Solar inverters

Grid-tie, hybrid, off-grid or microinverters

Topology is the first decision and the hardest one to reverse, because it determines whether storage and backup are possible at all.

• Grid-tie (On-Grid) units feed the house and export the surplus. They have no battery port and, by design, shut down during a grid outage so that nothing is backfed onto the lines. The simplest and cheapest option where storage is not planned.
• Hybrid units add a DC battery port and an energy manager. Solar covers the load first, the surplus charges the battery, and only the remainder goes to the grid or comes from it. Most residential installations in Europe are now built this way.
• Off-Grid units run without any grid connection, drawing from a battery bank and controlling its charging. Used for cabins, remote pumping, telecom sites and island systems where no connection exists.
• Microinverters sit under individual modules and convert at panel level. Each panel has its own MPPT, which suits complex roofs, several orientations and balcony systems, at a higher cost per Wp.

Single-phase or three-phase

Match the inverter to the grid connection you actually have. A three-phase inverter cannot be fitted to a single-phase supply, and a single-phase inverter on a three-phase house loads only one phase, which some distribution operators cap at a few kW of feed-in.

• 1-phase suits smaller systems, typically up to around 5 kW, on houses with a single-phase supply or where local rules allow single-phase feed-in at that level.
• 3-phase spreads generation evenly across L1, L2 and L3, keeps voltage rise lower and is the norm above roughly 5 kW, in commercial buildings and anywhere heat pumps or three-phase machines are present.
• Phase balancing matters for net metering. With a three-phase unit and a compatible smart meter, consumption and production are measured across all phases together.

Rated power against array kWp: the DC/AC ratio

The array is rated in kWp under standard test conditions that a European roof rarely reaches. Deliberately fitting a slightly smaller inverter than the array is called oversizing on the DC side, and it raises the number of hours the unit runs near its efficiency peak.

• DC/AC ratio 1.0 to 1.2 is the usual residential window. A 10 kWp array with a 8 to 10 kW inverter gives a good compromise between clipping and cost.
• Up to 1.3 is reasonable on east-west roofs, shallow tilts or northern latitudes, where peak output is reached for very few hours a year.
• Undersizing the inverter too far clips the midday peak on clear days, which is a permanent loss of energy. Oversizing it wastes money and leaves the unit running at low partial load, where efficiency is worst.
• Continuous load should sit around 80 % of rated power. Running permanently between 80 and 100 % shortens service life, particularly in warm, poorly ventilated plant rooms.

MPPT inputs, string design and shading

An MPPT (Maximum Power Point Tracker) continuously finds the voltage at which a string delivers most power. Every string on the same tracker is forced to the same operating point, so panels with different orientation, tilt or shading drag each other down.

• One MPPT per roof plane is the working rule. Two orientations on one tracker means the tracker compromises between them all day.
• Voltage window is decisive: check the string open-circuit voltage at the lowest expected temperature against the maximum DC input voltage, and the minimum start voltage against a hot summer afternoon.
• Current per tracker limits how many strings can be paralleled onto one input, which matters with high-current modern modules.
• Partial shading from a chimney, a mast or a neighbouring tree is better solved with optimisers or microinverters than by adding trackers.

Battery readiness, retrofit and backup output

Battery readiness is not a marketing label, it is a specific DC voltage range and a communication protocol. A high-voltage hybrid works with a high-voltage stack, a low-voltage unit with a 48 V LiFePO4 bank, and the two are not interchangeable. Approved pairings are listed by the inverter manufacturer, and using an unlisted pack usually voids support.

• DC coupling through a hybrid inverter converts once, from panel to battery, so round-trip losses are lower.
• AC coupling adds a separate battery inverter alongside an existing grid-tie unit. It is the practical retrofit route when the original inverter stays in place.
• Backup output (EPS) is a separate terminal that keeps a selected sub-board alive during an outage. It has its own power limit, usually below the inverter's grid rating, and needs a changeover arrangement in the distribution board.
• Black start capability decides whether the system can restart from the battery and the panels alone with the grid absent. Not every hybrid offers it.

Quick inverter comparison

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