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There are lots of books and articles written about how to size a solar powered system.  These are great if you want to power your home or your business but if you need to power a remote monitoring site, like a SIT65 Satellite System, water sampling system, or GL500 datalogging system they are a bit of an overkill.

Remote instrumentation sites are by nature low powered.  If you need to do significant control, you are at a powered site and don't need solar power.  You may need a battery backup system to temporarily power the site incase of power loss but these battery systems are kept charged by an AC charging system.

For remote monitoring and limited on-off control, you have a low powered site and you need a simple way to keep it powered at all times.  What you actually have is a battery-powered site with a solar charging system.  We never power a site directly from solar power since it is up and down and on and off and most systems need power all the time.

To power a remote site you need to know the battery size and the solar panel size.  Size the battery to run the site for 3 days without any solar charging.  Even on cloudy and stormy days there is still some charging going on.  So you are covered for several days of bad weather in a row.  First calculate the AVERAGE CURRENT draw for the site.  Most remote sites have a sleep or standby mode and come up to full power only when a reading is taken or a control is ordered.  For recording only or transmit only remote sites, the average current is usually very low (usually one milliamp or less).  Call this the STANDBY CURRENT.  For a 2-way radio system, the radio receiver must stay on continuously to receive commands from the central site and therefore the standby current is much larger (300mA or more).

When the remote sight takes a reading or transmits a radio signal, it usually comes on to full power, warms up the sensors for several seconds and then records and/or transmits the data.  Call this the ON CURRENT.


STANDBY CURRENT and ON CURRENT can be calculated from instrument and sensor specifications.

EXAMPLE: STANDBY CURRENT = 10 mA and ON CURRENT = 1 amp and the site is at high power (ON) for 10 seconds every 10 minutes.  So we have 10mA continuous plus 1 amp for 10 seconds out of 600 seconds or 1 amp/60 = 17mA.  So your AVERAGE CURRENT is 10mA + 17mA = 27mA

Batteries are rated in Ampere Hours (AH).  A one AH battery will supply one ampere for one hour.  In our example we want to power a site drawing 0.027 amps (27mA) for 3 days (72 hours) or 0.027 amps X 72 hours = 1.9 AH.  So we need a 2 AH battery.  We suggest that a 4 or 5 AH battery be used to be on the safe side.

Sizing the solar panel is a lot easier.  You have to consider weather, latitude, season, nighttime loss, etc., etc.  The rule of thumb for remote monitoring is to size the panel 10X the AVERAGE CURRENT to take care of all these factors.  In our example: AVERAGE CURRENT (0.027A) x 10 = 270mA.  So a 300 or 350mA panel will cover you.  Maybe a little bigger if you are in the far north or south.  Face the panel south at a 45% angle.  Any panel size above 80mA needs a voltage regulator, such as the FN0000 Smart Charger, so that you don't damage the battery by over charging.  Follow these rules and you will have a happy remote solar powered site.

For more information on the remote data transmission systems Global Water offers please visit our remote water monitoring systems page.
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