go for a 100W panel. It's very difficult to estimate the actual panel size you need as the amount of sunlight receive varies from places to places. For those of us staying in HDB flat, we may need to double the size of the panel as we are only getting the morning or afternoon sun.

After you have estimated the power of the panel required, you need to size up the battery required to store the solar energy if you are going to use the energy at night. We have calculated that you need 400Wh to power the fan for 8 hours. If you are going to use it only at night, you would need at least 400Wh / 12V = 33.3 Ah battery to store the energy generated by your solar panel. Battery is usually rated in Ah and I am assuming that we are setting up as 12V system. But it is better to get at least twice the capacity you need because it is no good for the battery to be always drawn down more than 75% most of the time. That will shorten the life span of the battery.

The last thing to size up is the charge controller. Charge controller is normally rated in Ampere (A).
A 50W solar panel will produce about 4.2A at peak power (50W / 12V). A 50W fan will also draws around 4.2A. If you want to power an LCD TV which draws about 100W, then you need a higher rating charge controller to handle the higher output of around 8.3A.

I hope this short write up will help you in sizing up your system. This is more of an art then science. You can always try and error to get the most ideal setup for your energy needs.

Also begin small and add on when needed.
If you find that your battery depleted within the same day (you get low voltage alarm and cut-off from the charge controller), then you need to increase your battery size. If you find that your battery depleted after a few days, then you need to add more panel. If you find that you battery is always full, then you are doing great, you can try to run more devices.

Cheers!