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Usable Energy

This section shows you how much electricity your panels will produce from your roofs annual 1,464 Peak Sun Hours (PSH).

We'll now calculate how many panels will fit on this roof and how much electricity the panels will generate.


The electricity is then converted from Direct Current (DC) to Alternating Current (AC) and feed into our homes electric panel.

This roof will generate $557 worth of electricity each year.


Scroll down to see how each step is accomplished.

Roof used to demonstrate available solar energy
Peak Sun Hours
We start with the usable Peak Sun Hours (PSH)



From our Roof Assessment in the previous section, we determined that our example roof will receive 1,464 Usable Peak Sun Hours (PSH) per square meter per year.


We've adjusted for the fact that our roof does not have perfect tilt and orientation and is subject to some shading.


PSH = 1 Kilowatt/per square Meter/Per Hour


We’ll now add panels to this roof and calculate the total energy production for the first year.

NREL US kWh/m2/day Map
Panel Count/Layout
General Panel Sizing/Layout Dimensions


This roof can easily hold 15 panels.


Solar Panels come in many shapes and sizes but for the purposes of estimating how many panels can fit on a Residential roof, a good standard size is 40” X 66”.  This takes into account the 1” gap between installed panels.


Note: Commercial panels are longer on  average (40” X 78”)


For this roof we’ve established that 15 panels can fit with enough buffer space around the array to accommodate most municipality setback requirements.


Note: Most municipalities don’t yet have roof edge setback requirements but this is changing. Your solar installer will take this into account when designing your panel layout.

Solar Panel Layout example
Solar Panel STC & PTC Rating Examples
Panel Energy Output
Panel Rating = Number of Watts produced per sq meter of panel


There are two main rating systems for solar panels:

  • Standard Test Conditions (STC) - Good for comparing panels

  • PV-USA Test Conditions     (PTC) - More accurate for energy production calcs.


The STC Rating (Nameplate Rating) is determined under ideal factory set conditions.

All manufacturers providing an STC rating which does provide us with a convenient number for apples to apples comparisons between panels. But STC ratings are not based on real-world conditions. 

The PTC Rating is determined under real-world conditions.

On average a PTC rating is about 9% lower than the STC rating.


It is important to know that when talking about the size of a solar panel array, the STC rating number is always used. As an example, our 15 panel array of 280 Watt STC rated panels is referred to as a 4.2kW array.  (15X280=4200Watts)
Solar Panel Nameplate sizing example
Array Size vs Array Capacity
To determine the nameplate Size of a solar array (Use STC)

To determine the rated size (nameplate size) of a solar system, you add up the STC Panel rating (Watts) for each panel.  For our example roof of 15 panels we’re going to assume a middle of the road STC rated panel of 300 Watts for each panel. 


Our example Solar System Nameplate Size is 4.2kW DC.


To determine the real-world production Capacity (Use PTC)

To calculate the real-world energy production capacity of this array we’ll reduce this system size by around 9% to more closely match the expected PTC rating. If your panel manufacturer provides the actual PTC rating then multiply the PTC number by the number of panels.

Our example System Capacity is 3.82kW DC.

SCT to PTC Reduction
Array Production

Expected DC Electricity Production at Panels


Our panels will generate 5,595 kWh Direct Current/Yr


From our Roof Assessment calculations we were able to determine that our particular roof will receive 1,464kWh of solar energy each year.


And from our panel layout and the panels PTC rating we determine our solar system has gathering capacity of 3.8kW.


By multiplying these two numbers (1,464 X 3.82) we now know our solar panels are expected to produce 5,595 kWh of Direct Electrical Current each year. We still need to convert this DC current to Alternating Current (AC) which we’ll do shortly.


NOTE: To make sense of these numbers, it’s helpful to think of the solar system capacity (3.82 kW DC) as the size of a bucket on your roof and think of the 1,464 kWh of annual solar energy as how much energy will fall on our roof filling the bucket. This is why we multiply these number together to determine energy production..

Solar Array Production Example Calc
Annual Production (Energy Lose)

Solar Panel DC current to Home AC Current.


We can expect to lose around 13% of our electrical energy as it travels down the wiring, through a DC->AC Inverter, and then connects to our home circuit panel.


Annual Production: 4,904 kWh

Value by offsetting grid use: ($557).


The amount of Electricity your panels produce, and as a result NOT needing to purchase from your public utility, is the main savings/income your panels will provide.

Now that we know how much electricity you'll be generating, let's see how your solar panels interact with your public electric utility.  See the "Utility Interaction" section for details.


Solar Panel Annual Production Calc
Energy Lose
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