Three-Phase Solar Inverters for Business: Sizing, Phase Balance and Power Factor
Updated 23 October 2026 · SEO Dons Editorial
Why do commercial sites use three-phase inverters?
Because the supply is three-phase. Electricity North West says domestic properties "have a standard supply of up to 20kW - this is a single phase supply", and that a customer who needs more than 20 kW "may need to apply for an amp increase or an increase from a single phase to a three phase supply". Business premises whose load exceeds that level sit on the three-phase side of that line, so the inverters that feed them are three-phase too. A three-phase unit connects to all three phases of the supply. How string, central and optimiser designs differ is covered in the inverter types guide; the commercial PV systems page shows where the inverter sits in a full specification.
What does the 16 A per phase limit mean for inverter size?
It sets the line between the simple notification route, G98, and the fuller route, G99. The Electricity Safety, Quality and Continuity Regulations 2002 describe the small-scale case as a source that "does not produce an electrical output exceeding 16 amperes per phase at low voltage". The Energy Networks Association (ENA) puts the same line in kilowatts: "A generator with a capacity less than 3.68kW/phase then EREC G98 applies. A generator with a capacity greater than 3.68kW/phase then EREC G99 applies."
| Supply | G98 ceiling (16 A per phase) | Above the ceiling |
|---|---|---|
| Single-phase | 3.68 kW | G99 |
| Three-phase | 11.04 kW | G99 |
The figures come from Electricity North West: G98 "applies to all generation installations up to 16 amps per phase (Single phase systems up to 3.68kW and Three phase systems up to 11.04kW). Anything larger will fall into a G99 application." Where the installation includes an inverter, "the rating of the installation is assumed to be the inverter's continuous steady state rating". A 50 kW inverter is more than four times the three-phase G98 ceiling, so arrays built from inverters of that size sit on the G99 side. SSEN adds that total generation counts: it lists "The total generation at a site exceeds the G98 limits" among the cases needing an application before connecting. This guide does not cover the application itself.
How does phase balance work?
The thresholds are written per phase, so where the output lands matters as much as the total. A hypothetical illustration: three single-phase inverters of 3.68 kW, one wired to each phase, put 16 A on each phase of a three-phase supply and total 11.04 kW. The same three units wired to one phase would put 48 A on it, three times the per-phase figure. A three-phase inverter feeds all three phases from one unit, so the question of spreading output arises mainly where single-phase units are used.
No numerical limit on how unevenly single-phase units may be spread was found on the ENA or network operator pages checked for this guide. Where single-phase units are proposed on a three-phase site, the permitted spread is a question for the network operator, to be settled before the units are specified.
How should inverters be sized against the supply capacity?
Start from two numbers on the datasheet and two on the connection. On the datasheet, read the AC rating in kW and in kVA. Fronius, for example, lists the Tauro 50-3-D as "AC nominal output (P ac,r) 50 kW" and "Max. output power (P ac max) 50 kVA". On the connection, find the agreed export capacity and the import capacity. Electricity North West defines Maximum Export Capacity as "the amount of electricity that may be exported onto the SP Electricity North West Distribution System", and explains that "kVA is a measurement of apparent power". The combined AC rating of the inverters, not the array's DC size, is what meets those limits. The inverter datasheet table shows how makers state these ratings.
A hypothetical case: an array that could reach 150 kW at peak, on a connection agreed for 100 kVA of export. Something has to close the gap: a lower inverter AC rating, enough on-site demand at that moment, or an export limitation scheme. The G100 guide covers schemes, and the DC to AC ratio and clipping sit in the inverter types guide above, not here.
Why can power factor fall after solar is installed?
Solar cuts the active energy a site buys but not the reactive energy it needs. Power factor compares the two. The IET's Wiring Matters article of September 2025 works an example: a site using 100 kW and 104 kVA at a steady load, with a solar array generating 1,200 kWh in a five-day week. "From the energy supplier's perspective, there would be a reduction of 1,200 kWh in active energy consumption, but no decrease in reactive energy demand."
| IET worked example, five-day week | Before solar | After solar |
|---|---|---|
| Active energy from the supplier | 4,000 kWh (100 kW for 8 hours, 5 days) | 2,800 kWh |
| Reactive energy | 1,143 kvarh | 1,143 kvarh |
| Power factor seen by the supplier | 0.96 | 0.93 |
| 33% of active energy | 1,320 kvarh | 924 kvarh |
| Reactive energy above that line | None | 219 kvarh |
The IET adds that the effect "can become more pronounced when a large PV system is installed to supply most of the on-site power during the summer months, while the installation continues to rely on the grid for reactive power".
How do reactive power charges arise?
On half-hourly metered sites, through the network charges the supplier passes on. The IET explains: "Where an electrical supply is metered at half-hourly rates, which is commonly carried out on commercial and industrial installations, suppliers and distribution network operators (DNOs) may charge for what they deem to be excessive reactive power usage." The common method it describes applies the charge "to any units of kvarh exceeding 33 % of the kWh usage", which corresponds to a power factor of 0.95 averaged over a half-hour. The IET calls this "a common methodology", not a universal one, so the threshold and rate that apply to a site should be confirmed with its supplier or network operator. The bill lines that solar does and does not move are set out in the business electricity bill guide.
What restores power factor: inverter settings or correction equipment?
Either, and the IET advises against leaning on the array alone: "A PV system should not be used as a remedy for poor power factor; instead, power factor correction should be implemented where necessary", and "the power factor of the installation should be measured before the PV system is installed". The options:
- Inverter reactive power settings. The IET says G98 and G99 "mandate that the inverter has a specified power factor operating range and may also require the PV inverter to provide reactive power control", and that "The DNO may impose a specific power factor setting in areas of the network with poor power factor or where voltage rise is an issue". Setting an inverter to supply reactive power is therefore a design item agreed with the network operator, not a switch to flip on site.
- Headroom. Many modern inverters "can supply reactive power without reducing active power output", provided there is kVA headroom. "If there is no available kVA headroom... the active power output will be reduced", which can happen when the array is large against the inverter.
- Dynamic control. SMA describes "dynamic closed-loop control of active and reactive power" on its Sunny Tripower X, and SolaX lists "night-time reactive power compensation" on its X3-MEGA G2. The IET notes some inverters can supply reactive power "even when there is no generation from the PV modules, such as at night".
- Voltage rise. A fixed inverter setting can raise voltage at the connection when the load is minimal and generation is high.
- Separate power factor correction equipment, sized to the site's measured load, which is the route the IET's guidance prefers to relying on the array.
Sources
- The Electricity Safety, Quality and Continuity Regulations 2002, regulation 22, legislation.gov.uk, checked 22 October 2026.
- G98 micro-generation and storage connections and Upgrade or reduce your supply, Electricity North West, checked 22 October 2026.
- Installation of up to 3.68kW per phase at one property, SSEN, checked 22 October 2026.
- Connecting generation to the electricity networks, Energy Networks Association, checked 22 October 2026.
- How embedded solar PV can affect the power factor of an installation, IET Wiring Matters, issue 107, September 2025, checked 22 October 2026.
- Maker pages, each used only for facts about that maker's own product, checked 22 October 2026: Fronius Tauro 50-3-D; SMA Sunny Tripower X; SolaX X3-MEGA G2.
Common questions
Is the G98 or G99 route decided by each inverter or by the total?
By the total at the site, per phase. SSEN lists total generation exceeding the G98 limits among the cases that need an application before connecting, and Electricity North West assumes the installation's rating is the inverter's continuous steady state rating.
Can an inverter correct a poor power factor on its own?
It can help, within limits. The IET says many modern inverters supply reactive power without reducing active output while there is kVA headroom, but advises that power factor correction be implemented where necessary and that the site's power factor be measured before solar is installed.
Do all businesses pay reactive power charges?
No. The IET says the charges are generally applied where half-hourly metering is employed, which is typical of commercial and industrial sites. A smaller site on non-half-hourly metering may not see them. The supplier and the network operator's charging statement show what applies.