System Sizing Calculations

If you’re thinking about designing a grid-tied PV system for your home, here’s a simplified overview of the steps required. System sizing relies on electricity consumption, site-specific solar insolation data, and array shading, tilt, and orientation specifics.

Step 1: Determine average daily AC electricity use. Grid-tied PV systems can provide some or all of your home’s electricity. Reviewing your past year’s electric bills will get you started. In our case, we wanted to offset 100 percent of our grid electricity with solar electricity.

2,520 AC KWH/year ÷ 365 days/year = 6.9 AC KWH/day

On average, we’d need our PV system to generate 6.9 AC KWH per day.

Step 2: Determine the initial array size (unadjusted for system efficiency) necessary to meet your average daily AC KWH solar-electric generation goal. You’ll need to know the average daily peak sun-hours at your location (visit http://rredc.nrel.gov/solar) and what percentage of the total solar resource is available, depending on shading at your site and array orientation. A solar resource evaluation tool, like the Solar Pathfinder, is needed to determine array shading. Our array faces true south, so no adjustment for orientation was necessary.

Average daily peak sun-hours at our location: 5.8 (Grand Junction, CO data)

6.9 AC KWH/day ÷ 5.8 average daily peak sun-hours = 1.19 KW (initial array size needed, unadjusted for system efficiency)

1.19 KW ÷ 0.90 (fraction of total solar resource available) = 1.32 KW (initial array size, adjusted for solar resource at site, unadjusted for system efficiency)

Step 3: Determine array size based on system efficiency factors. Precisely calculating overall system conversion efficiency depends on a number of variables, including module performance at elevated temperatures, the production tolerance specified for a given PV module, mismatch between individual modules wired in series, and inverter efficiency. Installation-specific details, such as array mounting as it relates to air circulation and cooling, transmission losses in system wiring, and PV output losses due to soiling/dust buildup, all come into play.

Overall efficiencies for grid-tied PV systems typically fall between 75 and 85 percent of the rated array output at standard test conditions (STC; 25°C, 1,000 W/m²).

Our estimation, considering the variables above, is based on a predicted system conversion efficiency of 77.5 percent.

1.32 KW (unadjusted PV array rating) ÷ 0.775 = 1.7 KW (specified array size)

Step 4: Determine the number of modules required to meet energy generation criteria. We were planning on installing Mitsubishi 170 W modules. Dividing our specified array size by 170 watts gives us the total number of modules required.

1.7 KW x 1,000 W/KW = 1,700 W 

1,700 W ÷ 170 W/module = 10 modules

Step 5: Determine array voltage based on compatibility with selected inverter model. Almost all modern grid-tied inverters are high voltage, with maximum DC voltages of 600 volts for some models. Pay attention to several variables when matching your PV array requirements to a specific inverter.

We decided to install a Fronius inverter, based on the product’s reputation for solid performance and reliability in the field. We chose a 2,000-watt IG 2000 model based on our calculated array size of 1,700 watts.  

The next step was to check what module string voltages are compatible with the inverter. Most grid-tie inverter manufacturers, including Fronius, have convenient string sizing calculators available online. Factors that affect array string sizing include maximum power, peak and open-circuit module voltage specifications, and the inverter’s maximum voltage limit and operating voltage range. Because array voltage increases as temperature decreases (and vice versa), string sizing calculators require the input of the record low and high temperatures at your site (visit www.weather.com/common/home/climatology.html).

The record low temperature in our town of Paonia, Colorado, is 31°F below zero. The Fronius configuration tool confirmed that, at our location, ten 170 W Mitsubishi modules in series were a good match for the IG 2000 inverter, and that the maximum DC input voltage would not exceed the inverter’s 500 VDC limit, even during record cold temperatures.