Many consumers considering solar electric systems for their homes believe it will offer them a back-up source of electricity during a power outage. In nearly all cases, when central-station power from the utility goes down, so does power from the solar system. The task of helping consumers understand this limitation often falls upon the local power supplier like Access Energy Cooperative; so here’s some background information.
Two very important characteristics of electricity produced from solar cells are:
It is DC (direct current) which must be turned into AC (alternating current) for use in the home;
The electric output from a solar module is not constant, it can vary continuously (clouds; sun angle, etc.).
Comparing these two characteristics to normal utility power clarifies the difference; central-station power is stable no matter what the weather conditions (industry standards require the voltage and current remain within a close range). Plus utility power is already delivered as alternating current (AC) as needed by motors, lighting and appliances.
In a solar installation, the device that converts the incoming and highly variable DC power into stable AC is called an “inverter.” This sophisticated electronic device requires a steady source of electricity to function. Since input current from the solar system constantly varies, the inverter needs a separate and constant power supply to do its job. If an outage occurs, the inverter won’t work unless another battery
source is in-place to supply it. Unless the inverter is powered separately, during a power outage the solar system won’t produce electricity for the home.
Adding a battery strictly to power the inverter during an outage sounds simple, but it’s not. It means the system must safely disconnect itself from the utility grid as required, but retain the ability to convert the direct-current (DC) electricity from the solar system into usable electricity for the home. Technology that is affordable and failsafe is still under development.
As a partial solution during outages, a few manufacturers include a separate plug-in outlet incorporated into an emergency power-supply system. This safely separates it from the utility grid and the building’s electrical circuits. It offers a small amount of power to serve cell phones or laptop computer charging.
When it comes to solar and other renewables, there is a major trade-off in reliability compared to conventional electricity generation. Sometimes the appeal of “clean energy” is so great that consumers tend to overlook these tradeoffs or other negative aspects. The challenge for power suppliers is to explain all the considerations so that members can consider all factors before making decisions.