Energy & Power

When you see the unit “Ah” (Amp hours), know that the “h” isn’t just a longhand way of expressing Amps.  The “h” is very important and distinguishes between Amp hours and Amps, which are as different as the units of Miles and Miles Per Hour. 

Amp hours is a unit of energy, which can be consumed or generated over a period of time.  Amps is a unit of power, which is an instantaneous measure.   The same goes for Watt hours and WattsWatt hours is a unit of energy, which is shown on your utility bill in kWh (kilo, or thousands of Watt hours), while Watts is the unit of measure used for solar panel ratings.  (FYI:  Amps x Volts = Watts and Amps x Volts x Hours = Watt hours)

To clarify some of the mystique involving Watts, Amps and Amp hours, let’s begin with a 100 watt solar panel that produces 5 amps during peak sun hours (a Solar Day).  At the end of this typical six hour “solar day" the panel will have generated 30 Amp hours (5 Amps x 6 hours = 30 Amps hours).

For an analogy, let’s use some familiar automotive terms:  “My 300 horsepower truck reached a top speed of 80 miles per hour during my 150 mile road trip.”  Here we see peak Energy (Watts) described as Horsepower and actual output (Amps) described as speed in terms of miles per hour, with Power, or the work being performed (Amp hours), expressed as a distance travelled in miles.

Knowing the difference between Power and Energy (Amps and Amp hours) is important when it comes to designing your system.  It is the Amp hour production of your solar panel(s) fed to your battery bank that must exceed the Amp hour consumption that you pull from the battery bank during a given period.

Having a solar array that produces more amps than your load consumes during peak sun hours is like having a car with enough gas to get up to freeway speeds, but not necessarily enough gas to reach your destination.  You must consider cumulative charge in Amp hours fed to your battery bank, not just the instantaneous peak amperage, since your loads may want to run regardless of whether the sun is shining with full intensity on your panels.

Things that don’t make sense:
“Amps per hour”  This is like saying miles per hour per hour.  We’ll assume you just mean “Amps”.

“It uses 3 amps.”  On average, how many hours per day will it use those three amps?

“I have two batteries”  What is their Amp hour rating?  Not all batteries are the same size. 

Extra Credit:
For those of you with a mathematics background, this is calculus.  Amp hours is the integral over time of Amps.  Meaning, if you plotted a graph of Amps vs. Time, Amp hours would be the area under that curve.