Home Photovoltaics Xantrex Inverters SMA Inverters After 7 Weeks After 18 Months

After 7 Weeks


Here's an early look at the performance data. Its not enough to draw any conclusions, though.


May 25, 2002

The system has operated for about seven weeks. A quick look at the chart shows production to be slightly higher than consumption. There has been no use of space heating during the chart period.



We're still using resistance water heating, and it is interesting to see that the isolated water-heating energy is nearly as much the amount consumed by the rest of this all-electric household. Yet we been able to bank a small reserve for the winter months, 5.6 average days usage at this point.


What about space heating?
We've lived in this house since November, and it is now nearly June. Electric consumption during the space-heating period (November 20 through March 25) was 209 kWh per month greater than the average since then. This implies an annual space heating load of 836 kWh, significantly less than the forecasted 1,339 kWh.
Why is space heating energy so low?
Our mild climate is the significant factor. The average annual temperature is 61 degrees. We rarely see overnight lows below 30 degrees, and more often they will be 40 degrees or higher.
Perhaps equal to this is the way I built the house. I did everything practical and many things impractical to make it energy efficient. Detour here to see more.
What about air conditioning?
Some allowance was made for this load. But early observations of the house thermal performance suggest that we may never need air conditioning. Here's why:
Unlike most areas, our evenings are always cool. On the hottest days, a dependable cool ocean breeze begins before sundown and high radiation cooling is a nightly phenomenon. Closing the house through the heat of the day keeps interior temperatures comfortable. Opening it in the evening for cross ventilation counters the small daytime temperature rise. This simple procedure maintains interior temperatures within a few degrees of a multi-day average.
We tried this during a hot spell last October. It really works.
This chart of production and consumption is only the introduction to a long story, but it inspires optimism. On the down side, the equipment is new and performing at its peak. It will degrade in years to come. And the evaluation period so far has been during longer days of the year. There will be shorter ones.

On the up side, production has been limited by our seasonal overcast at this time of year. The best weather is yet to come, and winter days here, though short, are most often clear and cool.

Based on the extrapolations shown in the sidebars, our total household, space heating, and DHW load might be 11,770 kWh. This is comfortably below the extrapolated production, but is 3,788 kWh more than the forecasted production of 7,982 kWh. Now consider this:

bulletConsumption includes resistance-heated DHW not originally forecasted but . . .
bulletProduction forecast was very pessimistic, may realize at least 2,000 kWh more.
bulletMust pay SDG&E for 365 kWh annually, whether used or not.
bulletSystem easily expandable by additional 1,000 kWh, or by 2,000 kWh with a little more effort.

From the above, it looks like an expanded system might serve our entire load including DHW, especially if we don't need air conditioning or are willing to sweat a little on the hottest days.

Early extrapolations
Extrapolating the chart data as of May 25 predicts the following annual amounts:
System production:  12,298 kWh
DHW consumption:  5,015 kWh
Other consumption:  5,919 kWh
System production will almost certainly be lower than the extrapolated amount as aging and abbreviated winter day length take their tolls.
My original sizing concept did not expect resistance water-heating and allowed only 551 kWh to support solar thermal collectors with COP of 8.0.
However the extrapolated household load, including clothes drying but not space heating or cooling, is significantly less than the forecasted 6,993 kWh.