Solar Inverter Reactive Power Compensation: Q(U) Modes

Last updated: November 2026

Grid operators now expect solar inverters to help hold voltage, not only feed in power. An inverter does that with one of three reactive power modes. A fixed cos(phi) holds one power factor at all times. cos(phi)(P) changes the power factor as active power output changes. Q(U), also called Volt-VAR, changes reactive power as the grid voltage at the inverter changes. Reactive power is power that moves back and forth in the grid. It does no work, but it helps hold voltage steady.

The grid operator decides which mode applies and which settings to use. This guide explains the three modes. It compares what five grid codes and operators publish. It shows how SMA documents the setup. It also works through the sizing arithmetic. Treat every value below as a published example. Use the settings letter from your grid operator for the site.

Country Grid Codes Compared

Each row comes from a document read for this update, named in the last column. These are the published requirements and example values from those documents. Other operators in the same country can differ. Standards are also revised. So confirm the edition and values with the operator.

Notes on the table:

• Germany. Older material, including SMA’s flyer on the 2011 rule, uses thresholds of 3.68 kVA and 13.8 kVA. The Thüringer Energienetze document applies the 2018 edition with a 4.6 kVA threshold. The operator chooses the method.
• United States. The IEEE 1547-2018 default is constant power factor at unity unless the operator specifies otherwise, per IEEE PES TR67. Unity means a power factor of 1. The JCP&L page shows what one utility specifies. Another utility may use other points.
• Italy. The cos(phi)(P) curve and lock-in voltage of 1.05 Vn are from the standard’s Figure 92 and test annex. Vn is the normal (nominal) voltage. The Q(V) function applies to inverters in plants above 11.08 kW. The standard says it is activated on the operator’s request, with a default activation delay of 3 s.
• Australia. The values above are the volt-var points in Jemena’s document, which matches AS/NZS 4777.2:2020. See the AS 4777.2 grid connection guide for the wider requirements.

For the export-limit side of the same jurisdictions, see grid export limitation rules by country. For Europe-wide rules, see Europe solar compliance.

P, Q, S Triangle: Real, Reactive and Apparent Power

Active power (P, in kW) does work. Reactive power (Q, in kVAr) shifts the phase between voltage and current. Apparent power (S, in kVA) combines the two: S squared equals P squared plus Q squared. The inverter is rated in kVA, so P and Q share one budget. SMA’s technical information describes the same triangle. It calls cos(phi) the displacement power factor. It is the cosine of the angle between active and apparent power.

For a single-phase load, P = U × I × cos(phi). For three phases, P = √3 × U × I × cos(phi), using line voltage and line current. Here are two checks on a 10 kVA inverter. At cos(phi) = 0.90 it delivers at most 9 kW and 4.36 kVAr (10 × 0.9 and 10 × sin(arccos 0.9) = 10 × 0.4359). At cos(phi) = 0.95 it delivers at most 9.5 kW and 3.12 kVAr. See the glossary entry on active and reactive power.

Sign rules differ between documents. CEI 0-21 defines positive reactive power as the generator supplying reactive power with current lagging the voltage. German documents use over-excited and under-excited. Before you enter a value, read the sign rule in the grid code and in the inverter manual.

How Solar Inverters Provide Reactive Power

A grid-tied inverter shapes its output current relative to the grid voltage. So it can shift the current ahead of or behind the voltage. Reactive power needs no extra hardware, only firmware and spare kVA capacity. The DC side does not supply it. Your panels do not work harder when you set cos(phi) below 1. But for the same active power, the inverter carries more current. Check the inverter’s datasheet for its reactive power capability and any derating, which means a cut in output.

What Is Q at Night (Reactive Power at Night)?

Some inverters can also provide reactive power with no sunlight. SMA’s “Q on Demand 24/7” lets a Sunny Tripower stay connected overnight. It draws a small amount of active power for its own electronics. It can provide up to 100% of its rating as reactive power. SMA lists which device types and firmware versions support it. Whether you need the function depends on the grid operator. A specification may ask for night-time reactive power. If so, confirm that the exact inverter model and firmware support it before you order. SMA’s table shows the availability varies by model.

What Is Fixed cos(phi) Mode?

The inverter holds one power factor value at every output level. If P is 5 kW and cos(phi) is 0.95, then Q is 5 × tan(arccos 0.95) = 5 × 0.3287 = 1.64 kVAr. The mode is simple to commission, and the operator checks the value once. Its limit is that it reacts to active power only, not to voltage. Use it where the operator specifies it. One example is the Thüringer Energienetze default of 0.95 under-excited for plants up to 4.6 kVA. See the grid code glossary entry for regional differences.

What Is cos(phi)(P) Mode?

cos(phi)(P) is a curve where the power factor depends on active power output. The Italian standard’s curve holds cos(phi) at 1 up to half of nominal power. Then it lowers it in a straight line to 0.95 at full power. It activates only when the voltage at the inverter terminals goes above a lock-in value of about 1.05 Vn. The aim is to avoid moving reactive power around when the grid does not need it. German operators use a curve of this type as the default for low-voltage connections above 4.6 kVA, per the Thüringer Energienetze document. Load the curve points exactly as the operator specifies them.

What Is Q(U) Volt-VAR Mode?

Q(U) is a voltage-to-reactive-power curve. The inverter measures grid voltage at its terminals and takes reactive power from the curve. In the JCP&L example above, reactive power is 0 from 0.95 pu to 1.05 pu. Above that, the inverter absorbs more as voltage rises to 1.09 pu, where it absorbs 44% of its VA rating. Pu means per unit, a voltage shown as a share of the normal voltage. In the Australia A set, the dead band runs from 220 V to 240 V. The inverter injects below it and absorbs above it. A dead band is a range where the inverter does nothing. It stops inverters from reacting back and forth against each other. Use the operator’s points unchanged. See the volt-var glossary entry and the active power guide for related concepts.

Response time is part of the setting. The JCP&L default sets a 5 second open-loop response time. CEI 0-21 gives an activation delay you can adjust from 0 s to 30 s, with a default of 3 s. Check the response time your operator requires.

Setting Reactive Power on SMA Inverters

SMA publishes step-by-step instructions for its Sunny Tripower range. The table summarizes two of them. Take the exact menu names from the manual for your firmware version.

SMA states that the grid operator must approve changes to grid-relevant parameters. They also need the SMA Grid Guard code. SMA also states that with Q on Demand 24/7 active, no other cos phi function can run between day and night operation. The documents are IPC-QoD24-7-STP-TI-en-17 and GMS-CORE2-EDMM-TI-en-11. For plants with many inverters, SMA’s Integrated Plant Control lets each inverter reproduce a Q(V) curve at the connection point. No separate measurement is needed there. The parameters are calculated in Sunny Design.

This update covers SMA only. For Huawei and SolarEdge inverters, use each vendor’s current commissioning guide. Load the region data set the operator requires before you adjust individual curve points.

Does Reactive Power Reduce Active Output?

If the inverter runs at full kVA, every kVAr of reactive power reduces active power. The rated active power at a given power factor is S × cos(phi). A 10 kVA inverter set to cos(phi) = 0.95 delivers at most 9.5 kW. So any array output above that, up to 10 kW, is clipped, which means cut off.

To keep full output at the required power factor, size the inverter for the array peak:

Real arrays rarely produce their full DC nameplate rating at the AC terminals. So the cost of clipping depends on the site and the oversizing already in the design. Model it in your energy yield tool before you decide. Also read the inverter sizing guide and the inverter sizing glossary entry for the DC to AC ratio. For systems with batteries, read the hybrid inverter guide and the hybrid inverter glossary entry.

For commercial sites, see the commercial solar audience page and the factory solar system design guide. For related topics, see the power factor correction post, the smart inverter glossary entry and the string inverter glossary entry.

Common Installer Mistakes

1. Leaving the default settings. SMA’s CORE2 document states that by default the inverter has no country data set. It also states that a wrong data set can cause problems with the grid operator. Load the right data set. Then read the active mode back and keep a screenshot for the as-built record.
2. Flipping the sign. Sign rules differ between documents, so check the direction at one operating point.
3. Missing night-time capability. If the operator asks for it, check the model and firmware before ordering.
4. Using a curve from another region. A US utility’s points (JCP&L) differ from the Australia A set and from the Italian defaults. Use the region data set and the operator’s letter.
5. Choosing a faster response than the operator specifies. Use the response time in the operator’s specification.

To see array and inverter sizing in one workflow, book a SurgePV demo or see the generation and financial tool for energy-yield modeling.

Frequently Asked Questions

What is reactive power compensation in a solar inverter?

It is the inverter injecting or absorbing reactive power (in kVAr) to help keep grid voltage inside limits. The inverter does this by shifting the phase angle between current and voltage. It needs spare kVA capacity, not extra hardware.

What are the three reactive power control modes?

A fixed cos(phi) holds one power factor at all times. cos(phi)(P) changes the power factor as active power output changes. Q(U), also called Volt-VAR, changes reactive power as the grid voltage at the inverter changes.

What cos(phi) do German operators ask for?

It depends on the operator and the plant size. Thüringer Energienetze applies VDE-AR-N 4105. It lets plants up to 4.6 kVA use a fixed cos(phi) between 0.95 under-excited and 0.95 over-excited. The default is 0.95 under-excited. Above 4.6 kVA, low-voltage connections get a cos(phi)(P) curve by default. The other choice is a fixed value between 0.90 under-excited and 0.90 over-excited.

What is a Q(U) curve and what are typical setpoints?

A Q(U) curve is a graph, made of straight line pieces, of reactive power against voltage. JCP&L’s IEEE 1547-2018 default is 0% reactive power from 0.95 pu to 1.05 pu. It absorbs 44% of VA max at 1.09 pu, with Vref at 1.02 pu. (Pu means per unit, a voltage shown as a share of the normal voltage.) The Australia A set in Jemena’s document is 44% supplying at 207 V, 0% from 220 V to 240 V, and 60% absorbing at 258 V. Your operator sets the points.

Do I need a Q at night function?

Only if the grid operator asks for reactive power outside feed-in hours. SMA’s Q on Demand 24/7 supports it on listed Sunny Tripower models and firmware. While it is active, no other cos phi function can run between day and night operation. Check the model before ordering.

Does reactive power reduce my active output?

Yes, when the inverter is at its full kVA rating. The active power limit is S × cos(phi). So a 10 kVA inverter at cos(phi) = 0.95 can deliver at most 9.5 kW. A bigger inverter removes that limit, for example 10.53 kVA for a 10 kW array peak at 0.95.

Who sets the Q(U) curve, the installer or the grid operator?

The grid operator specifies it and the installer programs it. In the UK, G99 says the generator agrees the control scheme and set points with the DNO (distribution network operator). They are recorded in the Connection Agreement. In Italy, the operator names the reactive power mode and parameters at connection. A US utility such as JCP&L publishes required default settings.