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Sizing a grid tied PV system

01 Sep 08 | Solar Electric

photovoltiac module

I get several calls per week from potential customers wondering how much or how little of a PV system they will need to off set their electrical use.  Grid tied PV sizing is pretty straight forward.  A Basic due south facing system should be tilted at latitude (~42 degrees in the Hudson Valley).  This is the bench mark for system sizing.  Unfortunately, most systems are not tilted at latitude and/or facing due south.  The benchmark system looks something like this:

  1. Annual kWh ÷ 365 days = kWh per day
  2. Percentage of electricity to offset (decimal)
  3. kWh per day ÷ sun hours (about 5 hours in the Hudson Valley)
  4. Figure in losses (temperature loss 88%, system derate 84%, inverter 94%)

For example, my house uses about 8,000 kWh (obtained from utility bills) per year.  Therefore:

8000 kWh ÷ 365 days = 21.9 kWh per day.

I want to offset 100 percent, so 21.9 kWh × 1.0 = 21.9 kWh

I have an average of 5 sun hours per day, so 21.9 kWh ÷ 5 hours = 4.38 kW

Calculate system temperature loss, 4.38 kW ÷ 0.88 = 4.98 kW

Calculate system derate, 4.98 kW ÷ 0.84 = 5.93 kW

Calculate inverter loss, 5.93 kW ÷ 0.94 = 6.3 kW

Therefore, I would need a 6.3 kW system facing due south, tilted at 42 degrees to off set 100% of my electrical use.  That is the simple answer.  It gets more complicated (and larger) as the azimuth and elevation of a typical installation are not usually ideal.  A site visit and investigation with a solar path finder will usually nail down the specifics.

Installed, with all federal and state credits and rebates, that system would cost about $4.50 per watt, or about $28,350.00.

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01 Sep 08 | Solar Electric | Comments (0)