Section 7: PV System Sizing

Question 1

What does PVWatts do?

Answer 1

NREL’s PVWatts® Calculator is a web application developed by the National Renewable Energy Laboratory (NREL) that estimates the electricity production of a grid-connected roof- or ground-mounted photovoltaic system based on a few simple inputs. To use the calculator, you provide information about the system’s location, basic design parameters, and system economics. PVWatts® calculates estimated values for the system’s annual and monthly electricity production, and for the monetary value of the electricity.

Question 2

What are some common loss factors?

Answer 2

Soiling, shading, snow, module mismatch, age (.5 to 1% per year)

Question 3

How do you size systems for grid-tied applications?

Answer 3

Assess the roof space to determine how much solar we can fit on the roof. We accomplish this through a site survey. Look at shading and other factors.
Electricity usage and rate schedule. 12 months utility bills would be helpful. Calculate monthly savings.
Determine customer’s budget, local incentive programs, federal tax incentives. Determine customer’s tax liability
Determine how much energy the system will make.
Will system fit on roof?
Is equipment appropriate for this location?
Make plans and pull appropriate permits

Question 4

How do you size a system?

Answer 4

If utility bill is 10,000 kWh per year…

1) Divide 10,000 kWh by kilowatt peak hour per year, as determined via PVWatts for that location

If you start with PSH…

1) Multiple PSH by derating factor for find kWh per day AC
2) Multiply kWh per day AC by 365 days to get kilowatt peak hour per year

Question 5

How do you do stand alone/off-grid sizing?

Answer 5

Site survey - is there enough room?
Calculate projected daily energy usage. No electric bill so go to each electric consuming device and determine total energy usage.
Look up insolation data. How much average sun does this location get? Use the critical design month, which is the period of lowest insolution.
Size battery bank
Size inverter. Inverter should be at least equal to, and more preferably greater, than the largest load.
Does the system require a generator? Should it be a hybrid system?