Grid-Tied System Sizing

Module 11Fall 2009

Sizing a Grid-Tied PV System

• Design Beginning Points– Annual Electric Usage– Available Space For System Panels and

Electrical– Budget Constraints

Sizing Process:

• Sizing begins at the load side and proceeds backwards to the array

• Grid-Tied systems are usually sized as large as possible within limits of budget and space

• Sizing of Stand-Alone systems involves a fine balance between energy supply and demand. Not covered in this class.

Accepted Method for Certification in CoSEIA

• Electric Load Estimation – 1. Approximate monthly and daily average energy

use• Yearly Average energy consumption: 7000 kWh/yr• Avg. kWh/yr ÷12mo = 583 kWh/mo• Avg. kWh/mo ÷ 30 days = 19.5 kWh/day

• This is Average Daily Load

Accepted Method for Certification in CoSEIA

• Electric Load Estimation – 1. Approximate monthly and daily average energy

use• Yearly Average energy consumption: 7000 kWh/yr• Avg. kWh/yr ÷12mo = 583 kWh/mo• Avg. kWh/mo ÷ 30 days = 19.5 kWh/day

• This is Average Daily Load

Step 1a

• 50% of power to be generated by PV

• 19.5 (AC)kWh/day x 50% of power from PV

= 9.75 (AC) PV system kWh/day

2. Array Sizing

• Average Sun Hours per day…. Denver (5.5 – 6 hours)

– See Irradiation chart– Or data in back of book

Solar Radiation Chart

3. Determine PV system kW needed

9.75 (AC) PV system kWh/day ÷ 5.5 (ASH/day) = 1.77 (AC) PV System Kilowatts

4. Factor in inverter efficiency

• 1.77 (AC) PV System Kilowatts ÷ .80 derate factor = 2.21(DC) PV array System kW needed

• 2.2 kW PV array x 1000 Watts/kilowatt = 2200 PV array watts

From the Book…

• Establish design kWh/day

• Divide by average peak sun hours

• Factor in Inverter efficiency

• Choose Modules

• Inverter Sizing

Inverter Sizing

• Calculate total watts of array

• Choose inverters that will pass that quantity of watts

• Check string sizing software to evaluate overall compliance with design criteria

• Decide how many strings your system will utilize

End of Day 2 presentation