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Stone Marmot Goes Solar: Part 2, Define System

By Sid of Stone Marmot

Nov. 27, 2009

Now that we have decided now is the time to buy, what kind of system should we buy?

Our electric use is incredibly low compared to the average American household. We typically use about 1100 kW-hr a year, or about 90 kW-hr a month, or 3 kW-hr a day. Using the online program PVWatts, we determined that a 1000 W of photovoltaic (PV) panels should meet all our electrical needs, even after rather severe derating.

Note that PV panels are rated when operating at 25 degrees C with no temperature rise above ambient. But they typically get rather hot when exposed to direct sunlight, as much as 35 degrees C or more above ambient. The power out of a PV panel also drops, typically about 0.48 %/degree C, as they heat up. So PV panels often put out a lot less power than expected, like 25 to 30 % of their rated power, during hot summer days, particularly here in Florida. When considered with other losses, such as voltage drops in the interconnects and wires, panel tolerances, inverter losses, etc, a system may put out only 50 to 60 % of its rated output during the hottest summer days.

The State of Florida will only rebate systems that are at least 2000 W. Since we calculate that a 2000 W system with the Florida state rebate is cheaper than a 1000 W system without the rebate, we want a minimum of 2000 W of PV panels. Most other people we know, who use considerably more energy, would probably need a 5000 W system to make a noticeable dent in their electric bills.

Our house also has a lot of shade to the east and west but little to the south. So we want the panels as close to the south end of the property as possible to avoid as much shading as possible. The house has its longest dimension running almost perfectly north to south. Since the roof is a hip roof, there is an angled section of the roof facing almost perfectly south, which is good. The problem is that it is also the shortest side of the house. Consequently, there isn't much room on the south roof for PV panels. The fact that this roof section is also triangular makes matters worse. So it might be a challenge to even fit 2000 W of PV panels on this south-facing roof. So 2000 W is also the practical maximum we can use.

We also want a grid-tied system, since it is required for the Florida rebate. It also saves having to buy an additional inverter for the larger 240 V loads. It will also be necessary to qualify for any potential future feed-in tariffs or to sell carbon credits earned by the system.

Florida is noted for its hurricanes. The first thing damaged in most all major storms is the electric power grid. It often takes a week or more to get it back up. We are also running these days with a lot less reserve capacity, in both generation and particularly in transmission, than we were used to during the last century, and it is getting increasingly difficult to get approval for new electric power generating plants and particularly right-of-ways for power transmission lines. Consequently, we expect the grid to go down more frequently and for longer periods of time in the future. So having battery backup for critical loads is also a desirable feature.

Three to five days of storage capacity in the batteries should tide us until we get enough sunlight to recharge the batteries. This would indicate that we need about 10 to 15 kW-hr of storage. We would want these batteries to be AGM (absorbent glass mat) deep cycle lead acid batteries, preferably a type specifically designed for solar battery backup situations. These AGM batteries are sealed and don't require any watering. Maintenance is minimal with these batteries, so they will probably outlast the typical flooded lead acid batteries which need to be constantly monitored and maintained for maximum life. AGM batteries cost about twice as much as equivalent flooded lead acid batteries, but will probably last about twice as long due to less critical maintenance requirements.

Battery back up doesn't get any rebates from the Florida government. It does still get a 30 % tax credit from the Feds. So battery back up is kind of expensive insurance. But we feel it is worth it and far more likely to be used than the other forms of insurance we buy, other than health insurance.

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