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Solar veteran Roger Perry discusses the pros and cons of One Tank Vs Two for Solar Back-up

All solar water heaters in Maryland, D.C. and the mid-atlantic provide a family of four with around 75% of their annual hot water load. Most customers use their existing electric or gas sources for the remaining 25% backup (usually needed dead winter). When installing a solar system one question that will come up is “Should I use my existing tank and have it fed by the solar system?” or “Should I remove my existing tank and use the electric back-up that comes in the solar tank?”. A couple of situations make this an easy decision;

1. An existing electric water heater with no room for another tank. This is especially satisfying and cost effective if the existing tank is leaking or on it’s last legs. It’s like getting $1500 off the cost of a solar system because that money would have needed to be spent anyway. In this case you would definitely chose a single tank system.
2. The other is if you have a gas water heater. While not as cut and dry as the example above, using the electric element would mean using a higher cost fuel for back-up (not so much with propane). Much of this extra cost would be mitigated because the single tank back-up would not run as much because it would be affected by solar input without running a faucet. You would need to run an electric circuit (30 amp, double pole breaker). This may be difficult or very easy depending on the breaker box location and available space in it. Most jurisdictions will also require a master electrician and a permit for this to be done. In this case, I think, most people would shy away from a single tank system unless there was just no room for another tank.

Let’s look at the pros and cons of each system;

SINGLE TANK PROS

• Smaller footprint
• Less heat loss
• Electric element can sense the solar output and not come on from stand-by losses
• Can go “all solar” just by turning off the element (essentially flipping a switch).

SINGLE TANK CONS

• Less back-up in cloudy weather (can be compensated with a larger solar tank which will have a larger back-up capacity).
• Less solar storage when the element is on (can be compensated for with a larger solar tank which will have more solar storage).

TWO TANK PROS

• Usually greater back-up capacity

TWO TANK CONS

• Greater heat loss
• Larger footprint

The secondary backup tank can’t sense the primary solar tank temperature unless a faucet is turned on, sending the water through the two-tank system.  This causes the second tank to turn on from stand-by losses when the primary solar tank is already plenty hot.

Going all solar requires operating valves as well as turning off back-up.

Personally I’m a fan of single tank systems. For the most part their two main drawbacks can be compensated for by installing a larger tank. An upgrade from a 80 to a 120 gallon solar tank is only a few hundred dollars. For a 50% increase in solar storage it is a small price to pay.

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