 Solar System Sizing
in General
The size of a solar electric system depends on the amount
of power that is required (watts), the amount of time it is used
(hours) and the amount of energy available from the sun in a
particular area (sun hours per day). The user has control of
the first two of these variables, while the third depends on
the location. To size a solar electric system you need to find
out how much power each appliance will consume when turned on.
Multiply this times the length of time you will use each appliance.
Add up the totals for each appliance and then add 15% to compensate
for inverter, wire and battery charging and storage losses. This
will give you an approximate idea of the energy your solar panels
and battery bank will have to produce and store.
Conservation
Conservation plays an important role in keeping the cost of
a photovoltaic system down. The use of energy efficient appliances
and lighting as well as non-electric alternatives wherever possible
can make solar electricity a cost competitive alternative to
gasoline generators and in some cases, utility power.
Cooking, Heating
& Cooling
Conventional electric cooking, space heating and water heating
equipment use a prohibitive amount of electricity. Electric ranges
use 1500 watts or more per burner, so bottled propane or natural
gas is a popular alternative to electricity for cooking. A microwave
oven has about the same power draw, but since food cooks more
quickly, the amount of kilowatt hours used may not be large.
Propane, natural gas and wood are better alternatives for space
heating. Good passive solar design and proper insulation can
reduce the need for heat. Evaporative cooling is a more reasonable
load, and in locations with low humidity, the results are almost
as good. One plus for cooling - the largest amount of solar energy
is usually available when the temperature is the highest.
Lighting
Solar powered lighting is one of the most cost effective uses
of solar energy.
The first decision is whether your lights will be run on low
voltage direct current (DC) or conventional 110 volt alternating
current (AC). In a small home, an RV, or a boat, low voltage
DC lighting is usually the best. DC wiring runs can be kept short
allowing the use of fairly small gauge wire. Since an inverter
is not required, the system cost is lower. If an inverter is
part of the system, the house will not be dark if the inverter
fails if the lights are powered directly by the battery.
In addition to conventional size medium base low voltage bulbs,
the user can choose from a large selection of DC fluorescent
lights, which have 3 to 4 times the light output per watt of
power used compared with incandescent types. Halogen bulbs are
30% more efficient and actually seem almost twice as bright as
similar wattage incandescent because of the spectrum of light
they produce. Twelve and 24 volt replacement ballasts are available
to convert AC fluorescent lights to DC.
In a very large installation or one with many lights, the
use of an inverter to supply AC power for conventional lighting
is cost effective. In a large stand alone system with AC lighting,
the user should have a back up inverter or a few low voltage
DC lights in case the primary inverter fails. It is a good idea
to have a DC powered light in the room where the inverter and
batteries are in case there is a problem. AC light dimmers will
not function on AC power from inverters unless they have pure
sine wave output. Small fluorescent lights may not turn on with
some "load demand start" type inverters.
Refrigeration
Using solar panels to power a typical residential refrigerator
is not a cost effective use of solar panels. Gas powered absorption
refrigerators are a good choice in small systems if bottled gas
is available. Modern absorption refrigerators consume 5 to 10
gallons of LP gas per month. If an electric refrigerator will
be used in a stand-alone system, it should be a high efficiency
type. SunFrost refrigerators use 300 to 400 watt hours of electricity
per day while conventional AC refrigerators use 3000 to 4000
watt hours of electricity per day at a 70 degree average air
temperature. The higher cost of good quality DC refrigerators
is made up many times over by savings in the number of solar
modules and batteries required.
Major Appliances
Standard AC electric motors in washing machines, larger shop
machinery and tools, "swamp coolers", pumps etc. (usually
1/4 to 3/4 horsepower) require a large inverter. Often, a 2000
watt or larger inverter will be required. These electric motors
are sometimes hard to start on inverter power, they consume relatively
large amounts of electricity, and they are very wasteful compared
to high-efficiency motors, which use 50% to 75% less electricity.
A standard washing machine uses between 300 and 500 watt-hours
per load. If the appliance is used more than a few hours per
week, it is often cheaper to pay more for a high-efficiency appliance
(if one exists), rather than make your electrical system larger
to support a low-efficiency load. For many belt-driven loads
(washers, drill press, etc.), their standard electric motor can
often be easily replaced with a high-efficiency type. These motors
are available in either AC or DC, and come as separate units
or as motor-replacement kits.
Vacuum cleaners usually consume 600 to 1000 watts, depending
on how powerful they are, about twice what a washer uses, but
most vacuum cleaners will operate on inverters larger than 1000
watts because they have low surge motors.
Small Appliances
Many small appliances such as irons, toasters and hair dryers
consume a very large amount of power when they are used but by
their nature require very short or infrequent use periods, so
if the system inverter and batteries are large enough, they may
be usable. Electronic equipment, like stereos, televisions, VCR's
and computers have a fairly small power draw. Many of these are
available in low voltage DC as well as conventional AC versions,
and in general, DC models use less power than their AC counterparts.
A portable stereo "boom box" that runs on 8 or 10 "D-cell"
batteries will usually work on 12 volts DC. Some have a DC input,
or you can connect wires from the battery contacts to the 12
volt system. This should be done by someone experienced in electronics
repair.
|
