Solar-electric systems use photovoltaic (PV) modules to make electricity, for use by the electrical loads. Between the PV module(s) and the loads, there are a number of different components and configurations, although four basic configurations are common.
1. PV-direct systems are the simplest, consisting of a PV module or modules connected directly to a load. There may be electronic controls or a linear current booster between the two. Having no storage capability, these systems operate only when the sun is shining. Common applications are water pumping and ventilation.
2. Off-grid or “stand-alone” systems pair PV modules with batteries for energy storage. This allows energy use when the sun isn’t shining, like at night or during storms. A charge controller prevents overcharging the batteries; it can also protect against too deeply discharging the batteries. These systems usually have an inverter, which converts the DC electricity generated by the PV modules and stored in the batteries to conventional AC electricity so that common appliances may be used.
Off-grid systems must supply all of the energy needed, so they often include other sources to supplement solar energy, such as a fuel-fired generator, or wind or hydro generator. Users must learn to live within their energy budget, since there is a limited supply.
3. Battery-based grid-tied systems are similar to stand-alone systems except they can use the grid for “selling” surplus energy (and earning credits for future use—net billing) and for backup battery charging. With these attributes, battery-based grid-tied systems have the best of both worlds. However, they are more complex and more expensive than batteryless grid-tied systems.
4. Batteryless grid-tied systems are connected to the utility. Compared to battery-based systems, they are simpler, more cost-effective and environmentally friendly, and require the least maintenance. Their drawback is that when the utility grid is out of service or out of spec, they cease operation until the grid problem is resolved. These systems usually have one major electronic component—the batteryless inverter, which converts the PV output to usable electricity for your home or for sending back to the utility. More recently, microinverter-based systems—in which an inverter is matched to each PV module—and AC modules (which incorporate a microinverter onto the module) have gained a foothold in the market. Microinverter-based systems are especially suitable in areas where partial shading of an array is a concern.
Most new systems installed in the developed world are batteryless grid-tied systems. Whether to have backup or not is a practical and personal decision, based on the number and duration of outages in your area and your personal preferences and needs.
How you use solar electricity depends on your application and your personal choice. One of the four configurations above will best serve your needs, allowing you to use the solar energy that comes up every morning.