How to size a solar PV system?

Let’s first check to see if your roof is suitable for a solar PV system. Look it up on a recent satellite image. If you can see your roof, so can the sun. If you want to conduct a more sophisticated analysis you can get in touch with a solar professional that has a Solar Pathfinder. This neat gadget will tell you what objects, such as adjacent trees and structures, will shade your solar system and for how long across the seasons.

Next, we can answer how to size your solar system. To calculate the size required to cover most if not all of your electricity needs, you need to know the following three variables:

  1. Your actual (or predicted) electricity consumption
  2. Average sun hours for your location
  3. System inefficiencies, or derate factor

Electrical consumption

Gather your electric bills for the last twelve months. Look up the total kWh you have used each month and add them up to get the total for the year. If you have a multi unit building like we have (i.e. you have more than one electrical meter), do the same for each electrical meter to reflect the total annual kWh consumption for the whole building.

For example, the consumption between January to December 2019 for all three units in our building was 10,648 kWh. We rounded it up to 11,000 kWh to be on the safe side.

If your solar system is meant for a new building, you have to come up with a predicted annual electricity consumption. This will depend on your habits and behaviors, which you may be able to extrapolate from your old electric bills, and the electrical appliances and their efficiency in the new building.

Average sun hours

It would be nice if this information would also be included on your electrical bill. Until then, you can look up it up online on the National Renewable Energy Laboratory’s (NREL) website:

Geospatial Data Science – Solar Resource Data, Tools, and Maps

You’ll find two options to determine the sun hours for your location:

  1. U.S. Annual Solar Global Horizontal Irradiance (GHI)
  2. U.S. Annual Solar Direct Normal Irradiance (DNI)

For a more cautious result, use the Direct Normal Irradiance, as it only takes into account solar radiation from the direction of the sun. The Global Horizontal Irradiance is the sum of Direct Normal Irradiance and Diffuse Horizontal Irradiance (DHI), i.e. reflection of sunlight from objects.

On the web page, scroll down to the Direct Normal Irradiance section and click on the U.S. Annual Solar DNI link. Find your approximate location on the map and determine your sun hours based on the color coding and the map key.

The sun hours for our location were a little tricky to read as we are right between two zones. It could either be 4.0 to 4.4 or 4.5 to 4.9. We ended up picking 4.0 sun hours to be on the safe side.

Side note: Don’t get intimidated by the lingo on the website. If there is a term you don’t know, it is probably explained in NREL’s glossary section:

Solar Resource Glossary:

System inefficiencies

Each solar system will have some losses due to shading, wiring, dust on modules, converting direct current (DC) into alternating current (AC), etc.

The recommended factor to account for system inefficiencies varies from 1.38 in a June 2019 paper published by the University of Arizona, Cooperative Extension (Calculations for a Grid-Connected Solar Energy System), to 1.2 according to a NREL publication in August 2002. The item that drives the inefficiency factor the most is probably the amount of shading.

We went with the default value of 1.14 (or 14%) from the NREL PVWatts Calculator.

Calculating the DC system size

Let’s quickly recap on what we have determined so far:

  1. Annual electricity consumption: 11,000 kWh
  2. Sun hours: 4.0 for our location
  3. System inefficiencies: 14%

We can now plug these values into the following equation to determine the DC size of the array:

(Yearly kWh Usage / 365 days / average sun hours) x inefficiency factor = DC solar array size required (kW)

(11,000 kWh / 365 days / 4.0 average sun hours) x 1.14 inefficiency factor = 8.59 kW DC

To cover all of our electricity needs we probably need a system sized at 8.59 kW DC. To double check the result and to adjust and play with various variables, I recommend to take this number to the NREL PVWatts Calculator. Here you can begin to fine tune the system based on type of modules (panels), tilt, orientation, shading, etc.

The PVWatts Calculator also has a function in which you can draw an area on your roof that you would like to set aside for a solar system, and it calculates the probable system size in kW DC for you.

How many modules do we need for a 8.59 kW DC system size?

Current modules (panels) produce between 310 to 350 Watts. If you read this and this post is a couple of years old, double check these numbers against current module product information.

If we assume a module production of 330 Watt (0.33 kW), we simply divide this by the system size.

8.59 kW DC / 0.33 kW = 26 modules

Actual system size

The above should give you the tools to calculate the probable solar system size and number of modules you may need. Look at this process as a feasibility study, but not a final determination.

For the latter, I recommend working with a solar professional whose analysis will be more thorough and grounded.

In our case, the numbers were very close. The professional analysis recommended a system size of 8.58 kW and an array with 26 modules.

Other resources:

Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors

NSRDB: National Solar Radiation Database

Related posts:

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

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