How to choose the right solar inverter size

September 29, 2026 · 5 min read · Danilo Nascimento

Understand the DC/AC ratio, why the inverter can be smaller than the panels and how to reach the right standard size, with an interactive calculator.

The inverter does not have to match the panels

Panels produce direct current (DC) and the inverter converts it to alternating current (AC). Two power figures enter the calculation: the DC power, the sum of the panels in kWp, and the AC power, the limit the inverter delivers in kW.

DC power (panels)

Sum of the panels, in kWp

10 panels × 550 W = 5.5 kWp
Measured under test conditions (STC)
Instantaneous power varies with sun and temperature

AC power (inverter)

Inverter limit, in kW

A 5 kW inverter
Applies to the alternating-current output
It is the ceiling of what goes to the home and the grid

Two power figures, two sides of the inverter.

The DC/AC ratio

The DC/AC ratio (nominal module power ÷ nominal AC inverter power) compares the two figures. It is also called the inverter loading ratio; some sources define it the other way round (AC ÷ DC), so check the definition used. A widely used range is 1.1 to 1.3: the inverter is a little smaller than the panels.

5.5 kWppanel power÷1.2DC/AC ratio=4.6 kWinverter for that ratio

Inverters are sold in standard sizes: pick the smallest standard size that is equal to or above the calculated value. Here that is 5 kW, which gives an actual ratio of 5.5 ÷ 5 = 1.10.

Why a smaller inverter is normal

Nameplate power is measured under test conditions: 1,000 W/m² of sun and 25 °C at the cells. Day to day, heat, the angle of the sun and clouds usually keep the peak below that, although brief increases in irradiance can exceed it. Sizing the inverter for 100% of the panels often means buying capacity that is rarely used.

Typical peak power of panels (% of nameplate power, illustrative values)

Test conditions (STC)100%

Clear summer day, with heat≈ 85%

Clear winter day≈ 75%

Overcast day≈ 25%

Size the inverter

Choose the number and power of the panels and the ratio you want. The calculator suggests the smallest standard size at or above the calculated value and shows the actual ratio.

What happens in each range

	Below 1.0	1.0 to 1.35	Above 1.35
Power clipping	Very rare	Rare	Frequent
Cost per usable kW	Higher	Balanced	Lower
Inverter utilisation	Low	Good	High, with losses
When it makes sense	If you plan to add panels later	Most installations	Roofs with poor orientation or shade

✓ advantage · ◐ depends on the case · ✗ point of attention

Other inverter limits

Maximum voltage and MPPT range: the number of panels per string has to fit the limits (see solar panels in series and in parallel)
Maximum current per input: high-current panels may need sturdier inputs
Phases: single-phase, split-phase and three-phase inverters must match the home’s connection
Utility rules: the power that can be connected to the grid depends on the connection
Backup: with hybrid inverters, check the output power during a grid outage, which is often lower than the rated power

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Ten 550 W panels (5.5 kWp) connected to a 5 kW inverter: a DC/AC ratio of 1.10.

Try it: Change the number of panels to 12 and redo the calculation: which standard inverter fits with a ratio close to 1.2? Write the result in the diagram text.

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