Grid-tie Solar System Sizing

The following steps will help you determine the array size for your grid tie solar photovoltaic system.

1.  Find you monthly average electricity usage from your energy bill.
This is the total kWh you pay for in a single month. Due to seasonal usage like air conditioning, space heating, it is recommended to look at bills from several months of the year. Using all the data available, determine your monthly average electricity usage.

2. Find your daily average electricity usage.
Divide your monthly average kWh found in step 1 by 30 days.

3. Find the daily average peak sun hours for your location.
You can use the maps shown in this page:  For example New Orleans is 4.5 kWh/sq.m/Day which is equivalent to 4.5 sun peak hours.

I f you can’t find your location or you need more information regarding the data source, please visit:

4. Calculate the solar system size (AC) to generate 100% of your electricity consumption.
Divide you daily average energy usage (step 2) by the average sun peak hours in your location. For example, if your average energy usage is 34 kWh/Day and you live in New Orleans (4.5 Peak Sun Hours) your solar system size (AC) should be : 34kWh / 4.5 hr = 7.55 kW. Multiply by 1000 to get Watts.

NOTE: The solar system size calculated in step 4 is in Alternative Current (AC), which is the output of a solar system.  The solar modules constitute the input of your solar system, therefore we need to include the system inefficiencies in order to estimate the number of solar panels you need. A common grid-tie solar system will lose about 14-22% during energy conversion, these accounts for cabling, inverter, connections, etc.

5. Calculate the number of solar panels needed for this system.
Considering a well designed solar system with 86% efficiency (14% loss), divide the solar system size (AC) in step 4 by 0.86. It looks like: 7.55 kW / 0.86 = 8.78 kW.

Let’s say you want to use a solar module with a nominal name plate power of 220 Watt. In that case you will need:  8.78 kW x 1000 / 220 W = 39.90 panels. Always round this number up.

In this example, you will require 40 solar modules at 220 Watt each to cover 100% of your energy needs.

This will be the largest system that would be cost effective to install.  A smaller system can reduce part of your bill, or eliminate higher cost electricity in locations that enforce higher electricity rate in the peak hours of the day.

There are other factors that will limit the size of your solar photovoltaic system some of the most common are roof space, budget, local financial incentives and local regulations. When you look at your roof space it is important to take into consideration obstructions such as chimneys, plumbing vents, skylights and surrounding trees.  Besides the solar modules, a grid-connected PV system consists of output cables, module mounting structures, AC and DC disconnect switches, inverter(s), grounding equipment and metering system.