Pump Up the Power: Page 4 of 5

rated to be installed outdoors must be kept shaded, even if it means installing an awning over them. Likewise, be sure that inverters installed in closets or small rooms have sufficient air circulation to help remove heat buildup.

Most modern inverters are rated at efficiencies of 90% or greater, but actual operating efficiency can vary. One factor that can affect power production is an inverter’s maximum power point tracking (MPPT) performance. PV module voltages fluctuate as light and temperature conditions change, and the inverter must be able to work efficiently within a range of voltages. If an inverter’s effective MPPT voltage range is too narrow, then production can drop accordingly.

Always consider the efficiency of inverters before buying. A 1% improvement in efficiency can mean thousands of KWH gained over the lifetime of your system, and more money in your pocket. Each inverter lists maximum efficiency in its specifications, but a more realistic value is the “weighted efficiency”—a useful comparison tool for designers and consumers. A weighted efficiency is estimated by assigning a percentage of time the inverter resides in a particular range of operation to approximate its efficiency over the full day. Because available sunlight and array operating conditions are constantly fluctuating, actual array power will vary throughout the day, so weighted efficiency can be a better predictor of system performance.

Miscellany

Line Losses. The amount of energy lost in conductors and electrical connections is known as line loss. The wasted energy from resistive losses—voltage drop in the electrical circuit—from source to load should be designed to be less than 5%. Since voltage drop is additive for each individual wire run within a circuit, keeping the overall voltage drop from source to load under 5% means voltage drops of the individual wire runs will have to be much lower (2% or less). Maximize performance by evaluating and sizing each wiring run individually. Of particular importance is the output circuit from a grid-tied inverter to the main service panel. This wire run usually needs to have a 1% or lower voltage drop to ensure that the inverter has enough excess voltage to be able to push its energy onto the utility grid and to make sure the voltage from the grid stays within the inverter’s AC operating window. Using higher voltages and larger conductors means less resistance losses. Additionally, reliable, low-resistance connections between conductors and equipment will help minimize losses.

Soiling. Dirt, dust, bird excrement, and snow can filter out some sunlight from the PV cells. According to the National Renewable Energy Laboratory, modules in areas that experience high levels of particulate pollution and infrequent rain can experience soiling losses of up to 25%, especially on flat-mounted arrays. Isolated soiling that remains for an extended time can cause “hot spots” that prematurely degrade or damage PV cells. It’s nothing a squeegee, some water, and a little elbow grease can’t conquer—keep your