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Renewable Energy Information for Everyone

DC to AC Power Inverters

dc to ac power inverters For alternative energy systems, inverters are the essential step between a battery’s DC power and the AC power needed by standard household electrical systems. In a grid*-connected home, an inverter/charger connected to a battery bank can provide an uninterruptible source of backup power in the event of power failures, or can be used to sell extra alternative energy power back to the utility company.

Inverter Function and Benefits
Types of Inverters & Waveforms

Modified Sine Wave Inverters
True Sine Wave Inverters
Inverter Sizing and Use
Related Product Information

Inverter Function and Benefits

Batteries produce power in direct current (DC) form, which can run at very low voltages but cannot be used to run most modern household appliances. Utility companies and generators produce sine wave alternating current (AC) power, which is used by most commonly available appliances today. Inverters take the DC power supplied by a storage battery bank and electronically convert it to AC power.

An inverter used for backup power in a grid connected home will use grid power to keep the batteries charged, and when grid power fails, it will switch to drawing power from the batteries and supplying it to the building electrical system. For an business or home office, a reliable power source is invaluable for preventing lost data on computer systems. Most modern inverters also include overvoltage and undervoltage protection, protecting sensitive equipment from dangerous power surges as well.

In some areas, grid connected homes can use inverters and alternative energy generators to sell power back to the utility company. With the inverter attached to solar, wind or water generators, the inverter can use the utility grid as its battery bank. Utility power will be used when alternative power sources are insufficient, but when power needs are low, excess alternative energy can be sent to the utility grid. Sellback power will then be credited to the user’s utility bill.

In a stand-alone renewable power system, whether residential, industrial, marine or RV, the inverter allows AC electrical appliances to be run from the storage battery bank. When the battery bank becomes discharged, the inverter can automatically start a generator to power the system while the batteries recharge.

Types of Inverters & Waveforms

types of inverter waveforms
Inverters, besides coming in a wide variety of power capacities, are distinguished primarily by the shape of the alternating current wave they produce. The three major waveforms are square-wave, modified sine-wave and true sine-wave. Square wave inverters are largely obsolete, as the waveform shape is not well suited for running most modern appliances, and prices have come down considerably for the superior modified sine wave and true sine wave types.

Modified Sine Wave Inverters The least expensive type of modern inverter produces modified sine wave power. The waveform looks like a stair-step, where the power rises straight from zero to upper peak voltage, straight back to zero, and straight to lower peak voltage, resting at each point for a moment.

Modified sine wave inverters will run many household appliances such a televisions, radios and microwaves with occasional minor electrical "noise" present. Sensitive equipment like battery chargers, tools with variable speed motors, laser printers and certain heating controllers (such as those used by Toyostoves) will run erratically or not at all with modified sine wave power.

For a remote cabin with only the "basics" running on the electrical system, a modified sine wave inverter is an economical choice. Modified sine wave inverters are also often well suited for solar-powered RV electrical systems.

True Sine Wave Inverters The power supplied by utility companies and engine generators is a true sine wave form. This is the most reliable waveform for household use. True sine wave power passes from the upper and lower peak voltages in a smooth curved wave, rather than the stair-step of the modified sine wave.

All appliances and electronic equipment will run as intended when using sine wave power. True sine wave inverters will produce AC power as good as or better than utility power, ensuring that even the most sensitive equipment will run properly. While sine wave inverters are more expensive than modified sine wave models, the quality of their waveform can be a definite advantage.

For office buildings considering a backup power inverter, a true sine wave model will allow proper function of all electronic office equipment and fluorescent lighting. For residential power, anyone using a Toyostove, battery chargers, electric drills, digital clock radios or other sensitive electronics should consider a true sine wave inverter to ensure proper function of all household appliances.

Various inverters may have different features making them better suited for different specific applications. Very small inverters are available that connect to a car cigarette lighter, with a single three-prong AC outlet as the output. Large inverters are generally designed to be hardwired into a building electrical system. Some inverters offer 240 volt output, others can be connected to a transformer to provide 240 volt power. The right inverter for any specific use can be found with the help of an experienced inverter dealer.

Inverter Sizing and Use

When choosing a specific inverter, the inverter’s output capacity must be matched to the size of the electrical loads it will run. By choosing which electrical circuits the inverter will power (all circuits or only selected "essential" circuits), the power draw of all electrical loads on each circuit can be added together to arrive at a minimum necessary inverter capacity. Extremely power hungry appliances such as electric water heaters and electric clothing dryers should either be replaced with gas-powered energy efficient models, or be run on non-inverter supplied power.

Inverters have two different capacity ratings. One is the inverter’s continuous output rating. This is the maximum wattage the inverter can output on a long-term basis. The second rating is the inverter’s surge capacity rating. This is the maximum wattage the inverter can output on a momentary basis. Surge capacity will often be 2x or more in excess of the continuous rating.

All appliances require more power to start than they use while running. Many appliances, such as refrigerators and water pumps, will require up to three times as much power to start as they require while running. The combined starting power required by all inverter powered appliances must be within the inverter’s surge capacity rating.

For home or office (permanent) installation, the inverter is connected between the circuit breaker panel and the power source. If the inverter is only running certain loads in the system, those specific circuits will need to be wired to the inverter through a sub-panel. Different wiring layouts are used depending on whether the inverter is in a remote power system, a grid backup system or a grid intertie system. Many inverters include built-in battery charging functions, which is especially useful in grid backup systems.

Most modern inverters are configured to automatically switch between functions and electrical sources as needed. In a grid backup system, the inverter will switch to providing battery power within milliseconds of grid power failure. When grid power returns, it will switch to recharging the batteries until the next time they are needed. In a stand-alone alternative energy system, the inverter can start the generator automatically whenever the batteries fall below a set voltage level, and switch back to battery power when they are recharged. In a utility sellback system, the inverter can be set to send power to the grid during specific times of day.