The most common power supply chosen for these sites is solar, but the decision to use solar power inevitably leads to the question -

How Much Power is Needed?
For many small systems that perhaps have few low-powered sensors and low sampling rates, almost any panel will be more than adequate. But what about those installations that have high powered (4-20mA) sensors,sat phones or switch high-power loads? Some use previous experience to judge their needs, others oversize everything to avoid running out - wouldn’t it be nice to have an easy precise way of calculating your exact requirements,saving you the guesswork? We'll get to that a little later in the article but it's important to note that there are 4 pieces of information that we need to collect in order to determine the size of the solar power supply:

1. Loads/Switching - What are the loads in the system and how often will they be switched on?

This is the most important of all the figures that you will collect.
Without knowing the current drain of the different parts of the system, there is no way to calculate the required power supply . In this step, you need to consider anything that will use electrical power – including your data logger, sensors and any communications equipment (cellphones etc) that may be attached.  Some sensors need not be included because they consume very little current, but care needs to be taken to ensure the power needs are what you expect. For example, the power consumption of a standard 03001 cup-and-vane wind sensor is negligible and could safely be ignored, but on the other hand, a 03001L sensor that has a 4-20mA output will draw a continuous current of up to 20mA which would be significant in many systems.
When considering how often a device will be switched on (the duty cycle), it isn’t necessary to convert all the time periods into the same units. For example, in your system the cellphone might be turned on for 4 hours every day, while your RH sensor might be powered for 2 seconds in every minute. In this case all you need to record is that the cellphone duty cycle is 4 in 24 and the RH sensor is 2 in 60. It doesn’t matter that one is in hours and the other is in seconds.

2. Insolation - Where will the system be situated (roughly)?

This information will allow you to estimate the insolation. Insolation is simply amount of solar radiation that will strike the solar panel on average throughout a year. The manufacturer’s power rating for a solar panel is calculated assuming a solar radiation level of 1000W/m2 with the panel perfectly aligned. Unfortunately, a solar panel installed in the field will rarely have perfect alignment and depending on the location may only experience 1000W/m2 of solar radiation for a few hours each day, if at all. As a result, a panel rated at 5W will generally produce significantly less than this when installed. Luckily, there are some simple, colour-coded insolation charts available for the whole globe, which can help you to estimate the average number of “full sun” hours (1000W/m2), that the site will experience each year. We have included the Australian chart in our spreadsheet (see below for more details)

3.Autonomy - How many rainy days should the system be able to handle ?

It seems pretty obvious, but it is often overlooked or ignored. If the system needs to continue to operate when there is no sun, then the batteries will need to hold enough energy for the system to operate during these periods.

4.Recovery - How long should the system take to recover after a period of rain?

For some installations this figure is unimportant because the period between storms can be quite long, but in some tropical places during monsoon season it can be quite important to recharge the batteries within a short period, due to volatile weather.  This is the period (in full-sun days), that it will take for the batteries to return to full charge after being completely exhausted.

You could use the long and complicated mathematical formulas to calculate solar panel size but Campbell Scientific offers a simple, easy to use spreadsheet specifically designed for sizing your solar panel - suitable for all Australian clients.  To obtain a copy of the spreadsheet please contact Bree on bree@campbellsci.com.au.