Fuse box wiring.
more than 20%. This is ideal for someone with limited space.
On the downside, a windy day will cause the panels
to “feather” by laying flat. Also, any moving part will
eventually require maintenance and, as panels have a 40 to
50 year life expectancy, at least one replacement should
be factored into the system life cycle costs. Tracking
systems generally use top-of-pole mounting and require a
monolithic footing for structural integrity.
live in an above average area with 5. 8 hours of full sun per
day. A single axis tracker would increase that amount by 36%
to 7. 9 hours, and a dual axis tracker by 41% to 8. 2 hours.
The size of the PV system is:
<Daily Consumption (k WH)> / <Hours of full sun> =
<Size in k W>
Sizing the PV System
For a grid attached home, add the last 12 month’s
consumption from utility bills and divide by 365. This is
your average daily consumption.
Next, consult http://rredc.nrel.gov/solar/pubs/red
book/ and find the group of readings that match the type of
system you plan to install (fixed, single axis, dual axis). On
the line that indicates the correct tilt angle, read across to
the year column for the average and min/max readings. This
is the average number of hours of full sun available each day.
If you are evaluating trackers, use these figures to evaluate the effectiveness of a single or dual axis tracker by dividing those numbers by the fixed panel number. In my case, I
In my case, 20 k WH / 5. 8 = 3.448
At this point, begin searching for system kits on the
Internet. Generally, a kit will include panels, one or more
inverters, an AC disconnect, and a DC disconnect. Sometimes
the disconnects will be bundled inside the inverter or mounted
together on a board, and will generally be called a power panel.
The easiest way to compare systems is by using COST/WATTS.
Be sure to check for UL listings and NEC compliance.
Derating the System
The system must now be derated for panel temperature,
inverter inefficiencies, and wiring losses. Panels are rated by
the manufacturer using the STC method. STC stands for
Standard Test Conditions of 1,000 watts per square meter,
70°F panel temperature, and no wind. Additionally,
a temperature performance curve is developed
for each size of panels and this becomes very
important for cold weather performance.
Because of complaints that panels were not
performing as expected, California developed
the PTC rating. The amount of light is the same
■ PV — Photovoltaic
■ HOA — Home Owner’s Association
■ Grid-Tied — A house attached to the utility grid
■ Stand-alone or Off-Grid — House not attached to the utility grid
■ Panel — A collection of solar cells group together with one
positive and one negative lead
■ Array — A collection of PV panels in series
■ Charge Controller — A device that uses power from a solar
array to charge batteries
■ Inverter — A device for converting DC to AC
■ System — One or more inverters with one or more arrays attached
■ NEC — National Electric Code
■ UL — Underwriter’s Laboratories
■ PTC — Practical Test Conditions, California testing standard
■ STC — Standard Test Conditions,used by manufacturers to
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