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Solar Basics: Hot Water & PV
While there are many different ways that the sun can be harnessed to produce energy, two that are
used most often for residential homes are solar photovoltaics (PV) and solar hot water (SHW). Both
technologies can be excellent investments for the property owner to lower the costs of their energy
bills.
What is Solar PV?
Solar PV technologies convert the sun’s energy into electrical energy. PV uses materials that “give up”
electrons when struck by the sun’s rays. These electrons leave the solar module to do work somewhere
else (light a bulb, power your air conditioner, etc.). This simple technology has no moving parts. PV
systems have a lifespan of 25+ years when installed properly.
What is Solar Hot Water?
Solar hot water (SHW) systems trap the heat from the sun (solar radiation) and transfer the heat to
water for use as thermal energy. SHW systems are composed of three main elements: solar collector(s),
a hot water storage tank and a circulation system to transfer the heat from the collectors to the tank.
While there are many design variations, essentially the solar collector gathers the heat from the sun by
heating up a fluid (either water or a non-toxic heat transfer fluid) running through them. This heated
fluid flows out of the collector to a hot water tank, and is used as necessary.
OR A LESS-TECHY ALTERNATIVE
Solar hot water (SHW) systems trap the heat from the sun (solar radiation) and transfer the heat to
water for use as thermal energy. This technology is very reliable and efficient at gathering the sun’s rays
to heat water. SHW systems save energy so that you don’t have to use as much electricity or gas to heat
your water.
What does a typical installation look like?
The most common solar PV systems (“grid-tied”) consist of modules on your roof (or near your house on
the ground) that are wired together. This wiring then goes to one or more “inverters” which change the
electricity into a form that your house and the rest of the electric grid can use. From the inverter(s) the
electricity goes to your main electrical panel. Whatever energy you don’t use in your home when it’s
produced goes out to the electric grid where other homes nearby can use it.
A typical residential solar hot water system consists of collectors that are also either on your roof or
near your house on the ground. These collectors are connected by piping through which a fluid flows
that gets heated up by the sun. The fluid then goes back to a “heat exchanger” which transfers the heat
of the fluid to the water you use in your home.
IS SOLAR THERMAL RIGHT FOR YOUR HOME?
Did you know that water heating is the second largest consumer of energy in your home, behind only
space heating? In fact, it accounts for as much energy consumption as all lights, appliances, computers
and other gadgets and gizmos put together. If you are interested in reducing your energy footprint and
greenhouse gas emissions, addressing this is one of the most impactful ways to do so.
Solar thermal technology, of which solar water heating (SHW) is a sub-category, can mitigate up to 75%
of these costs and emissions by heating up your water with free solar energy instead of gas or
electricity. As with any improvement to your home, there are a number of important factors when
considering a SHW system:
 Costs: upfront costs for a SHW system can vary significantly based upon the system type,
collector area and configuration of your home, among other things. Some areas have attractive
incentives that, when taken together with the federal tax credit, can cover over 75% of the
installed cost.
 Savings: annual potential savings are primarily a function of the fuel being substituted for;
electricity, heating oil and propane are expensive ways to heat water, so the savings can be
substantial. They are less so with natural gas given its current lower than historical average cost.
Note that I didn’t mention “payback”, which is in the eye of the beholder (what is the payback
on your existing water heater or granite countertop?). People buy things for different reasons.
 Collector Siting: while SHW collectors don’t necessarily have to be situated on a roof, this is the
typical application. A roof should be in good condition (i.e. not requiring replacement within the
next 10 years), with roughly a 100ft2 area of southerly, mostly year-round unshaded exposure.
 Hot Water Demand: a typical person uses 20 gallons of hot water a day, so your potential for
savings with SHW goes up with more users per household. This also impacts the sizing of the
system for your home.
 Tank Replacement: regular water heaters have an average lifespan of 13 years; most
homeowners only find out how old their tank is when it unexpectedly (and unpleasantly) fails on
them! If your water heater is over 10 years old and in need of replacement, going solar is a way
to get a new tank and lower your bills at the same time. Put another way, when calculating the
cost of SHW consider only the additional cost of solar above and beyond the $1500 or so you’d
be spending anyway on a new tank. Solar water heaters are usually a bit bigger than standard
tanks, and thus require slightly more utility room space; the good news is that you likely get
more hot water storage capacity than you had before! If you decide on a two-tank
configuration, you will need room for an extra tank.