Sizing a solar inverter: how much panel power can it take?

An inverter has to match your panels in three ways: power, voltage and current. Too small and you lose output on bright middays; too large and you pay for capacity that mostly runs at part load. And if the string voltage sits outside the inverter’s window, the system underperforms or will not start. This guide explains the numbers that matter, using the spread across our inverter database.

1. DC/AC ratio: panels can be bigger than the inverter

Panel power (kWp, DC) and inverter power (kW, AC) are different things. Panels rarely hit their full rating in real conditions, so inverters are often chosen smaller. Fraunhofer ISE notes that inverters are frequently undersized for better utilisation; across Germany the DC rating averages just under 10% above the AC rating, and for large plants the AC/DC ratio is typically around 85% ¹.

How far you can go is set by the manufacturer’s maximum PV array power:

Of the 72 models with a stated maximum, 68 (94%) accept at least 1.5 times their AC rating. That is a technical ceiling, not a recommendation. East–west arrays can go further than south-facing ones because the two faces never peak at the same time. Power above the AC limit is clipped.

2. The UK twist: 3.68 kW and G98

Most British homes have a single-phase supply. Engineering Recommendation G98 covers installations up to 16 A per phase: the installer connects first and notifies the DNO within 28 days of commissioning. Above 16 A per phase, G99 applies and you need the DNO’s permission before installation ². On a 230 V single-phase supply, 16 A is 3.68 kW.

That is why a 3.68 kW inverter is so common here, often paired with a larger array: the inverter’s AC rating keeps the installation within G98 while the extra panel power improves output in the mornings, evenings and winter. In our database 13 of 36 single-phase inverters are rated at 3.68 kW or less.

Going bigger on single phase means a G99 application. In some cases a DNO will accept a larger system with an export limitation scheme under ENA Engineering Recommendation G100 ³. Adding a battery also counts towards the limit — see grid connection: G98, G99 and the ENA register.

3. MPPTs: how many and how to use them

An MPPT continuously finds the operating point where a string gives the most power. 76 of 86 inverters in our database have two MPPTs, seven have three, and microinverters have one per panel.

• One unshaded roof face: one MPPT is enough, two give flexibility.
• East + west, or a shaded dormer: put each face on its own MPPT. Mixing orientations in one string pulls the whole string down.
• Many small faces or moving shade: microinverters or optimisers are often easier; see string vs micro vs hybrid.

22 of 71 models with this data accept two parallel strings per MPPT; the rest take one.

4. Voltage window: does your string fit?

Panels in series add up their voltage. The string voltage must sit within the inverter’s MPP voltage range:

The spread is wide. With a lower limit of 230 V and panels at around 34 V maximum-power voltage, you need at least 7 panels in series — and more margin in summer, because hot panels produce less voltage. Models with a lower limit of 40–90 V cope with short strings on small roof faces.

At the top end, what counts is the open-circuit voltage in cold weather. On a clear, frosty morning panel voltage is at its highest and must never exceed the inverter’s maximum input voltage. Your installer calculates this from the temperature coefficient on the panel datasheet; ask to see it.

5. Input current

Each MPPT input has a maximum current, from 12 to 32 A across our database. Large modern panels deliver more current than older ones. If the panel current exceeds what the inverter accepts, the inverter limits it and you lose power. Two parallel strings on one MPPT double the current.

Worked example

14 panels of 430 Wp, seven east and seven west (6.0 kWp), on a single-phase supply:

1. G98 or G99? A 3.68 kW inverter keeps the installation within G98 ²; the DC/AC ratio is then about 1.6, so check the inverter accepts 6.0 kWp.
2. MPPTs: east and west on separate trackers.
3. Voltage: seven panels in series must stay above the lower limit on a hot day and below the maximum input voltage on a cold morning.
4. Current: panel current below the per-MPPT maximum.

Compare candidates on these specs in our inverter comparison, and see inverter efficiency explained for what the efficiency figures mean across the load range.