When taking on a new commercial project, it’s important to be able to do back-of-the-napkin calculations to inform key stakeholders of hypothetical system sizes, and thereby the potential Return on Investment (ROI) or Internal Rate of Return (IRR) of the project. Seeing these numbers on paper can be quite exciting, and it’s easy to sell a project when the sky’s the limit.

However, it’s also important to ensure these projections are realistic when considering other project-limiting factors.

# Maximum System Size: Limiting Factors

From a physical standpoint… It's often the site itself that will dictate how many solar panels can be installed. Whether working on a rooftop, ground mount, or carport system, the amount of available area often places an upper limit on the amount of solar that can be installed, even if all other constraints are removed.

Other considerations include the required clearances from nearby features (such as parapets or an HVAC unit on a rooftop, or a highway near a ground mount), the distance between rows to account for inter-row shading, the presence of wetlands or protected animal species, and the structural integrity of the roof or the characterization of the soil.

Several reports or studies may be required, including but not limited to geotechnical, structural/roof, topographical/land, and environmental/wildlife.

From an electrical standpoint, in several cases, the utility/Authority Having Jurisdiction (AHJ) will have set a maximum DC/AC system size or a maximum amount of backfeed current.

For example, they may set an arbitrary upper limit of 1MW DC, or they may restrict the maximum backfeed breaker size to 2000A.

Negotiating with the utility/AHJ is a lengthy process, so it’s important to engage them as soon as possible to ensure that your project complies with any such regulations.

For the purposes of this article, we’ll put these thoughts aside and focus on just the electrical limitations.

The key formula when calculating maximum inverter output power is:

*Power=Current×Voltage×Power factor×**√**3*

Where

*Power** *is the maximum inverter output power;

*Current** *is the maximum inverter output current when taking 125% OCPD/conductor oversizing into consideration;

*Voltage** *is the L-L voltage of the service (e.g. 480V on a 480/277V service or 208V on a 208/120V service);

*Power factor* is the ratio of real power to apparent power (typically we assume unity, or 1); and √3 (or ~1.73) is derived from conversion between L-L and L-N voltage in a 3ph system.

The above formula can be manipulated to yield maximum inverter output current by solving for *I*.

Example 1:

Given: A commercial service is at 480/277V. The site has an 800A main busbar with an 800A main breaker. Assume the power factor is 1 and the desired DC:AC ratio is 1.3.

Find: The maximum DC and AC system size assuming a load side connection.

Solution: The maximum backfeed breaker size is dictated by the 120% rule.

*Breaker size*=800A×120%-800A=160A

This breaker must also, in turn, be 125% greater than the maximum inverter output current.

Finally, using the first formula given previously,

*Inverter power,AC*=128A×480V×1.73

To convert this to DC power, multiply by the DC:AC ratio of 1.4.

*PV power,DC*=128A×480V×1×1.73×1.4

Answer:

*Inverter power,AC=106kWac*

*PV power,DC=137kWdc*

Example 2:

Given: A commercial service is at 208/120V. The site has a 400A main busbar. Assume the power factor is 1 and the desired DC:AC ratio is 1.4.

Find: The maximum DC and AC system size assuming a supply side connection.

Solution: Since we have a supply side connection, we are not affected by the 120% rule. However, the maximum inverter output current still must be 125% less than the rating of the main busbar to protect the conductors in between.

Using the first formula given previously,

*Inverter power,AC*=320*A*×208*V*×1.73

Again, to convert this to DC power, multiply by the DC:AC ratio of 1.4.

*PV power,DC*=128*A*×480*V*×1×1.73×1.4

Answer:

*Inverter power,AC=115kWac*

*PV power,DC=161kWdc*

Example 3:

Given: A 100kW inverter operates at 480/277V. Assume the power factor is 1.

Find: The maximum inverter output current.

Solution: Using the second formula given previously,

Answer:

*Current=*120*A*

This can be verified using a real-life inverter’s datasheet, such as the SE100KUS. The maximum continuous output current per phase at 277V is 120A.

Example 4:

Given: A 66kW inverter operates at 480/277V. Assume the power factor is 1.

Find: The maximum inverter output current.

Solution: Using the second formula given previously,

Answer:

*Current*=79.5A

This can be verified using a real-life inverter’s datasheet, such as the STP62-US-41 which has a maximum apparent power of 66,000VA. The maximum continuous output current per phase at 277V is 79.5A.

If you need help designing or quoting your commercial project, please email **info@renvu.com** to speak with a Sales Engineer!

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