“A competitor says his system has an AC-DC derate of 95%. Why is yours so low?”

On his blog, Dave Beumi notes: “Derates are the various locations and instances in a PV system where power is lost from DC system nameplate to AC power. This includes inverter loss, resistive factors, environmental conditions and issues relating to maintenance… The PV industry and its financing partners rely on simulation modeling software, which provides a fairly accurate multiple year forecast of energy production and economics, including [solar] financial payback. These models are thorough, sophisticated software packages which take into account the many variables which affect a PV system’s performance including weather, environmental conditions, technology and product performance, government subsidies, and cost of money among others… While the model takes into account the derate factors and a detailed weather history for a given location, its important to note that annual fluctuations in weather conditions is an important variable which can be significantly different year to year. Overall, simulation models are quite accurate and are a fairly good gauge for finance companies to make an informed investment decision.”

PVWatts makes it calculations using whatever derate number the user inputs. Our experience has born out that 81% is a reasonable but conservative derate. There are technical reasons why we could use a higher or lower factor, but that would take several pages of explanation and probably put you to sleep.

Suffice it to say, that 95% total derate is a physical impossibility. Any solar installer telling you that is not being truthful.