Solar system performance: 1, 5 & 10 years down the road

Published October 21, 2016

When you buy a solar electric system, you are actually buying over 25 years’ worth of solar electric production. Thus, accurate production estimates are one of the most important components of a solar project proposal. To calculate solar production, LightWave Solar inserts project variables into the National Renewable Energy Lab (NREL) tool, PV Watts. Following industry best practices, we aim for estimates that are not too conservative nor too aggressive.

Since we installed our first system in 2006, we have paid close attention to estimated production versus actual production. We consistently meet our estimated production numbers and routinely outperform our estimates, even for our oldest systems.

Caldwell Travel Solar
Our installers at Caldwell Travel in 2007.

For example, in 2007 we installed a 12kW roof mount system for Caldwell Travel, now Direct Travel, Inc. in Brentwood, TN. Nearly 10 years later, the system has generated 104% of what we estimated! This includes a solar panel degradation rate of 1/2 of one percent (.005) per year, a fairly conservative assumption that accounts for a decrease in panel efficiency over time.

We also checked in on a system we installed about 5 years ago: a 37kW ground mount system for Wilson Farms in Hickman, KY. Similarly to the system at Direct Travel, Max Wilson’s system has generated 103% of our estimates, even with 3 months to go before the system’s 5 year anniversary.

Commercial Solar Systems in Tennessee
Solar system at Keg Springs Winery

Lastly, we checked in on a system that has operated for only one year. We installed a 31kW roof mount system for Keg Springs Winery in Hampshire, TN last summer. After one year, their system hit 95% of our estimated production. Each year we expect to be within 5% of our estimates, so the system is still right on track.  PV Watts averages airport weather data from the last 30 years to estimate solar irradiance for a particular place, so solar generation estimates are an average – some years will see less irradiance, some will see more.

Since our customers are essentially buying 25 years of electricity in advance, our mission is to install high performing systems that meet and exceed expectations, not just in the first couple of months or in the first year, but over the long haul. To read more about our customers’ experiences with their systems, check out our page on