A remote diagnostic confirms whether your inverter topology is wrong from 7–14 days of monitoring history — usually in 30 minutes. We give you a clear written report to take to your installer.
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A CT clamp (current transformer) wraps around a live cable and measures the current flowing through it without breaking the circuit. Your inverter uses its CT data to decide when to charge the battery, when to discharge, and when to push power back to the grid. If the CT misses load, the inverter misses load. Misplace a CT, install the wrong topology for the supply, and the system makes the wrong decisions — every second of every day.
On a single-phase supply the choice is simple: one phase, one CT, one inverter. On a 3-phase supply (most large UK homes, almost all commercial property, and any property with an 11 kW or 22 kW EV charger) the picture splits into four common topologies. Three of them have failure modes that show up only after install, when the homeowner notices the battery emptying while the kitchen on a different phase is running flat out.
UK SMETS2 polyphase smart meters measure import and export per phase, then sum the three phases algebraically before reporting an import register and an export register to your supplier. The export register only ticks up when the whole-house net across all three phases is exporting at that instant. The import register only ticks up when the whole-house net is importing.
L1 exports 2 kW. L2 imports 1 kW. L3 imports 0.5 kW. Net = 0.5 kW of export → SEG register accrues 0.5 kW. Now flip it: L1 exports 2 kW, L2 + L3 import 3 kW total. Net = 1 kW of import → import register accrues 1 kW. The export register doesn't move. One inverter cannot tell what's happening on the other two phases unless its CT setup measures all three.
This is the single most important fact in this guide. Everything below — why topology 1 fails, why topology 2 wastes battery cycles for no SEG credit, why topology 3 works but stresses one phase — derives from how the smart meter sums.
The most common wrong-spec install. The inverter only knows what's happening on the phase it's wired to.
The inverter is wired to L1, neutral, and earth. Its single CT clamps around the L1 incomer at the consumer unit. The inverter sees L1 only — it has no visibility of L2 or L3, no awareness of cross-phase loads, and no ability to inject or absorb current anywhere except L1.
When L2 has the kettle on (3 kW) and L1 is idle, the inverter sits there with a full battery while the meter charges you for 3 kW. When L3 is charging an EV at 7 kW, you import the full 7 kW from the grid — the battery doesn't fire because the inverter doesn't see the load. When L1 has its own load, the inverter does what it's designed to do, but only on a third of the household.
Typical self-consumption on a UK 3-phase domestic supply with this topology is 20–45% of generated kWh, against 75–88% achievable with topology 4. There is no software fix. The fix is to change inverters.
It can also breach DNO rules. A single-phase 5 kW inverter on one phase is 22 A on that phase — above the G98 16-amp-per-phase limit, so the install needs a G99 application that's rarely granted on a 3-phase supply where a balanced product exists. The over-export onto one phase also raises L1 voltage and triggers the inverter's own G99 over-voltage trip sooner than a balanced install would.
Looks balanced on paper, fails on UK net metering.
Three single-phase inverters, each with its own DC array or battery, each wired to one phase, each with one CT measuring its own phase. The three inverters operate independently — none of them has any visibility of what the others are doing.
Each inverter tries to zero its own phase. Locally, each phase is balanced. But the smart meter is netting all three phases, and the inverters don't know it.
The failure mode is wasted cycling. L1 inverter exports 2 kW to cover its own zero-target. L2 inverter imports 2 kW from the grid because L2 has a 2 kW load and an empty battery. The meter sees: net zero. You're billed nothing for that instant — but no SEG credit either, and the L1 battery just discharged 2 kW for nothing.
Worse, the cross-phase asymmetry can shift hour to hour. SEG export only registers when the whole-house net is exporting. With three independent inverters chasing per-phase zero, the system will routinely create flows that the meter nets to a wash, paying neither party. Typical self-consumption: 55–70% — better than topology 1, worse than 3 or 4.
It's also the most expensive of the four. Three inverters + three CTs cost roughly 50–100% more than a single 3-phase unit of equivalent total kW. The only times it's the right call: heritage upgrades being done one phase at a time, or where each phase needs its own backup-power island.
One inverter, all output on one phase — but it sees all three phases. Works on UK SEG.
Electrically the inverter is wired to one phase (typically L1). Instead of a CT clamp it uses an external 3-phase energy meter sitting on the supply tails, reading all three phases and reporting whole-house net via Modbus RS485 back to the inverter.
The inverter's control loop targets total imported power = 0 across the sum of L1+L2+L3. If L2 has 2 kW kettle and L3 has 1 kW load, the inverter pushes 3 kW out onto L1. The smart meter sees L1 = −3 kW, L2 = +2 kW, L3 = +1 kW → net 0. Exactly the behaviour SEG was designed to reward.
Self-consumption matches topology 4 in steady state — typically 70–85%. The catch is what happens when the inverter is exporting hard onto one phase.
A 5 kW inverter exporting at full tilt onto L1 = ~22 A on that phase. That exceeds G98's limit of 16 A per phase, which means the install needs a G99 application. Some DNOs accept this on weak networks; others won't. Below ~3.68 kW it's G98-eligible. Above it, you're applying to the DNO and they may refuse if the local LV network can't tolerate the imbalance.
It's the right answer when a 3-phase inverter isn't available in the desired battery ecosystem, when the inverter capacity stays under 16 A on one phase, and when the DNO accepts the imbalance current. It's the wrong answer when you need more than ~3.68 kW of continuous output and the DNO won't grant G99.
The default-correct topology. Each phase metered, each phase balanced.
The inverter has three independent phase outputs (or a firmware-bonded 3-phase cluster). It can source or sink current on each phase independently. It uses either three CT clamps (one per phase) or an external 3-phase Modbus meter to measure all three phases.
Each phase is balanced to zero against its own load. If L1 has 2 kW load, the inverter supplies 2 kW on L1. If L2 has 1 kW load, it supplies 1 kW on L2. If there's surplus PV after household load, it exports balanced kW across all three phases.
Self-consumption is the highest of the four topologies — typically 75–88% of generated kWh — and the residual loss is just the standard battery-empty edge cases that affect any storage system.
Balanced by design means no P29 voltage-unbalance risk and no G98 imbalance breach. G98-eligible up to 3 × 16 A = ~11 kW total inverter output on a 3-phase service; anything above goes G99.
The premium over single-phase: typically 20–40% on inverter hardware, plus ~£150–250 for the 3-phase meter if it isn't bundled. For any 3-phase supply that's getting more than ~3 kWp of PV or a battery system intended to cover the whole house, the premium pays back inside the system lifetime through self-consumption gains alone — never mind the resilience of getting the topology right first time.
| Topology | CTs | G98 ceiling | SEG net works? | Self-consumption | DNO risk |
|---|---|---|---|---|---|
| 1 — 1-ph inv, 1 CT | 1× on L1 | 16 A on one phase | Partly — meter nets but inverter is blind to it | 20–45% | Voltage rise on one phase; imbalance |
| 2 — 3× 1-ph inverters | 3× (one per inverter) | 16 A/phase, balanced | Partly — cross-phase waste | 55–70% | Multiple-G98 acceptance varies by DNO |
| 3 — 1-ph inv + 3-ph meter | External 3-phase meter | 16 A on host phase | Yes — designed for it | 70–85% | Imbalance current on host phase |
| 4 — True 3-phase inverter | 3× CTs or 3-phase meter | 3×16 A ≈ 11 kW total | Yes — designed for it | 75–88% | None notable |
The UK distribution code from ENA (Energy Networks Association) sets two thresholds that decide which paperwork applies:
Up to 16 A per phase. At 230 V that's ~3.68 kW single-phase or ~11 kW total on a balanced 3-phase install. You install first; the DNO is notified within 28 days. No pre-approval.
Anything above 16 A per phase, or any installation with multiple G98 units at one premises that breaks the imbalance rules. You apply to the DNO before commissioning. The DNO can take up to 45 working days to reply on the standard route, or 10 working days on the Fast Track.
The imbalance rule is the part most installers miss. Multiple G98 single-phase units at one premises are only acceptable when the imbalance between phases does not exceed 16 A. Topology 3 (single-phase inverter pushing all output onto L1) is simpler: an inverter rated above 3.68 kW is over 16 A on that phase on its own, so it is G99 even with export limited.
If you want the full picture on the paperwork, see our G98 vs G99 explained guide.
A 3-phase EV charger draws roughly 16 A on each of the three phases (11 kW) or 32 A on each (22 kW). For the single-phase EV-charger CT placement story, see our EV charger CT placement guide. This is where the topology choice shows up most aggressively on the homeowner's bill.
5 kW single-phase inverter covers ~1/3 of the EV load if it happens to share a phase. Solar self-consumption during EV charging: 0–33%.
Inverter pushes 5 kW onto L1 to offset 11 kW total. Meter nets to 6 kW import. Solar covers ~45% of EV load.
9–11 kW 3-phase inverter fully covers an 11 kW EV charge. Solar self-consumption: ~100%.
No domestic inverter has the capacity. Topology 4 still minimises grid draw the most.
If you're adding an EV charger to a 3-phase house — or planning a new solar install for one — topology 4 is the only choice that makes the numbers work.
It isn't usually malice — it's friction. The reasons are mundane and they stack up:
The van carries single-phase hybrids. They don't normally stock 3-phase units.
20–40% inverter premium, plus a 3-phase meter at £150–250, plus more complex CT routing.
Several popular UK battery stacks were single-phase-only until recently. The installer wants the chemistry they know.
Single-phase + one CT can be commissioned by one electrician in half a day. 3-phase needs phase-rotation checks and balanced-output verification.
G98 single-phase notify is 28 days post-install. G99 usually needs approval first: up to 45 working days on the standard route, 10 on the Fast Track. Installers under quota pressure choose the path that ships fastest.
None of these reasons survive contact with the homeowner's actual bills, but they explain why so many 3-phase supplies end up with the wrong topology in the first place.
Before commissioning, or as a post-install audit:
If you don't know, check your meter. Three meter tails coming in (excluding neutral and earth) = 3-phase.
Read the model number off the inverter itself. “1P”, “1-phase”, or “single-phase” in the datasheet means it generates on one phase only — regardless of how many CTs it accepts.
One CT on a 3-phase supply = topology 1, and you need to know that going in.
If the installer can't answer this, the chances are it's topology 1.
If the inverter is over 3.68 kW on one phase and there's no G99 reference, the install may not be DNO-compliant.
The fastest diagnostic without lifting a panel:
Run a known load on a phase that's not the inverter's phase — e.g. plug a 2 kW heater into a socket you know is on a different phase circuit.
Watch the inverter monitoring app. If the battery doesn't respond and the grid import jumps by 2 kW, the inverter can't see that phase. Topology 1 confirmed.
Cross-reference with the smart-meter in-home-display (IHD) reading. If the IHD reports by 2 kW of import while the inverter says “self-consumed”, the inverter is operating on bad data.
A remote diagnostic from us pulls 7–14 days of monitoring history and looks at the cross-phase pattern automatically — usually catches a wrong-topology install inside the first 30 minutes. For the wider “CT clamp installed wrong” symptom set (reversed readings, battery not charging, monitoring inverted), see CT clamp installed wrong.
A remote diagnostic confirms the topology from monitoring data and gives you a written engineer report — clear enough to take to your installer or to a DNO. £125, no fix no fee.
Due to unprecedented demand we’re currently only carrying out on-site work in Yorkshire, and replies are taking up to 7 days. Remote diagnostics continue for the whole UK as normal.
Every message is read and answered, so please don’t send it twice; that only slows the queue.
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