First a definition of some of the concepts involved in solar systems followed by a definition of the main components.
- Power: The available "pressure" instantaneously available to run appliances. An inverter is characterized by, among other things by its power which may be from watts to kilowatts. Although, just because a power supply is rated at a certain wattage, it does not mean that it will necessarily output it, that depends on what its load asks of it.
- Energy: The total available power available for a sustained period of time. It is measured in watt-hours, that is watts times time. A battery's capacity is measured in watt-hours or ampere-hours which, when multiplied by the voltage yields the energy available from that battery.
- Insolation: The amount of sunshine falling on a solar panel. Typically, on a sunny day, it is about 1000W per square meter of solar panel; on overcast days the available energy would be about 10 to 30% of that amount. About 18% of this incident solar energy is converted into electricity and is available to charge batteries and run appliances.
- DC voltage: A steady output of voltage, available from a solar panel or battery. Typical voltages available from a single solar panel are from 12V to 40V; for batteries, typically available voltages are 6V and 12V. Voltage increases as batteries or solar panels are connected in series.
- AC voltage: A continuously varying output of voltage, available from the utility company or an inverter connected to a battery. The job of the inverter is to convert DC voltage to AC. Typical current available from a single solar panel is from 4 amperes to about 8 amps. From a battery, anywhere from 5 amps to thousands of amps are available for varying periods of time thus the need for the aforementioned fuse used for safety purposes.
- Current: The amount of electricity flowing through a wire or a device such as a solar panel. It is measured in amperes. Roughly speaking, current flows out of solar panels only. With batteries, the current can flow either into or out of them. With loads, the current only flows into them. Lastly, in order for current to flow, there must be a path into the device and out of it; thus, it always takes two wires for current to flow.
- Charge/discharge cycle: When the sun is shining, the solar panel(s) convert the available light into electricity and, after proper regulation, this current is fed to batteries to charge them. At night, with no sunshine available, the reverse happens: the energy stored in the batteries flows out, into the inverter to run appliances. This continues until the sun comes up again. The cycle repeats every day. Therefore, it is better to run demanding appliances such as washing machines during the day so that the load is supplied from the solar panels rather that by the batteries alone.
- Inrush current: When starting, some appliances need, for a few seconds, much more power than when they are running. Examples of such devices: anything with a motor such as an air conditioner or a fridge.
- Depth of discharge: A battery has a limited amount of energy available, the more the battery is used between charges, the deeper the depth of discharge. This is reflected in the battery voltage, although the two are not directly related. Typically a lead acid battery can be discharged to a depth of 20% for about 500 times before it can no longer function properly. The lower the depth of discharge (DOD), the longer the battery will last. This is not true for the so-called "Edison batteries" which are much more durable and resistant to deep discharge.
Possibly the most critical components of an off-grid solar system is the battery fuse, resettable or not, which should be placed as close as possible to the positive terminal of the battery or batteries. Its purpose is to protect, not the electronics or the batteries, but the wiring. This is crucial as batteries can, for a certain period, provide an almost limitless amount of current possibly causing the connecting wiring to catch fire!
Regarding the charge controller whose function, you will recall, is to regulate the charging of the batteries from the panel(s), there are two main types: PWM and MPPT. The MPPT controller is favored over the first type for its greater efficiency in extracting the maximum of energy from the panels regardless of the sun's output.
Batteries are generally of the lead acid type although so-called "Edison batteries," made of nickel and iron with a potassium hydroxide electrolyte, are longer lasting by far, being usable for as much as several decades and are not toxic, whereas lead acid batteries typically do not last more than a dozen years and contain noxious chemicals. The lead acid batteries are easily the most expensive and troublesome component of a solar system, costing even more than the labor involved in installing a system. Also available are lithium-ion batteries with their higher performance although they cost significantly more per watt-hour than traditional lead acid types.
Inverters are electronic components which convert DC voltage (generally from 12V to as much as 48V) to AC, usually 110V which is useful in powering appliances. They come in three types: 1] square wave output 2] modified sine output and 3] pure sine output (this in order of increasing quality and cost). For residential application, they commonly range in power output from 100W to 15,000W, continuous. When selecting an inverter, one mush be careful to not only select a device which can handle the continuous power requirement but the expected (if any) inrush current as well; the latter can be as much as ten times the continuous requirement!
Solar panels are the main, if not the most critical, component of a system. They are solid state devices (oversized semiconductor diodes, really) responsible for generating the power which is stored and later used. They typically range is size from watts to about 350W with an efficiency in converting incident solar energy into electricity of about 18%, for the commercial variety. Durable when manufactured properly, and because they are semiconductor devices, they can last twenty five years with only minimal loss of power output. The glass cover, if not damaged, protects them from moisture and the elements.
Circuit breakers are basically switches charged with interrupting current from the solar panels and batteries while properly handling the arcs which occur when large currents are abruptly stopped.
Metering equipment is used by the owner of a solar system to assure the proper functioning of the system as a whole. It monitors: 1] Voltage/current and wattage coming from the solar panel(s) 2] Voltage/current/wattage charging the batteries 3] Current/wattage going from the batteries to the loads, whether they be DC or AC. Battery voltage is the most important of these since depth of discharge and the associated battery voltage level strongly determine the lifetime which can be expected of the batteries.