Almost all small hydro and wind turbines produce alternating current (AC) but this is converted to DC for battery bank storage. Let's look at the conversion of DC back to AC for your home electrical appliances with the extra energy sold back to the grid.

In the olden days of renewable energy, a system could only be built with 12V DC appliances that are commonly found in many recreational vehicles. Grid-connected renewable energy systems were still just a pipe dream. But today, times have changed: high-quality, low-cost inverters that on- and off-grid systems can use are freely available.

The basic inverter converts low-voltage DC from a battery bank or PV panel to AC of similarly low voltage. It then "steps up" the voltage to fit 120V/240V home installations, as supplied by your utility company.

There are also smaller, cheaper inverters for grid connection without batteries. The design is similar but the required DC input voltage is higher.

Inverters have a host of features and functions such as:

• Battery charging
• Transfer of your home's power from the generator to the inverter
• High and low voltage detection and disconnection (LVD function) at the battery
• Sleep mode on the inverter for more energy savings; the inverter 'wakes up' at the first flick of a power switch
• Auto start and stop for backup generator
• Maximum power point tracking available with on-grid systems
• Complete safety protection for homeowners and utility workers

The schematic on this page shows an inverter installation for battery-backed on-grid operation. Energy from the renewable source is kept in the battery bank to be redirected to the internal components of the inverter.

A controller and power supply must be available to ensure that the unit functions as desired. Once the inverter starts, the controller powers up a high-powered oscillator or power bridge to generate alternating current at 12V, 24V or 48V, depending on the battery bank or renewable source voltage.

Low-voltage AC in North America usually runs at 60 cycles per second (60 hertz) whereas AC in Europe or Asia runs at 50 hertz. This means the current's polarity switches direction 50 or 60 times every second, just like turning the battery around in a flashlight.

The main advantage of using AC instead of DC is the ability to step the voltage up and down using a transformer.

A transformer inside the inverter receives low-voltage AC power and steps it up to the utility-standard 120V or 240V. In the inverter shown in our schematic,  the transformer is designed to accept a nominal 12V AC input from the power bridge and convert it to 120V output. This power is then fed through a protective circuit breaker and into the power panel, which distributes it throughout the house.

If the system is grid-interconnected, the inverter automatically synchronizes its waveform to that of the utility and excess electricity is "exported" to the grid via an electrical energy meter.