Inverter Sizing Calculator

What this inverter size calculator actually does

This page answers two practical early-design questions:

1. How large should the inverter AC nameplate be for the PV array?
2. How many modules can reasonably go in one string before cold-weather Voc reaches the inverter DC input class?

It does not pretend to be a full interconnection study, off-grid load inverter selector, or permit package. It is a fast planning screen for PV array to inverter matching.

For the wider solar path, pair this page with the solar system sizing calculator, the inverter sizing chart, and the solar planning hub so PV sizing, strings, and worksheet references stay aligned.

The four topology workflows on this page

The inputs that drive the result

• Module STC power and module count set the array DC power in kWdc.
• Module Vmp, Voc, Imp, and Isc come straight from the module datasheet.
• Voc temperature coefficient and coldest design ambient drive the cold-weather open-circuit voltage screen for string-based topologies.
• Inverter max DC input class sets the ceiling for the string-voltage screen. This page supports 600 Vdc, 1000 Vdc, and 1500 Vdc classes.
• MPPT minimum and maximum voltage let you check whether the screened string Vmp lands inside the inverter tracking window instead of only below the hard DC ceiling.

How the inverter AC size is screened

Array DC power (kWdc) = module power x module count / 1000

Recommended inverter AC size (kWac) = array DC power / target DC/AC ratio

The page also returns a practical low-to-high AC size range so you can see when the array starts looking heavily loaded versus lightly loaded. That range is a planning range, not a manufacturer approval.

How the cold-weather string screen works

The module open-circuit voltage is adjusted upward for the selected cold design ambient:

Cold-weather module Voc = module Voc x [1 + |temp coefficient| x (25 C - cold ambient)]

Then the page screens the maximum modules per string against the selected inverter DC input class:

Max modules per string = floor(max DC input voltage / cold-weather module Voc)

The suggested modules-per-string value is then pulled toward a nominal operating-string target for the chosen DC class so the result is useful as a real planning screen rather than only a hard voltage ceiling.

How the MPPT window check works

After the page suggests modules per string, it compares the resulting string Vmp at STC with the entered inverter MPPT minimum and maximum voltage. This matters because a string can stay below the cold-weather maximum DC voltage and still sit outside the inverter tracking window during normal operation.

If the result returns Below MPPT minimum or Above MPPT maximum, treat the screen as a planning warning and confirm the final string count with the manufacturer tool and datasheet.

What the current outputs mean

The page reports both array operating current at module Imp and a parallel output current x 125% planning value based on module Isc. That 125% figure is useful context for PV output-circuit review, but it is not the same thing as a finished conductor or overcurrent device design by itself.

What this page does not claim

• It does not replace the inverter manufacturer string-sizing tool.
• It does not certify compatibility with every module pairing list.
• It does not size batteries, off-grid load inverter surge, conductors, disconnects, or utility export controls.
• It does not replace the final review of NEC 690.7, NEC 690.8, rapid shutdown, or local utility requirements.

Worked rooftop example

Suppose the array uses 20 modules at 400 W. That is 8.0 kWdc. In the string-inverter workflow on this page, the first screened inverter size is about 6.67 kWac because the target DC/AC ratio is about 1.20.

If each module has 49.5 V Voc and a -0.29%/C Voc coefficient, the cold-weather module Voc at -10 C screens at about 54.52 V. On the 600 Vdc inverter class, that leaves a hard ceiling of 11 modules per string. The page then suggests 10 modules per string, which gives a practical 2-string layout and a cold-weather string Voc of about 545 V.

That is the level of answer this page is built to give: a transparent first screen that you can still audit against the actual inverter and module documentation.