Home Sizing the System The Structure The PV panels Field Wiring Tilt Angle

Sizing the System




bulletThe PV plant is to provide 100% of the energy needed except . . .
bulletWater heating will be provided by solar thermal collection.
bulletResidence is all electric, no propane or natural gas.
bulletResidence is new construction - no historical records available.
bulletExpected energy demand derived from records at previous residence.
bulletPrevious residence used propane - conversion required.
bulletElectric loads will remain the same.
bulletLifestyle and conservation habits have stabilized, however . . .
bulletOccupants are in their senior years with escalating chill sensitivity.
bulletThe new house has 10% more floor area and 22% more volume.
bulletThe new house is better sealed and insulated.
bulletIncreased use of spot heating and space heating is expected.
bulletSpace heating and cooling by ground-source heat pump.
bulletMild summer climate, cool evenings, air conditioning rarely needed.
bulletSystem designed for expansion should needs increase.

Historical data from previous residence

bulletHistorical energy usage from 10/14/98 through 2/25/01 is considered.
bulletHistorical annual electric energy consumption: 4,864 kWh.
bulletAverage annual propane consumption:  498 gallons (13,351 kWh).
bulletAllocation of 13,351 kWh propane energy equivalent:
Space heating (27%): 3,605 kWh
Clothes drying (17%): 2,270 kWh
Water heating (56%): 7,477 kWh.
bulletAdjust allocations to effective amounts realized after burner losses:
Space heating (AFUE = 80): 2,884 kWh
Clothes drying (AFUE = 75): 1,703 kWh
Water heating (AFUE = 59): 4,406 kWh

Derivation of average annual energy consumption at new residence

bulletSpace heating with COP = 3.0: (2,884/3) = 961 kWh
Expected increase space heating of 30% = 288 kWh
Additional load for resistance spot heating = 90 kWh
Clothes drying, COP = 0.8:  (1,703/0.8) = 2,129 kWh
Water heating with COP = 8.0:  (4,406/8) = 551 kWh
Carry forward historical electrical usage:  4,684 kWh
Note: Non-infinite COP for solar water heating due to electrical pumping energy and occasional backup by resistance heating.
Forecasted annual electric consumption:  8,703 kWh

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.

Capacity = (8,703kWh/yr)/(0.17 x 8760hrs/yr) = 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.

80 x .0846kw x 88% = 5.96 kW

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

(5.96kW x 0.17 x 8,760hr/yr) = 8,876 kWh/yr

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.


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.

(72 x .0846kW x 88%)(0.17 x 8,760hr/yr) = 7,982 kWh/yr

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.