# What are the differences between IEC solar calculator and the PV Watt calculator findings?

The two calculators are actually very similar in results but approach the problem from different angles. The National Renewable Energy Laboratory’s (NREL) PVWatts calculator starts with the DC rating of the solar array while the Iowa Energy Center Solar calculator begins with the AC rating delivered to the load. The difference between the two calculators is the point at which the system losses or derating is applied. Determining the appropriate loss factor is not a trivial matter and should be calculated by an experienced professional. The system losses are site specific and may be comprised from a combination of factors associated with the wiring, inverter(s), soiling, orientation, panel mismatch, panel nameplate rating, etc.

Here is an example:

PVWatts requires that the DC rating of the solar array be inputted after which it is derated (by default or user input) for the system losses to arrive at the AC rating of the array. Assuming a 1 kW dc rated solar array, oriented at due south (180 degrees) with a fixed tilt angle of 30 degrees, located in Des Moines, IA, and derated by the PVWatts default factor of 0.77 (23% for losses); the PVWatts program calculates an array annual generation of 1289 kWh.

Using the Iowa Energy Center’s solar calculator for the same conditions, the annual generation for a 1 kW AC output array is calculated to be 1650 kWh/year. Using a similar derate factor as used in the PVWatts example of 0.77, the corresponding DC array rating would need to be 1.0 kW AC/0.77 = 1.3 kW DC. If you want to calculate the output of a comparable 1.0 kW DC rated system using the Iowa Energy Center Solar Calculator, simply multiply the 1 kW AC output generation of 1650 kWH by the appropriate loss factor to give 1650*0.77 = 1,270 kWh which is very similar to the PVWatts calculation.