Solar Inverter Sizing: AC, DC and Battery Limits

Short version. Size from four inputs: the AC output the site needs, the DC power the model permits, the temperature corrected string voltage and current, and any backup load. A rule matching inverter kW to panel kWp skips the three checks that actually cause failures.

The four checks

These interact. A string layout that satisfies voltage may exceed per tracker current. An array within DC limits may need a different tracker arrangement because the roof faces two directions. The MPPT guide covers that distinction.

Why kWp does not pick the inverter

Ten 550 W modules give 10 times 550, which is 5,500 Wp, or 5.5 kWp of rated DC capacity.

That figure is measured at standard test conditions, which an Indian rooftop rarely sees. Real output is usually below rated, which is why installing somewhat more DC capacity than the inverter’s AC rating is a normal design choice rather than an error. The upper bound is the inverter’s maximum permitted DC input power and voltage, stated on the datasheet.

What the kWp figure cannot tell you is how those modules are wired, what voltage the strings reach on a cold morning, what current each string carries, or whether your connection permits that AC output.

String voltage is set by the coldest morning

This is the check that protects the equipment, and the one most often skipped.

Module open circuit voltage rises as temperature falls. The highest DC voltage an inverter will ever see occurs at first light on the coldest day of the year, when the array is cold and suddenly illuminated.

The calculation needs the module’s open circuit voltage, its temperature coefficient for voltage, and the lowest expected module temperature at the site. Multiply the per module corrected voltage by the number of modules in series and compare with the inverter limit, keeping margin.

The opposite bound matters too. On the hottest afternoon, string voltage falls. It must stay above the bottom of the MPPT operating window, or the tracker cannot hold the string at its maximum power point and output suffers.

So string length is bounded at both ends: short enough to stay under maximum DC voltage when cold, long enough to remain inside the MPPT window when hot.

Current limits constrain layout separately

Each tracker has a maximum input current. String current is driven by module short circuit current, and can exceed standard test figures under high irradiance conditions.

This limit is independent of power. A design can sit comfortably within the inverter’s DC power rating and still exceed the current limit on one tracker because too many strings were paralleled onto it. Check current per tracker, not just total array power.

A worked check against a real datasheet

The Qbits QB-4/5/6KTLD datasheet specifies two MPPTs, 20 A maximum input current per tracker, 550 V maximum DC voltage and an 80 to 550 V MPPT operating range. Its 50 V start up voltage is a separate figure describing when the inverter begins operating, not a design limit.

Against those numbers, the questions become concrete. What is cold Voc per module at your lowest expected temperature, and how many can go in series before approaching 550 V. What is hot Vmp per module, and does the string stay above 80 V. What is string current against the 20 A per tracker limit. And does the roof layout justify using both trackers independently.

Use the string sizing calculator as an initial screen, treating its stored model data and assumptions as something to verify against current project documents rather than as final design.

DC oversizing and clipping

Installing more DC capacity than the inverter’s AC rating is a deliberate design choice, not a mistake, and it is worth understanding before you push back on a quote that proposes it.

An array reaches its rated output only under conditions a rooftop rarely sees: full irradiance, clear sky, cool modules, clean glass. For most of the year it produces well below rated. Sizing the inverter to a peak that occurs for a few hours a year means paying for conversion capacity that sits idle the rest of the time.

Oversizing the DC side lifts output across the whole of the morning and evening, and across cloudy days, at the cost of clipping the top of a small number of peak hours. Clipping simply means the inverter holds at its maximum AC output while the array could briefly have delivered more.

Two boundaries apply. Total DC power must stay within the inverter’s maximum permitted DC input, and string voltage must stay within the limits already described. Within those, the right ratio depends on orientation, tilt, local irradiance and tariff structure, which is why a single recommended figure would be misleading.

AC output and the sanctioned connection

The array can be perfect and still fail here.

List the simultaneous loads or the planned grid connected output, and check the sanctioned connection on the electricity bill. Your permitted capacity is usually tied to sanctioned load rather than roof area, and a three phase inverter requires a three phase service. The single versus three phase guide covers the consequences of getting this wrong.

Battery sizing is a separate calculation

A hybrid inverter’s 5 kW rating is a power ceiling. It does not mean a 5 kWh battery runs for one hour, and it has no bearing on runtime at all.

Battery energy is calculated from the load, the required runtime, the usable fraction of nameplate capacity and the conversion losses, then checked against the current limits of both the battery and the inverter. The battery sizing worksheet works through a full example.

What to send an installer

Provide the electricity connection details including sanctioned load and phase, recent consumption data, the roof and module layout, the module datasheet, and any backup load list.

Ask in return for the exact inverter model designation, the string schedule with calculated cold and hot voltages, current per tracker, the protection design, and the utility approval path with responsibility identified.

A model selected by a nominal kW label alone is an incomplete answer. Contact Qbits with those documents for a model specific equipment enquiry.