How to Determine What Size (Watts) Solar Panels You Need

A great way to understand electricity is to liken a solar setup to a rainwater system. Let’s say the roof of a house that gathers the water is the solar panel and the water tank is the battery.

The bigger the roof, the more water that can be gathered in the same way the more watts a solar panel is rated at, the more electricity it can capture.

The water tank is connected to the roof via a pipe, as water runs down the roof into the gutters and then eventually into the pipe, more pressure will build up as the more water will be in a smaller area, think of it like a funnel. The water pressure coming from the roof represents voltage, and the flow of water in the pipe attached to the tank represents amps.

With the same analogy we have a tap at the bottom of the tank which we will draw water from. The fuller the tank is the more water pressure we will have out of this tap.

Watts - the amount of power the solar panel is producing, with the total wattage of the panel able to be achieved in perfect conditions. (A solar panel at the ideal temperature and aligned with perfect sunlight) Similarly how much power flowing out of the battery can be also measured in watts.

Volts - the pressure of electricity being produced by the solar panel, similar to the height of water stored in a water tank: the fuller the water tank is, the more water pressure you have.

Amps - the actual amount of electricity flowing through the solar panel and cables. We call this flow current, which can be compared to the amount of water flowing into and out of a water tank.

The flow of electricity will vary depending on the amount of sun on our solar panel or if the battery is full or empty, large or small.

Amp-Hour (Ah) - the steady flow of 1 Amp for one hour, 5 Amp-hour (5Ah) is a flow of 5 amps for one hour or 1 amp for 5 hours.

Ohm’s Law states that: Volts x Amps = Watts

This can also be changed around so that Watts / Volts = Amps or Watts / Amps = Volts

We have a constant as we know that our system is 12.8 volts at full charge. But when we are talking about solar panels, the unregulated voltage will be between 19 and 21 volts. You will hear this being referred to as VOC or Open Circuit Voltage.

So How Much Power do I Need?

This brings us to how we work out what size system we will need. The easiest way to do this is determine how much power your appliances are drawing out of the system over a 24 hour period. This can either be determined in watts or amp hours. Generally when we are talking about 12 volt systems we use Amp Hours but they are interchangeable.

The easiest way to do this is buy using a Kickass Watt meter on the load side of the system to to give us a figure in either watts or Amp hours on what you a drawing and remember that you will need to do this over a 24 hour period.

Generally the most common and biggest drawing item on your system will be a 12 volt compressor fridge which can account for up to 60% of your total power draw as they are running the whole time you are away. While appliances such as power inverters and air compressors can draw a lot of power, most of the time they are only operated for minimal amounts of time. LED lighting is preferable as it has very low power draw.

Fridge Draw Reference

If used as a freezer, it will draw approx 25% more AH)

Fridge Size (L) Compressor Current Draw Fridge Draw (on avg over 24 hrs) Total Amp Draw (on avg over 24 hrs) Total Amp Hour Draw over 24 hrs
30L - 50L 5 Amps 6.5 hours 1.3 AH 33 AH
50L - 60L 5 Amps 7.5 hours 1.5 AH 38 AH
60L - 80L 5 Amps 9 hours 1.8 AH 45AH
80L - 110L 5 Amps 11 hours 2.2 AH 55 AH
110L - 210L 5 Amps 14 hours 2.9 AH 70 AH

Once we have a figure, for this example 70 Amp hours or 896 Watt hours, we can work out what we need to balance this power draw with power generation, this is where our trusty solar panels come in. To work out how big our panel needs to be we need to work out how much sunlight we are going to be able to capture. Below is a chart that shows us the average daily full sun available across Australia.

If we are travelling across queensland and we will get on average 7 hours a day of full sun, if we divide our 70 amp hours used over 24 hours by our time in full sun we know we will need to generate at least 10 amps an hour during the day in order to keep our system continually running.

So if we look at our panels VOC of 21 volts and we need 10 Amps per hour we will get the equation 21 volts x 10 amps = 210 watts. So in a perfect world in perfect conditions all we will need 210 watts worth of solar panels. But as we all know it is not a perfect world so it is always a good idea to give yourself some headroom.

Conditions such as overcast days and extreme heat will affect how much power that you panels will generate so it is always a great idea to install a system slightly bigger than you will need. If we put a 300 watt panel on this system and we have a few bad days weather the extra output of the panel will allow the system to catch back up when the weather turns good again.

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