I am trying to determine the relative benefits of solar PV and solar hot water for an upcoming residential building in NE Georgia. We are located at 34 degrees 28 minutes latitude. The house has an 1870 sf footprint. There is a walkout basement, main floor and a loft on the north side of the building. The roof is a shed roof, 2:12 pitch, oriented to true south, with an area of ~2425 sf, subject to final overhang adjustments. It will be occupied by two retirees with low to moderate hot water usage. Current plans call for R30 walls and an R60 roof. These values are approximate until construction details are finalized.

We are currently planning on Mitsubishi ducted/mini-head units for heating and AC as well as an ERV.

We are unable to base our decisions on our current utility usage since we have lived in an RV for the past ten years – we have no historical information for a baseline. Rate information from the local EMC shows that we should plan on spending between 10 and 11 cents/kwh.

To complicate matters, the house will be unoccupied for significant portions of the year. We plan to leave by November 20 every year and to return approximately March 20. We will also spend at least another month each year traveling. During the time when we are traveling we will design the house to minimize parasitic usage from microwave, televisions, etc.

We will have very good passive heat during the winter months, with ~28% glass on the south-facing side. The house temperature while we are gone will be determined by the need to keep the humidity in the proper range. Overnight temperatures in the 50-degree range are acceptable as long as the humidity is controlled.

My question: since solar hot water is primarily for heat and domestic hot water, and since our need for these is minimal, am I not better off simply installing solar PV to offset the home’s electrical use? Even though solar hot water panels are more efficient than solar PV my initial impression is that it’s better to have a less efficient system that meets my needs, rather than a more efficient system which doesn’t.

Any input would be appreciated!

Roger

On a purely cost basis, this decision will come down to a very small difference in terms of money. Given your "absentee" lifestyle, the best choice is to put the solar hot water capital into additional PV as opposed to creating a more complex system to maintain.

I third that. Thermal on a house where you will be gone months out of the year (even winter months) is a non-starter. Put the $ into a PV system and call it a day.

You said 28% glass on the south. Do you mean 28% of your floor area in ft^2 is glass on the south side? or that 28% of the houses glass is on the south side? If it's the former, then you will have major temperature swings in the house during the winter if you do not include a LOT of thermal mass in the right places. If you meant the latter, then that doesn't really tell us much.

Solar OH asked: "You said 28% glass on the south. Do you mean 28% of your floor area in ft^2 is glass on the south side? or that 28% of the houses glass is on the south side? If it's the former, then you will have major temperature swings in the house during the winter if you do not include a LOT of thermal mass in the right places. If you meant the latter, then that doesn't really tell us much."

Even worse: I didn't say either one! What I was trying to say was that the south wall consists of 28% glass and 72% solid material.

What I should have said is that the south wall glass is 9.9% of the total square footage of all three levels.

Roger

Got it. 10% of your sqft in southern glass doesn't raise a flag for me, but I,m a bit north of you. North GE is between zones 3 and 4, where I'm between 4 and 5.

Be careful as even 10% southern glass can cause significant temperature swings if there is no mass to moderate it. But as you will not be in the home, you may not care as long as the temp swings do not effect the home in an adverse way.

Being in what I'd consider a cooling dominated climate, installing a PV system, and a heat pump water heater will be far more effective than PV and a separate solar thermal system. The water heater will help cool/dehumidify the air in the summer when you are there, and will not appreciably effect the home in the winter when you are gone (it will actually heat the home slightly since there will be no hot water usage in the winter).

Depending on just how low your hot water usage will be, a heat pump water heater may not be worth the investment either. As there can easily be a $700 up charge from standard electric resistance to a heat pump. And if you don't use a lot of water, the extra efficiency doesn't save you a lot of $ even over 15yrs.

Since your building the home, build it right first, before PV. Your basic specs are promising. Just make sure you include an air tightness spec to be verified via a blower door test. Anything below 5ACH50 (air changes per hour at 50 pascals) is considered "tight" by a lot of builders, but getting below 1.5-2ACH50 is where I personally think people should be trying to get to, and it's totally achievable today (passive house spec is 0.6ACH50) Once that's done, spend your remaining budget on PV.

FYI: I'm a PV/solar thermal system designer, not a home builder and I still think the envelope is the first thing to worry about, even considering the plummeting price of PV systems.

rlsmith,

You may need to consider multiple, small, local water heaters to take care of each floor if the runs are long or a http://www.chilipepperapp.com/Artcl...-S-70T.htm that turns on a pump only when hot water is needed at any of the locations.