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Does the idea of harnessing the Sun's energy for your electrical needs appeal to you? Have you listened to the wind and seen it's effects and wondered how you could utilize that energy for yourself? We have the equipment and expertise you need to make the most of the energy of wind and sun for your home, farm, or business.

Have you looked into building a house away from the local electrical companies power lines, only to be shocked at the cost of running lines and transformers out to your property? Solar and wind power can provide an economical alternative to being tied to your electric company, and provides freedom from monthly bills and many weather-related power outages.

Energy independence can, at times, be critical to survival. In the event of a prolonged utility-grid power failure your home can continue to have light, heat, refrigeration, water, etc. Whether that lapse is due to weather or a technological failure, having energy available to you can mean the difference between just "scraping by," and "surviving with style."

Achieving energy independence requires thoughtful planning, and a willingness to rethink how you do the things you do. On this page, I will guide you through much of the process of selection. By following the links below, you will have access to the data you need to make those decisions.

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Solar (Photovoltaic) Electrical Systems

Small Stand Alone
DC Only System

The small stand-alone DC system is an excellent replacement for propane or kerosene lights in a remote cabin, recreational vehicle, or a boat. The size of the photovoltaic array (PV Array) and battery will depend upon individual requirements. The actual sizing methods are discussed on a link at the bottom of this page.

The PV array charges the battery during daylight hours and the battery supplies power to the loads as needed. The charge regulator terminates the charge when the battery reaches full charge. The load center may contain meters to monitor system operation and performance, and fuses to protect wiring in the event of a malfunction or short circuit in the building.

Stand Alone
AC-DC System

This system is the same as the previous system, except that it includes a DC to AC inverter to convert direct current to alternating current. Using the inverter, commonly available household appliances such as power tools, vacuum cleaners, and washing machines may be used.

High quality DC to AC inverters are available with power outputs ranging from 100 watts to 12 kilowatts, and with conversion efficiencies in excess of 90%. To ensure reliable system operation, the inverter should be carefully matched to the loads that will be run. Further information about inverters can be found by following the link below.

As the loads on the system are increased, a larger PV array and more battery storage will be required.

Stand Alone AC-DC System

Stand Alone
AC System

A stand alone AC system will usually have a PV array of 10 or more modules, battery bank, and one or more inverters. Two or more cascadable inverters are an excellent choice for this type of system since they can work together to supply power to large loads and if one fails, the other(s) can continue to operate at a reduced output until repairs are made. This redundancy is important for continuously operating critical loads, such as refrigerators and freezers.

In large homes, an AC only system simplifies wiring by allowing the use of low cost, readily available switches, outlets, and fixtures. Savings in wire cost are significant, because the large gauge wire required for efficient transmission of low voltage DC power over long runs is avoided.

Stand Alone AC-DC System

PV - Generator Combination
This may be an economical alternative to a large stand alone PV system, because the PV array does not have to be sized large enough for worst case weather conditions. A gasoline, propane or diesel generator combined with a battery charger can supply power when the PV array falls short.

If the PV array is sized for average conditions, then during extended overcast situations or periods of increased load, the generator can be started. When the batteries are low, the generator will power the AC loads in the house as well as a battery charger to help recharge the batteries.

If the PV array is sized much smaller than needed for normal use, the generator can power peak loads such as doing laundry or pumping water and simultaneously run the battery charger to charge the battery bank.

In addition to allowing for a smaller PV array, a back-up charging system may also allow use of a smaller battery bank. Generator and battery bank size must be chosen carefully for reliable system operation.

Stand Alone AC-DC System

Utility Interface
This is used in a grid-connected house. Electricity generated by the PV array is stored in a battery bank which provides all or some of the home's power needs. If the batteries become low, an automatic transfer switch reconnects the house to utility power until the PV array has recharged the battery bank. When charging is complete, the automatic transfer switch reconnects the house loads to the batteries and inverter.

Any size PV array can be used in this type of system, since utility power may be available whenever PV-produced power is not. Array size can be increased incrementally, requiring less and less use of utility power. However, consideration should be made to having enough PV-generated power available, with a large enough battery bank, to power critical loads in the event of an extended grid-related power outage.

This type of system can have a lower initial cost than a utility intertie system, it will function during power outages, and it can be enlarged to make the home independent of the utility.

Utility Interface

Utility Intertie
This is also used in a grid-connected house, but instead of storing power in batteries it is sold to the utility company. The Utility Intertie System employs a special type of inverter, which inverts DC into low distortion AC, acceptable for use by the local utility company. Batteris are not needed for storage. The power is delivered through a kilowatt-hour (kWh) meter to the utility grid as it is produced by the PV modules. A second kWh meter is used to measure the power consumed by the loads in the house.

The largest disadvantage to this system is obvious: If the utility company is not producing power for some reason, you will only have power available during daylight conditions.

Utility Intertie

The size of a solar electric system depends upon the amount of power that is required (in watts), the amount of time it is used (hours), and the amount of energy available from the sun in a particular geographic area (sun hours per day). The user has control over the first two variables, while the third depends upon the location.


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 whenever possible can make solar electricity a cost-effective alternative to gasoline generators, and in some cases, utility power.


Conventional electric cooking, space heating, and water heating equipment use a prohibitive amount of electricity. Electric ranges use about 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 and wood are better alternatives for space heating. Good passive solar design can reduce the need for heating. Evaporative cooling is a more reasonable load, and in locations with low to moderate humidity, the results are almost as good. One plus for cooling is that the largest amount of solar energy is usually available when the temperature is highest.


Lighting requires the most study since so many options exist in type, size, voltage, and placement. The type of lighting that is best for one system may not be right for another.

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, RV, or a boat, low voltage DC is usually best. DC wiring runs can be short, allowing 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 an inverter fails since the lights are powered directly off the battery. In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of low voltage fluorescent lights, whicdh have 3 to 4 times the light output per watt of power use compared with incandescent. 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 larger stand alone syustem with AC lighting, the user should have a backup inverter of a few low voltage DC lights in caser the primary power fails. 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.


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.


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. The inverter will get warm or hot when running these loads, which may shorten its life. The 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 400 watt-hours per load. If you use the 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-presses, etc.) their standard electric motors 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 upon 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.


Many small appliances such as irons, toaster, 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.



Static inverters convert DC power stored in batteries to AC power that can be used as needed. Selecting the static power inverter that is needed for a stand alone power system requires the determination of:

  • Maximum load
  • Maximum surge
  • Output voltage required
  • Input battery voltage
  • Optional features needed.

The size of an inverter is measured by it maximum continuous watts output. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. The size of the inverter can be determnined from a tag or label on the appliance, usually located near where the power cord enters, or from the owners manual. If the inverter is expected to run induction motors, like the ones found in automatic washers, dryers, dishwashers, and large power tools, it must be designed to surge, or deliver power many times its rating for short periods of time while these motors start.

This has one function, which is to change DC power to AC power that will be fed into the utility grid. A power system with this type of inverter uses the utility company as a storage battery. When the sun is shining, your electricity comes from the PV array, via the inverter. If the PV array is making more power than you are using, the excess is sold to the utility power company through a second electric meter. If you use more power than the PV array can supply, the utility company makes up the difference. This type of system makes the most sense if you have utility power, because there are no batteries to maintain or replace, but it has a very long payback period and may not be cost-effective at today's electric rates.

Another factor in the choice of an inverter is the output waveform. Inverters deliver power in one of three basic waveforms: square wave, modified square wave (sometimes called modified sine wave), and pure sine wave. Utility power is delivered as pure sine wave.


Square wave inverters are the lowest in cost, and efficiency and are not sold by this company. The price of the better inverters in low enough to make square wave inverters an unattractive choice.

Trace DR-Series inverters have modified square wave output with harmonic distortion of around 40%. They are usually the best choice for a "whole house" inverter because their high surge capacity allows them to start large motors while their high efficiency make them economical with power when running small loads like a stereo or a small light. Unfortunately, this type of inverter may destroy some low-cost rechargable tools and flashlights, laser printers, and copiers. Some light dimmers and some variable speed tools may fail to operate. Some audio equipment will have a background buzz that is annoying to music connoisseurs.

Sine wave inverters have recently become much more economical, and are an excellent choice for power systems running audio equipment and other electronics that is waveform sensitive. Many people install large modified square wave inverters to run common household loads, and a small sine wave inverter to run stereos, televisions, and computers. Loads run more efficiently on a sine wave inverter. Motors run cooler, fans run at their proper speed, and loads are quieter on sine wave power.

Most inverters cause interference on AM radio!


All stand alone and utility interface PV systems require battery storage. Photovoltaic modules charge the batteries during daylight hours, and the batteries supply the power when it is needed, often at night and during cloudy weather. Since utiltity intertie systems supply power directly to the utility grid, no battery storage is needed for these.

The two most common types of rechargable batteries in use are lead-acid and nickel-cadmium. These two types of batteries get their names from the materials that they are made of. Lead acid batteries have plates made of lead, mixed with other chemicals, submerged in a sulfuric acid solution. Nickel-cadmium batteries have plates made of nickel and cadmium, submerged in a solution of potassium hydroxide. We now have nickel-iron batteries, which are similar to nickel-cadmium batteries.

Lead-acid batteries are the most common in PV systems because their initial cost is lower and because they are readily available nearly everywhere in the world. There are many different sizes and designs of lead-acid batteries, but the most important designation is whether they are deep-cycle batteries or shallow cycle batteries.

Shallow-cycle batteries, like the type used as starting batteries in autos, are designed to supply a large amount of current for a short time, and stand a mild overcharge without losing electrolyte. Unfortunately, they cannot tolerate being deeply discharged. If they are repeatedly discharged more than 20%, their life will be very short. These batteries are not a good choice for a PV system.

Deep-cycle batteries are designed to be repeatedly discharged by as much as 80% of their capacity so they are a good choice for power systems. Even though they are designed to withstand deep cycling, these batteries will have a longer life if the cycles are shallower. Try to routinely stay above 50% capacity. All lead-acid batteries will fail prematurely if they are not recharged completely after each cycle. Letting a lead-acid battery stay in a discharged condition for many days at a time will cause sulfation of the positive plate and a permanent loss of capacity. An electronic desulfater can be added to a battery to minimize sulfation and greatly extend battery life.

Nickel-cadmium (ni-cad) batteries are nearly ideal for all types of power systems, but their initial cost is very high. New ni-cad batteries cost 10 times as much as similar capacity lead-acid batteries, and reconditioned ni-cads cost about 3 times as much as lead-acid batteries. Ni-cad batteries can last 10 times as long as lead-acid batteries and a smaller capacity battery can be used.

Since ni-cad batteries can be discharged 100%, and they can stay in a discharged condition for long periods of time without damage, a smaller battery can be used. The fact that their capacity does not get lower at low temperatures, and that their voltage is very stable from full charge to full discharge, even under large loads, makes a smaller battery a workable choice. Depending upon loads and weather conditions, a 30% to 50% smaller battery many be used.

Ni-cads can operate down to -10 degrees F with only a small reduction in capacity and they will not be damaged if they freeze. This allows them to be used in areas with severe temperature swings without having to oversize capacity and without providing a heated space.

Charging efficiency of ni-cad batteries is about the same as that of new lead-acid batteries, and it stays the same for the life of the battery. Their overall eficiency is higher, because they have an extremely slow self-discharge rate. if the power system is increased at a later time, additional ni-cad batteries of the same type can be added to the old bank without any problem, something that should never be done with lead-acid batteries.

Ni-cad batteries require slightly higher charging voltage than lead-acid batteries to bring them up to full charge. This may cause some difficulty with equipment designed for lead-acid batteries. Many AC power driven battery chargers will not go up to the 16.5 volts required to completely charge them. Solar modules on the other hand may be usabel without regulation, since their peak charging voltage is usually between 16 and 17 volts. This higher full charge voltage will cause most inverters to turn themselves off, so they may not work near the end of a charging cycle. Even though these batteries reach this voltage near full charge, the voltage drops immediately upon discharge, so the condition is only temporary.

Nickel-iron batteries are very similar to nickel-cadmium. They use an alkaline electrolyte similar to ni-cad batteries that allows them to withstand low temperatures. Their charge and discharge voltage is about the same, as is their long life. Many old nickel-iron batteries that were made 50 years ago in the U.S. are still working fine.

One major difference between nickel-iron and ni-cad batteries is discharge rate. Nickel-iron batteries cannot deliver the extremely high currents that ni-cads can, so if you plan tohave heavy loads, you will need to have a larger capacity battery bank.

One of the most interesting aspects of nickel-iron batteries is that they are made withour toxic lead or cadmium, which solves a future disposal problem.

The size of the battery bank required will depend upon:

  • the storage capacity required
  • the maximum discharge rate
  • the minimum temperature that the batteries will be stored at (lead-acid only).
When designing a power system, all of these factors are looked at and the one requiring the largest capacity will dictate battery size.

Temperature has a significant effect on lead-acid batteries. At 40 degrees F they will have 75% of rated capacity, and at 0 degrees F, that drops to 50% of rated capacity.

The storage capacity of a battery, the amount of electrical energy it can hold, is usually expressed in amp-hours. If one amp is used for 100 hours, then 100 amp-hours would have been used. A battery in a PV system should have sufficient amp-hour capacity to supply needed power during the longest expected period of cloudy weather. A lead-acid battery should be 20% larger than this amount. If there is a source of back-up power, such as a standby generator with a battery charger, the battery bank does not have to be sized for worst-case weather conditions.


Phone Number: 734-552-2074
Fax: 734-758-0732

Copyright 1998 - 2000 by GeneSys Resources, Inc., and Mark Tank

For additional tutorials, see:

On-Line Renewable Energy
Education Module

Welcome to the first version of CREST's online renewable energy education system! Based on an interactive kiosk also developed by CREST in 1994, this system uses words and pictures to teach the theoretical and practical basics of renewable energy. It takes an introductory approach, not getting so complex as to baffle the first time viewer.