Inverters for Grid-Tie & Off-Grid Solar Power

Stand-alone Inverters convert DC power stored in batteries to AC power that can be used as needed. Selecting an inverter for your power system based on the maximum load you will be powering, the maximum surge required, output voltage required, input battery voltage and optional features needed. High quality stand-alone inverters are available in sizes from 100 watts, for powering notebook computers and fax machines from your car, to 500,000 watts, for powering a commercial operation. The size of an inverter is measured by its maximum continuous output in watts. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. The size of the inverter can be minimized if the number and size of the AC loads is kept under control. Wattage of most AC loads can be determined from a tag or label on the appliance, usually located near where the power cord enters, or from the owner's manual. If the inverter is expected to run induction motors, like the ones found in automatic washers, dryers, dishwashers and large power tools, it must be designed to surge, or deliver power many time sits rating for short periods of time while these motors start.

Stand-alone inverters are available with three basic power output waveforms: square wave, modified square wave (often called modified sine wave) and sine wave. Intertie inverters and utility companies deliver a sine wave. Square wave inverters have the lowest cost and efficiency and are not sold in this catalog. The price of the better quality inverters is low enough to make square wave inverters an unattractive choice.

Intertie Inverters

Intertie Inverters change DC power into AC power to be fed into the utility grid. A power system with this type of inverter uses the utility company as a storage battery. When the sun is shining, your electricity comes from the PV array, via the inverter. If the PV array is making more power than you are using, the excess is sold to the utility (power company) through an electric meter. If you use more power than the PV array can supply, the utility makes up the difference. This type of system makes the most sense if you have utility power, because there are no batteries to maintain or replace, but it has a very long payback period and may not be cost-effective at today's electric rates. The Trace SWPV, UT and microsine, AEI GC and Omnion 2400 inverters are examples of an intertie inverter. Using a multifunction inverter allows you to sell excess power to the utility, and also maintain a battery bank for standby power in the event of a utility power failure.

Multifunction Inverters

Trace Engineering Company produces a line of sine wave inverters called the SW line that can operate as a stand-alone inverter and as an intertie inverter at the same time. In a typical installation, the Trace SW inverter is connected to a battery bank, the utility power lines, a standby generator and the house load center. When batteries are in a charged condition, the SW inverter supplies AC power to the house from the batteries. If the batteries become discharged, the inverter supplies the house loads from the utility lines, while charging the batteries. If the batteries become fully charged by another power source, such as photovoltaic modules or a wind or hydroelectric generator, excess power may be sold back to the utility. If utility power fails, the inverter can still operate, supplying critical loads. If a standby generator is started, it can also supply power to loads. The inverter will synchronize to the generator and allow loads to be powered that are too large for either the generator or inverter to supply alone. Multifunction inverters are not the most efficient intertie inverters because the system must have a battery, but they allow system flexibility that intertie inverters do not.