Considerations

Historical data from previous residence

Derivation of average annual energy consumption at new residence

Note: Non-infinite COP for solar water heating due to electrical pumping energy and occasional backup by resistance heating.

Determining the PV system capacity

The PV array will be supported on a semi-fixed structure with adjustable elevation angle. Published long term photovoltaic capacity factors for fixed-mount arrays in the nearest city (San Diego) average about 0.21. This site is in the coastal plain about 60 miles north of San Diego and is subject to a micro climate with more late morning overcast and late afternoon fog. Furthermore, the acknowledged long-term accuracy of published capacity factors is (+,-) 12%. For this exercise a worst-case capacity factor of 0.17 is estimated. Based on this, the 8,703 kWh average annual demand will be properly served by a PV system capacity of 5.84 kW.

The PV panels selected are Kyocera model KC-120-1 with PTC rating of 0.1057 kW each. These panels can be expected to stabilize at about 0.0846 kW (80% of their original capacity) after five years of operation.

The inverter selected is Trace model ST-2500 with peak efficiency rating of 0.94%, however this efficiency is not always realized in practice. Performance curves of this inverter used in monitored systems seem to reveal that 88% is a more realistic value.

Panels must be utilized in series-wired strings of 4. The lowest multiple of 4 that satisfies the design goal is 80 panels.

A power level of 5.96 kW should satisfy an average annual demand of 8,876 kWh, which is adequate for the expected load.

Reconciliation with budget limitations

The installed cost of the 5.96 kW system exceeds the available budget, which leads to the following process of rationalization:

Heat pump

Abandon the ground-source heat pump in favor of less predictable but much less expensive air-source heat pump. The approximately $11,000 avoided cost can be transferred to the PV system budget to support a 72-panel 5.36 kW system. This is a reasonable compromise for our mild climate sunbelt region. In a cold northern region, however, money might be better spent on the energy-conserving ground-source heat pump instead of photovoltaics.

Worst-case parameters

The pessimistic values used for capacity factor, inverter efficiency, and PV output degradation may not in their aggregate represent reality. And some advantage may be gained from the adjustable elevation feature of the array.

Contingency plan

Producing surplus energy brings no benefit to the system purchaser. In fact, since the power utility (SDG&E) charges a minimum billing for 365 kWh annually, an optimum system should incur an annual deficit of this amount. The best approach is to err on the short side and design the system for future expandability. Capacity can be increased if and when it is determined to be necessary and funds are available.

Conclusion

It is therefore decided that the 5.36 kW system shall be expandable, consisting initially of 72 KC-120-1 PV panels and four ST2500 inverters. This initial system should satisfy an annual demand of 7,982 kWh.

Each inverter will be configured to serve five strings of four PV panels each. Three of the inverters will be fully implemented, and the fourth will serve three strings with provision to accept two additional strings if the system is expanded.