An unsuitable backup power supply is the Achilles' heel of an off-grid energy system. The problem does not lie in the renewable energy equipment's reliability; a well-designed system is often more reliable than the electrical grid.

Problems are more often related to the weather than the equipment. When the winter months are dark without any sun or wind, your system will require more batteries.

Grid-tie systems work similarly; when the grid is down, so is your home system. Systems that have battery backups are only slightly better as they are incapable of lasting through more than a couple of days of grid outage or dark skies.

The fossil-fueled backup generator is the antithesis of renewable or clean energy technology but, as one of life's many tradeoffs, it's often introduced into the off-grid setup.

An off-grid system that is designed well will operate 90% of the time without the need for a backup generator. But in poor weather or when the system requires major servicing, you will have to shut everything down or start up the generator.

The backup generator fully charges the battery bank before shutting down quickly. Generator power is usually noisy, expensive and polluting compared to renewable or grid-produced electricity so the shorter its running time, the better. Generators perform best at full load: they consume the same fuel whether they charge a battery bank or run a single light bulb.

The generator's running time is limited by the inverter's internal battery charger to fully "load" the generator at the bulk-charging step. When the battery is normally charged, the inverter converts to absorption mode before going to float mode for a complete cycle with an auto switch-off. If you desire battery equalization, the inverter will perform the high-voltage equalization charge before returning to float mode and then shutting off the generator.

Charge controllers or inverters with automatic controls can get the generator to start and stop automatically to complete the charging cycle efficiently and quickly. The entire process takes about 5-10 hours for one cycle depending on the capacity of the generator, battery size and discharge depth.

A typical renewable energy system can operate three quarters of the year without switching on the generator. During the cold winter months, the generator must be in full working order: it's pointless having a cheap unit that will fail you just when you need it!