Field calculator

Solar PV production calculator (kWh estimate)

This calculator gives a quick estimate of how much energy a solar array will make, the starting point for sizing a system, checking a proposal, or sanity-testing a payback. The energy equals the system size in kilowatts DC times the peak sun hours per day times a derate factor, multiplied by 365 for the year. Enter the system size, the site peak sun hours, and a derate percentage. The derate, commonly about 75 to 85 percent, is where the real-world losses live: inverter efficiency, high-temperature derating of the panels, soiling, wiring and mismatch losses, and any shading, so the AC energy delivered is always less than the nameplate DC times sun hours. Peak sun hours are not daylight hours; they are the equivalent hours of full-sun irradiance and they vary by location, tilt, orientation, and season. For a number you can put in a proposal, pull the site's real solar resource from a tool like NREL PVWatts and use a modeled derate rather than an assumed one.

anvilfield.com/calculators/solar-pv-production-estimate-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Solar production FAQ

What is NEC 690 rapid shutdown?

NEC 690.12 rapid shutdown lets a firefighter de-energize a rooftop array's conductors from one switch. On initiation, conductors inside the array boundary drop to a controlled level and those outside fall to about 30 V within 30 seconds. Module-level electronics or a UL 3741 system meet it. Verify the limits against the adopted edition.

What is the difference between a string inverter and microinverters?

A string inverter converts the DC from series strings at one central box, so shade on one module drags the whole string. Microinverters convert at each module, isolating shade and mismatch and providing module-level rapid shutdown, at higher cost and with roof-level units to service. Match the choice to shading and roof complexity.

What is the 120% rule for solar interconnection?

The 120 percent rule limits load-side solar backfeed under NEC 705.12. With the PV breaker opposite the main, the main rating plus 125 percent of inverter output cannot exceed 120 percent of the busbar ampacity. A 200 A bar with a 200 A main leaves about 40 A for PV. Verify against the adopted edition.

Why does PV string voltage matter in cold weather?

Cold raises a module's open-circuit voltage, so a string sized at room temperature can exceed the inverter maximum and the system voltage limit on the coldest morning. NEC 690.7 makes you calculate maximum voltage at the lowest expected temperature using the module's coefficient. Size the string for the cold Voc, not the spec sheet.

Can I mix MC4 connectors from different brands?

No. Mate connectors of the same brand, or ones specifically listed as compatible. Two MC4-style connectors from different makers can click together and look seated while the contacts barely touch, and a high-resistance DC connection in full sun makes heat and starts fires. If pigtails and module leads differ, change the pigtails.

More in the PV system wiring (NEC 690) field guide.