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Inverter manufacturers offer online tools to make sure series strings of modules are optimally matched to an inverter, maximizing system performance.

utility bills from the previous residence, past tenants, and/or similar homes. The more data the better—if possible, view several years' worth to take into account occasional aberrations, such as a holiday when lots of guests increased the load.

A thorough profile is necessary because of the various utility accounting methods for grid-connected PV systems, particularly with surplus generation. The most progressive programs use feed-in tariffs (a type of production-based incentive), which pay premium prices for RE-generated electricity. But most utility customers have only net metering—a kWh-for-kWh exchange. Net metering values PV energy at the retail price but usually does not pay for production beyond what the customer uses. Any excess production is usually carried over between billing cycles and for up to a year. After that it is lost—given away to the utility. In some states, the utility pays for excess generation at "avoided cost," closer to the wholesale price per kWh.

Any time PV production is undervalued in this manner, the effective cost per kWh of the system increases. In contrast, many utilities have tiered rate plans, where the cost per kWh for utility electricity increases as more energy is used, especially during the hot summer months. During these times, increased PV output, whether by design or by the simple fact that there is more sun, increases the average value of each kWh generated by the system. In this scenario, a smaller system may have a lower price per kWh because a larger percentage of the energy generated will be credited at a higher rate—effectively "shaving" the most expensive kWh purchased during the year. Time-of-use rate plans further complicate the analysis (see "Time-of-Use Metering" sidebar). Coordinating between monthly consumption patterns, expected system output, and the ins and outs of specific utility requirements and programs can be daunting—just remember PV energy is pricey, so there's no reason to give it away.

Selecting Equipment to Maximize Performance

The list of available PV modules, inverters, and other equipment literally grows by the day. Ten-year inverter and 25-year module warranties are the industry norm, with some exceptions. Competing claims of performance and efficiency are complicated by standardized ratings that don't duplicate realistic operating conditions (see "Realistic Expectations" sidebar). Cost, usually expressed in dollars per watt when pricing modules or inverters, always plays a role, along with availability and familiarity—look for the good deal, but in all cases, choose equipment that has a good reputation in the field.

Time-of-Use Metering

Especially during the cooling season, the load on the electric grid varies greatly at different times of the day. Peaks coincide with people getting ready for or coming home from work, and with the hottest part of the day, when the most air conditioners are running full-blast.

Utilities need to be able to safely meet the peak demand, and they accomplish this through a combination of "peaker" generating facilities and energy purchase contracts. But peak energy is the most expensive for the utility to maintain since these power plants have to be ready to supply energy only during peak usage times, and sit idle other times.

To encourage decreased use during peak periods, time-of-use (TOU) rate plans charge more for electricity when there is more demand and less when demand is reduced. Lower-than-normal rates in off-peak times compensate for the increased on-peak rates, potentially reducing electric bills and reducing the amount of peak capacity the utility must have available.

The details for TOU rate plans—including whether or not net-metered PV systems can take advantage of them—can vary considerably and can be complicated to decipher. But often, a PV system's highest production—during the peak sun-hours between 9 a.m. and 3 p.m.—also coincides with the highest peak grid usage. If excess PV energy is credited in dollars at the higher on-peak rates, then the credit can be used to purchase lower-priced, off-peak kWh at night, when the PV system is offline. This can tremendously lower the system's cost per kWh.

Another possible TOU scenario is when excess energy can only be used as a credit during the period in which it was generated. This is the most difficult situation to assess, and requires careful analysis of system production and load by period, as well as how annual surplus is accounted for.

For customers who already have TOU metering and are adding a PV system, detailed billing history will usually break down total consumption into on- and off-peak periods. Customers with higher on-peak usage are most likely to benefit from the combination of PV and TOU. This includes home offices, commercial and industrial facilities, and households with large daytime loads.

Mounting thermally sensitive fuses or breakers on hot roofs or in other sunny locations can cause nuisance-tripping, increasing downtime and reducing the system's lifetime energy production.

Mounting thermally sensitive fuses or breakers on hot roofs or in other sunny locations can cause nuisance-tripping, increasing downtime and reducing the system's lifetime energy production.

Module Considerations. One of the biggest differences between modules is power tolerance, which is the expected variation from a module's rated output. For example, a 200-watt module with a tolerance of +/-7% could actually produce from 186 to 214 W. Many modules are in the +/-3 to +/-5% range; some have a wider range, as high as +/-10%. Others guarantee a positive power tolerance, stating that the module will produce at least its rated power.

The wider the power tolerance in the module, the more likely it is that the PV array will have modules with different operating characteristics. For instance, the 200 W module mentioned previously has a power tolerance spanning nearly 30 W. Individual modules in this array are likely to operate at

Voltage Drop

Even though it's not a calculation required by the NEC, ignoring voltage drop—caused by a conductor's internal resistance as current flows through it—can result in sizable energy losses as well as increase the possibility of the array output falling below the inverter's minimum DC input voltage.

Good system design limits total voltage drop on DC circuits to 2% or less. Voltage drop between the inverter and its connection with the grid should usually be 1.5% or less to ensure that the inverter has enough voltage to push back the AC voltage from the grid. Voltage drop on this circuit will result in the inverter "seeing" a voltage above that measured at the point of connection; if this is too high, it can exceed the AC operating voltage window of the inverter, causing it to go offline. (See "Back Page Basics" in this issue and "Voltage Drop after NEC Requirements" in HP80.) Numerous voltage-drop calculators, such as those on inverter manufacturer Web sites, are available—though it is always wise to doublecheck results.

Inverter

PV Array

Inverter

Grid Connection significantly different maximum power points. Because they are often wired together to an inverter capable of tracking only one maximum power point, the lower-output modules will tend to "pull down" the ones with higher outputs. When wired in series, the current of the string will be closer to the current of the lowest-rated module. When wired in parallel, the voltage of the strings will tend to be the average of the strings' combined voltages. Since most grid-tied systems have source circuits wired in series and parallel, this results in lost power and means that a +/- power tolerance is usually only a minus. To combat this issue, modules with a narrow (or positive only) power tolerance can be used. Or consider using microinverters, where each module is paired with its own inverter or with inverters that can track more than one MPPT.

Matching the number and model of a PV module to an inverter requires careful consideration of local temperature extremes and the overall size and wiring configuration of the array (see "String Theory: PV Array Voltage Calculations" in HP125). Coupling an inverter to an array that barely meets the inverter's DC input voltage will result in disappointing performance: In addition to the decrease in voltage due to temperature, modules will also lose some power output over time due to dust, degradation, corrosion, and increased connection resistance. Though it may take several years, these combined factors can result in a low-enough voltage to shut down the inverter. (See the "Grid-Tied Inverter Buyer's Guide" in this issue for more information.)

Installation Considerations. Regardless of how well a system is designed, improper installation can result in poor performance. PV systems should operate for decades, and the materials and methods to install them should be selected accordingly. Wire, conduit, and associated hardware typically make up a small percentage of PV system cost—skimping on them may result in decreased output and, therefore, a higher cost per kWh.

Loose connections are a common and potentially serious installation issue. They lead to increased voltage drop, lost output, and added maintenance costs. At worst, the increased resistance leads to heat buildup and fire. Troubleshooting

Roof overhangs can provide some shade, but the inverter may still be subject to direct sunlight and elevated operating temperatures at different times. A shading analysis tool can also be used to determine inverter placement.

loose or sporadic connections can be time-consuming and frustrating, so minimize their likelihood from the start: All connections in the system should be tightened to the specifications of the device, and should be appropriate for the size and type of wire, as well as for the location. Torque wrenches are fairly common (torque screwdrivers much less so), but specs for tightening screw terminals are provided for components and should be followed.

But They're Supposed To Be in the Sun

Both PV modules and inverters operate more efficiently at cooler temperatures. While most grid-tied inverters are designed for outside installation and housed in outdoor-rated enclosures, they should not be mounted in direct sunlight, as this will cause them to operate less efficiently. In addition to the lost output, inverter life is likely to be shortened. While the expectations built into most PV financial modeling programs include inverter replacement, "burning" through several expensive inverters will dramatically increase the system's cost per kWh. The LCD display in most inverters also can be rendered useless after too much sun exposure.

Placing overcurrent protection devices in excessively hot and sunny locations can also lead to unexpected downtime and the loss of energy production. Fuses and breakers are thermal devices—they rely on the heat generated by current running through them to "trip" and disconnect, protecting the wiring from overcurrent conditions. When operating in high ambient temperatures, the ratings of fuses and breakers are effectively lowered, meaning that they may "nuisance trip" even when carrying less current than they are rated for.

Until the fuse is replaced or the breaker reset, the output of the PV source circuit or array connected to that overcurrent device will be lost. Because grid-tied systems operate "silently," the building will still have power even if the inverter is offline—lost production may not be noticed until the next electricity bill reports higher-than-normal usage. Keeping combiner boxes off hot roofs and out of direct sunlight and wiring roof-mounted arrays with home runs from each source circuit back to an inverter mounted in a shaded location (build an awning if necessary) can be good strategies to ensure that the system stays online.

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Brian Mehalic ([email protected]) is a NABCEP-certified PV installer, with experience designing, installing, and servicing PV, thermal, wind, and water-pumping systems. He is currently an instructor for Solar Energy International and works on curriculum development for SEI's PV program from his home in Prescott, Arizona.

Further Reading:

"Solar Survey," by Justine Sanchez, HP130

"Optimizing a PV Array" by David Del Vecchio, HP130

"String Theory: PV Array Voltage Calculations," by Ryan Mayfield,

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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