Solar energy, facts and figuring


To the Editor:

I couldn’t agree more with Tim Twombly’s assertion [Letter to the Editor, “Solar solution to the mystery, January 13] that solar thermal is a very effective strategy to protect oneself from rising fuel costs and reduce the amount of fuel or energy that needs to be purchased.

As installers of both photovoltaic (PV) and solar thermal systems, we’ve learned that a building’s occupancy greatly influences the decision to use one or the other, or both. Seasonal buildings don’t have a use for domestic hot water in the winter and therefore often realize a greater benefit by installing PV electric generation and taking advantage of the Massachusetts’ net metering laws to credit the off-season production to the summer bills.

I believe Tim’s statement “64 square feet of ST is equal to 400 square feet of PV” came from the Solar Ratings Certification Corporation Solar Facts document (

This document is dated October 17, 2001, when most photovoltaic panels were in the range of 10-12 percent efficient. Most of the PV panels South Mountain Company installs today are the highest efficiency on the market at 18-19.5 percent. So, taking currently available technology into account, 64 square feet of solar thermal collector is roughly equal to 200 square feet of PV. If there is a year-round need for hot water, solar thermal is still more cost effective to install than PV.

Another important difference today versus nine years ago — PV installation costs have dropped dramatically. Nine years ago, a typical residential PV system would need to produce electricity for 15-20 years before the savings would equal installation cost. PV systems have a 25-30 years life expectancy, so the incentive wasn’t there for the majority of the population. Today, Massachusetts net metering laws, a utility sponsored state rebate program, 30 percent federal and $1,000 state tax credits, and a new production-based incentive, Solar Renewable Energy Certificates (SREC) bring that term as low as five years.

For example, a $21,000 total cost to install a 3kw high-performance system that produces 3,600 kWh per year would be reduced to $11,800 with available incentives. If the cost of electricity stays the same as it is today, and SREC revenue is the minimum of $0.285 per kWh, six years of savings and revenue amount to $11,800. If electric rates increase just 3% per year, and SREC sales continue at the $0.50 per kWh they realize now, the savings and revenue total $11,800 early in the fifth year of operation. The SREC incentive became available in January 2010 and has proven to be a boon to the PV market in Massachusetts.

For comparison, 3,600 kWh is equivalent to 12.3 million Btu and could be produced on Martha’s Vineyard with 64 square feet of solar thermal collector, a two-collector system. The economic benefit of the solar thermal system depends on the type of fuel used to heat water — electricity is most costly, oil is the cheapest (for now). The solar thermal system costs less to install than a 3 kW PV system. However, when rebates and production incentives are factored in, the PV system is a stronger economic performer.

Furthermore, only PV-generated energy has the benefit of being exportable. If a grid-tied PV system produces more than is needed on-site, the surplus feeds into the grid and turns the meter backwards. Because power plants are approximately 30 percent efficient at making power and sending it to us over the transmission lines, every kWh of electricity made on-site with PV is equivalent to 3 kWh of energy used at a power plant.

So, per kWh produced, the carbon reduction of site-generated electricity exceeds the carbon reduction when a solar hot water system displaces propane or oil used on site.

The Evacuated Tube (EVT) collectors Tim installs are an amazing technology. The strength of EVT collectors is their ability to collect solar energy in extremely cold conditions. However, field data shows that in climates such as Martha’s Vineyard, flat-plate collectors and EVT collectors will produce similar amounts of energy per gross square foot of collector annually, and the SRCC collector ratings bear this out. We’ve found that in terms of energy delivered per dollar invested the flat-plate collectors are a clear winner. However, we analyze each project individually and certainly look at EVTs in that process.

It’s clear that there has never been a better time to install solar in Massachusetts: The financial incentives are strong, our fuel and electric costs are high, and the cost of installation is as low as it’s ever been.

Deciding which technology to install, however, depends on the building and its use, and the type of energy being replaced. Rob Meyers Energy Services Manager South Mountain Company Inc.