How Solar Works

It’s important for our customers to understand the basic functions of their new solar system to maintain and prolong the life of their system. Solar electric systems are simple to operate and have no moving parts. However, photovoltaic (photo = light and voltaic = voltage producing) or PV cells, employ sophisticated semiconductor devices, many of which are similar to those developed in the integrated circuit industry over the last 35 years.

These are the “work-horse” behind your system, feeding your battery-bank, allowing you to utilize the sun’s energy for your comfort. Please read further to better understand how solar power works.

How Much Solar Power Do You Need?

  • Determine your solar charging goals and estimated power use
  • Continuous dry camping
    • Refill what you use each day, or at least each week
    • Verify light loads and no inverter, perhaps 75 – 150W
    • Small inverter w/light use (small TV, laptop, etc.), perhaps 150 – 300W
    • Large inverter may require large system, perhaps 300 – 500W+
    • Solar system should produce at least 3.5A per 100AH of battery
  • Non-continuous or long weekend dry camping
    • Refill only a portion of what you use each day
    • If you can last 3 days on 225AH (=70AH/day), one 100W module
      producing about 30AH per day can extend this to perhaps 5 – 6 days
  • Storage or maintenance only
    • 50 – 75W typically sufficient

Some System Basics

  • Batteries don’t make electricity, they are simply a “storage tank”
  • Electrical energy is stored as chemical energy
  • Batteries store electricity in units called Amp-Hours (AH)
    Ah IS THE PRODUCT OF current (in amps) -times- TIME (in hours)
    10AH of consumption could be:

    • 5A for 2hr, 0.1A for 100hrs, or 100A for 1/10hr
    • Loads Remove AH from the battery
    • Charging sources Replace AH back into battery
  • Load AH consumption examples;
    • Typical coach standby – 0.5A x 24hrs = 12AH
    • 2 overhead lights for 4 hours 2.5A x 4hrs = 10AH
    • 2 hour movie w/satellite – 8A x 2hrs = 16AH
    • Microwave TV dinner via inverter – 130A x 0.1hrs (6 min) = 13AH
    • Inverter left on while mostly doing nothing – 1.5A x 24hrs = 36AH

Pv modules

  • PV Modules are energy converters, turning light into electricity
    • In full sun 1m of OV modules will produce = 130 - 160W
  • Typical 12V PV module
    • Constructed with 36 cells in series
    • Typical V = 17V, V = 22V
  • 60 Cell “Grid Tie” modules
    • Typical V = 25V, V = 30V
    • Cost less in $/Watt compared to 36 cell module
    • May require more expensive charge control solution
  • Tipica full sun amp-hours delivered daily
    • Estimate daily AH by; 5hrs x (PMAX17v)
      Pv Module Watts Typical Maximum Amp-hours Per Day
      50W 15 AMP - Hours
      50W 30 AMP - Hours
      135W 40 AMP - Hours
      540W 160 AMP - Hours

Lead-acid Batteries

  • A 12V battery is six 2V cells wired in series
    • May be a 12V battery, all 6 cells in the same case
    • Or may be two 6V batteries in series, 3 cells in each case
    • May be floored, or sealed Gel or AGM
Combined - 225ah/12v
Golf Cart - 225ah/6v
  • Battery voltage vs. state of charge
    • Voltage shown is valid w/battery open circuit for 24 hours
    • Will be lower if battery is being discharged or higher if battery is being charged
    • Under light load you can subtract = 0.3V for reasonable accuracy
Combined - 220ah/12v
Group 27 - 110ah/12v
  • Batteries in series
    • Voltage adds, AH do not
  • Batteries in parallel
    • AH add, Voltage does not
Combined - 450ah/12v
Golf Cart - 225ah/6v
  • Proper battery size is important
    • A bigger battery is not always better.
      A battery too large for the charging source will die from sulphation
    • Minimum battery size:
      Large enough to limit DOD to < 50% on a regular basis, never > 80%
      Light loads like coach background needs & lighting can discharge more
      Heavy loads like a large inverter perform better with less discharge
Remaining Battery Capacity






Liquid Electrolyte
12v Open

> 12.73 V

12.43 V

12.10 V

11.74 V

11.38 V