A Simple Explanation of Off-Grid Power
To select a solar electric system for your off the grid homes or RV, you need to know what the major parts are called, what they do, and how they work together. Here is a quick overview followed by drawings explaining the whole process. Individual parts will be described in more detail within each section of the website.
Sun shining on solar modules produces DIRECT CURRENT electricity, or DC, the only kind of off grid power systems stored in batteries. Often this is 12 volt DC, the standard used in cars and RVs. Larger systems may be designed for 24 volt DC, or sometimes 48 volt DC. This just means combining the same 12v solar modules in pairs for 24 volts, or groups of four to get 48 volts. Windmills and micro-hydro generators in this website also produce DC for charging batteries.
Grid Tie System without Batteries
This DC power is stored in DEEP CYCLE LEAD-ACID BATTERIES, which give back the off the grid electricity as needed, even when no power is being produced. Like a bank account, power put into batteries over a period of time can be taken out more quickly if a lot is needed. Like a bank account the total amount of power you withdraw cannot be more than you put in, or the account will be depleted. Moreover, lead-acid batteries need to be frequently 100% fully charged to remain in good condition. They should never be drawn completely down to empty. Because of these needs, to get the most years from your batteries requires some supervision by the owner.
Grid Tie System using MicroInverters
The INVERTER is a major component that converts the 12, 24, or 48 volt DC current from the battery into 120 volt AC current, the same as utility power for standard household lights, outlets, and appliances. Most solar homes use primarily 120 volt AC produced by the inverter. A few DC circuits are usually added where using DC can save a lot of energy. Sometimes a small solar electric RV, boat, or cabin may have no inverter, and use only DC wiring and appliances.
If there are a number of consecutive days without sunshine, the owner, being aware of the weather, checks the batteries. If the charge level is low, an engine driven generator may be started to recharge the batteries in order to keep the whole system working. A battery charger plugs into 120 volt AC from the generator producing low voltage DC to charge the battery. The generator is shut down after the batteries have been recharged. This process is automated in some power systems. (Battery chargers in Recreational Vehicles are called converters).
SOLAR MODULES are installed in groups of 1 to 18+ modules on a solar mount, which in turn attaches to a building, to the roof of an RV, or atop a metal post in the yard. Together this is called a solar array. Each solar module is wired to the other modules in that array by sunlight tolerant solar interconnect wiring. Several arrays may be wired to a solar combiner box where they are all connected to heavier underground wires taking the power to the battery and equipment room.
A CHARGE CONTROLLER is a small wall mounted component receiving the power from solar, wind, or micro-hydro generators, and controlling the flow of power to the battery. To prevent battery damage from overcharging, the charge control automatically cuts back, stops, or diverts the charge when batteries become full. A charge control may have manual control switches and may have meters or lights to show the status of the charging process.
BATTERIES receive and store DC electrical energy, and can instantly supply large surges of stored electricity as needed to start or run heavy power appliances that the solar panels or hydro electric generator alone could not power. This large power capability can be a fire hazard just like utility company power, so fuses and circuit breakers on every circuit connected to a battery are essential. Battery size is chosen for both surge power requirements and for the amount of reserve power needed. Typically, 2 to 12 square feet of batteries weighing 150 to 5000 pounds are enclosed in a battery box with a vent pipe to the outside.
The INVERTER is the major electronic component of a power system. It converts DC power stored in batteries to 120 volt AC, standard household power. Short, heavy cables with a large fuse or circuit breaker carry battery power to the inverter. After conversion to AC, power from the inverter usually connects into the circuit breaker box of the house in place of utility lines. The house breaker box routes power to lights, appliances, and outlets of the house. The inverters we offer for home power come in ratings from 300 to 8000 watts.
A STANDBY INVERTER/CHARGER is an inverter that also has a battery charger and transfer relay built in. When the input terminals of a standby inverter/charger receive power from an outside source of AC (a generator or utility power) the inverter stops producing AC power from the batteries, and instead passes generator or utility AC power straight through to the house. At the same time it uses the generator or utility power to recharge the batteries. Some standby inverters even auto-start the generator when batteries need charging. A separate battery charger can be used instead of (or in addition to) a standby inverter/charger.
Off-Grid System with Batteries & Generator
An ENGINE GENERATOR producing 120 volt or 120/240 volt AC power is usually part of the system. This is a second source of AC power and a backup for charging the battery when there is a shortfall in solar or wind power, a temporary need for additional power for construction or visitors, or in case of breakdown of other equipment. Just starting the generator begins the standby inverter charging process. The best generators start automatically or by push-button from the house.
A generator is located outside, usually in its own shed at least 30 feet away to avoid noise. For reasons of health and safety, it should not go in a basement or garage. AC power from the generator goes through a circuit breaker, then is wired into the power room to run battery charger/s as well as supply the AC power to the house whenever the generator runs. Since both battery charger and AC transfer relay are usually part of a standby inverter, the generator power usually connects only to the AC INPUT terminals of the inverter, NOT to the house breaker box!
FUSES or CIRCUIT BREAKERS are necessary in all DC wiring between the batteries and other power system components described, but not shown in the drawing. This prevents fires and equipment damage in event of a malfunction. Breakers may be separate components in their own box, or might be built into a power center. In contrast, the AC breaker box for household wiring is part of the house wiring, not usually included with power generating equipment.
METERS, like the gas and temperature gauges in a car, are necessary to show everything is working. Solar charge indicating meters are often built into the charge controller to confirm the charging process instantly. Other meters show how much power is being consumed, and confirm how much power is available. These battery system monitors can be located in the power room, or at a convenient spot in the home for easier checking.
A POWER CENTER is a product including system meters, DC circuit breakers, and wiring connections for batteries, inverter, solar and other charging sources. Power centers are easier to install and to pass building code than would be selecting, buying, and installing all those parts separately. The power room is simplified, with just a few main components: powercenter with charge control attached, a standby inverter-charger, and a battery box on the floor. Some powercenters, like the Outback, Magnum or Midnite E-Panels, are shipped as a completely assembled power system.