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The
Value of Energy Efficiency in Stand Alone PV Systems:
A Simple Approach

How Good an Investment
is a Compact Fluorescent? If you replace a 100watt light bulb with a 20watt compact fluorescent and the bulb is used 4 hours per day your energy savings will be .32 KWH/day. If you have to expand your system to produce this energy, choosing instead to reduce your load by .32 KWH/day will result in a savings of $1794.00; $1090.00 initially and $704.00 in operating costs for the life of your system. If a compact fluorescent has a life of 10,000 hrs and an incandescent bulb a life of 750 hrs., over 24 years 3.5 compact fluorescents or 46 incandescent lamps will be required. The 3.5 compact fluorescents will cost roughly as much as the 46 incandescent lamps; so for essentially no additional investment you can reduce the total cost of your power system by $1794 – that’s a good deal. 
The Cost of Generating
One KWH/day
“CFLs (compact fluorescents) on the ceiling before PVs on the roof!”
is a Home Power adage. But just how much is efficiency worth? How much
should an offgrid homeowner be willing to invest in more efficient appliances
(refrigerator, washer/dryer, lighting, well pumps etc.) to save a KWH?
The answer depends on how much capital it takes to generate offgrid power.
We will consider the investment necessary to produce an additional 1 KWH/day
from an existing PV system.
The cost of the PV system will be divided into two parts, the initial investment and operating costs. The initial costs will include PV panels, mounting hardware, original battery set etc. Operating costs will include battery replacement and the cost of backup generator power during the winter months. The system is sized so that it can provide 1 KWH/day of AC power with 4.2 hours of insolation, the average yearly insolation for Arcata, a Northern California coastal town. (Insolation is the average amount of usable sunlight available to a nontracking solar system). It will be assumed that the system is being increased in size and that the inverter, charge controller, load center, and battery housing are large enough to handle the increase in size.
Cost per KWH – A Minimum Value
If you want to get an estimate of how much a KWH of electricity costs
from your PV system. You could divide the total investment needed to produce
1KWH/day by the number of KWHs those PVs will produce in 24 years ($6606.00
/ 8760 KWH= $.75/KWH). If a more detailed financial analysis is carried
out considering interest rates and Net Present Value of money, etc., this
cost will increase, $.75 is as low as it will get.
The following table summarizes the assumptions made in calculating
the initial capital costs and the operating costs of the PV system.
System Assumptions:
The Cost Of A KWH/day Over The Life Of A PV System (24 yrs.):
Initial
Investment: 

Solar Panels  $1963.00  393 Watts at $5 /watt 
Pole Mount  $393.00  Pole & foundation 
Batteries  $500.00  500 amp/hr batteries (8 year life) L16 or equivalent  $1/amp/hr @ 12 volts 
Wire and Misc. hardware  $100.00  
Installation  $450.00  
Subtotal  $3406.00  
Lifetime
Operation Cost: 

Batteries  $1000.00  2 sets 500 amp/hr batteries (8 year life) L16 or equivalent  $1/amp/hr @ 12 volts 
Generator Power  $1200.00  Required backup power 50 KWH/yr. @ $1/KWH 
Subtotal  $2200.00  
Total  $5606.00 
Refrigeration
What would the savings be in purchasing a DC Sun Frost rather than an
energy efficient AC refrigerator such as the one reviewed in HP 84: Aug/Sept.
2001? Sun Frost’s refrigerators are produced in both AC and DC models.
If a DC model can be conveniently hooked up it has the advantage of eliminating
inverter losses and increased reliability because an inverter failure
will not leave you without refrigeration. In addition, there will be more
inverter capacity left for other applications. A DC Sun Frost unit will
typically consume about .5 KWH/day. With 10% inverter losses, the fridge
reviewed in HP 84 consumed about 1.5 KWH/day including inverter losses:
The savings with a Sun Frost are then 1KWH/day. This will result in an
initial saving of $3406 in cost of the PV system and an additional saving
of $2200 in operating costs for the life of the system. This savings of
$5606 will be approximately three times cost difference between the two
refrigerators, making the Sun Frost an excellent investment. If the expected
life of the appliance is factored in the savings will be even greater.
The expected life of a Sun Frost is over 24 years and the expected life
of a conventional refrigerator is only 15 years. Over the life of a PV
system this will increase the savings by an additional factor of 1.6.
Modifying this approach to your Personal
System
We know that conservation is important, but exactly how many dollars should
be invested to save a KWH? We answer this question by comparing the investment
necessary to produce one KWH/day with the cost of conserving one KWH/day.
Even though a KWH is relatively expensive, with effective investments
in conservation you can substantially reduce the total cost of operating
a home with a standalone power system.
You can also modify our results to fit your personal situation. For example, if you live in the Southwest and receive 20% more sunlight you could reduce the cost of producing a KWH by about 20%, or you may choose to do your own installation and deduct that cost from you solar investment. As for rebates, we have found that unfortunately they are seldom applicable to stand alone systems.
The investment for increasing the capacity of the system by 1 AC KWH/day is $5606.00: $3406.00 for the initial cost of the equipment and $2200.00 for the operating costs. If you include the cost of all the equipment to set up a system, the inverter, load center, charge controller, and battery housing add about $1000.00, the total cost to produce 1KWH/day is $6600.00.
Conservation will be a good investment if you can purchase a device that can save 1KWH/day for less than $5606.00. If the device will last less than 24 years, its replacement cost should be considered. If you spend $1000.00 on an energysaving device that saves 1KWH/day and lasts 12 years, you’ll buy 2 over the life of your PV system and you’re investing $2000.00 that will save you $3606.00.
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