Richard's solar spreadsheet
a guide to solar & battery requirements

This spreadsheet was designed for my benefit so that I could choose the solar panel array that I would need to match the battery capacity in the Prado and the anticipated draw by the fridge etc. The spreadsheet reflects what we typically do, that is arrive at a camping site late in the afternoon with the battery "fully" charged by the car's alternator, and then spend one or more days camped.

The spreadsheet contains calculations for both day and night periods as the power requirements are different for each and of course, there is no solar power generated at night. Power generated by the solar panels is used by the fridge etc with any surplus fed to the battery until the battery is 100% charged.

The arithmetic in the spreadsheet is correct, but it relies on assumptions and these are up to the user. There is considerable flexibility to do “what if” type scenarios – ie change the panel size, number of sun hours, battery size etc.

Input assumptions are made in the cells formatted in yellow. The other cells are not protected from alteration and the calculations can be changed if desired. Some of the formulae are complex so as to allow for the green/orange/red formatting that indicates whether or not there is sufficient power available.

I have seen claims that a car’s alternator will charge a battery anywhere between 70% and 100%. Choose your initial state of charge either by picking a figure between 70% (pessimistic) and 100% (optimistic) or measure the open circuit voltage of your battery after a decent drive and after allowing the battery to 'rest' for several hours, ie nothing running off it. The table in the spreadsheet gives 'State Of Charge' for various voltages for wet standard/low maintenance and maintenance free/AGM/Gel batteries. This data is taken from William Darden's website 'Car and deep cycle battery answers to Frequently Asked Questions (FAQs)'  http://www.batteryfaq.org

The solar panels are unlikely to operate at 100% efficiency due to temperature, occasional shade, not directly facing the sun etc. The 70% figure I have in the spreadsheet was taken from a book on solar written by Collyn Rivers using a PWM solar regulator. http://www.caravanandmotorhomebooks.com/ 

The figure for system losses is a pure guess & you may wish to change it. If using a MPPT solar regulator you could also change the panel efficiency on line 13 to 80%.

You will find an excellent article written by Collyn Rivers covering all aspects of solar battery charging here http://www.exploroz.com/Vehicle/Electrics/Solar.aspx   

All loads (fridge, lights etc) are assumed to be at 12volt and battery charging is assumed to be at 14volt.

The spreadsheet is provided “as is” and no responsibility will be accepted by myself for the use of this spreadsheet or for any outcomes that may arise by using the spreadsheet. User beware – it is a guide only!!!

I would like to thank other members of the Campertrailers Group, especially Jeff Pethybridge, for their constructive criticism.

Richard's  Microsoft Excel spreadsheet 'Solar Power' can be downloaded here

Richard Graylin

Peak Solar Sun Hours

Peak Solar Sun Hours is the number of hours when solar irradiance averages 1000 watts per square meter. For example in Sydney around the September equinox when we get twelve hours day & twelve hours night, there is only 5.6 Peak Solar Sun Hours. You will find a Peak Solar Sun Hours chart below for locations around Australia.

Peak Solar Sun Hours