C-20 — How the power actually behaves

Godrej Retreat, Sector 83, Faridabad · Deye SUN-12K-SG04LP3-EU · 10 kW solar · 2 × SE-F5 battery
Two sections. A — what happens when the grid fails. B — what happens when the sun shines.

Section A

Load shedding — how a floor turns its own ACs off, by itself

This is the circuit that keeps the house alive through a night power cut. The inverter watches the battery; when it drops to 30%, it pulls a signal wire dead, a contactor in each floor board springs open, and the ACs and geysers drop out — while lights, fans and the fridge carry on. Built up here one piece at a time, with the full working circuit at the end. Everything is live — press the buttons.

Grid failureBattery SoCContactor + coil4-hour overrideMCB protection

1 · The control core 2 · The override 3 · Everything together

The control core — signal → contactor → AC

Just three things: the inverter's signal wire, the contactor it drives, and the AC it switches. Nothing else yet.

Inverter signal:

Signal LIVE: 230V on the red wire energizes the coil (A1–A2). The coil becomes an electromagnet and pulls the moving contact shut, bridging poles 1→2 — so supply phase flows straight through to the AC and it runs. Flip the signal DEAD and watch the coil drop, the contact spring open, and the AC stop.

Add the override — a manual way to force the AC on

Same circuit as before, plus a push-button + timer relay that feeds the coil directly — so a floor can keep its AC even when the signal is dead.

Inverter signal:

Try it: set the signal DEAD — the coil drops and the AC stops, just like Section 1. Now press override: the push-button feeds the coil directly through the timer relay, so the coil re-energizes, the contacts close, and the AC comes back on — even with a dead signal. The timer holds it for 4 hours, then releases it back to automatic.

Everything together — the full working circuit

All parts combined: grid/inverter supply, MCB protection, signal automation, the override from step 2, battery drain and automatic load-shedding.

Supply:

Battery %: 85%

Live / phase (230V)

Neutral

RED signal wire (control)

De-energized

Energized + current flowing

✅ Normal — coil energized, contacts closed, AC running

Grid present. The 230V signal on the RED wire energizes the contactor coil (A1–A2). The coil's magnet pulls the moving contact shut, bridging poles 1→2, so grid phase reaches the AC.

The control chain

Inverter puts 230V on the RED signal wire when battery is healthy.
Signal reaches coil terminal A1; A2 returns to neutral → coil is energized.
Energized coil = electromagnet → it pulls the moving contact against poles 1 & 2.
Contacts closed → grid phase flows through to the AC. It runs.
Battery ≤30% in an outage → inverter removes the signal → coil de-energizes → spring pushes contacts open → circuit breaks → AC off.
Push-button feeds the coil directly for 4 hr, overriding a dead signal.

Why the MCB, if it's always on? It isn't a switch for daily use — it's protection. The contactor does the automatic switching (open/close many times a day on the signal). The MCB stays closed and only breaks the circuit on a short-circuit or overload, and gives the electrician a safe manual isolation point to work on the AC circuit. Hit "Simulate AC fault" to see it trip and cut power upstream of the contactor.

Section B

The CT — the one part that decides whether your solar reaches the whole house

Section A is about the grid failing. This is about the grid being perfectly fine, the sun blazing at noon in June — and the EV charger, the lift and the servant AC still being billed at full rate, because the inverter cannot see them. A CT is a split ring that clips around a cable. It carries no power. It only senses how much current is flowing and which way. Get it right and roughly ₹25,000–35,000 of solar a year stops being thrown away — and the thing about getting it wrong is that nothing looks wrong. Everything below is live.

Zero Export to CTWhole-home self-consumption4 m lead vs 15 m runRS485 energy meterOpen action — not yet decided

1 · What it can see 2 · The blocker + 5 ways out 3 · The meter route 4 · Power-flow simulator 5 · The proof test 6 · Settings + checklist

What the inverter can see — and what it is blind to

One switch in the inverter menu. It decides whether solar reaches only the circuits wired through the inverter, or the entire house.

Inverter work mode:

Midday, June, sun at full · EV on charge

Solar reaches

Inverter side only

the raw-grid side stays dark to it

Grid import at noon

~12.9 kW

EV + lift + servant AC, bought

EV charged at midday

22 units bought

every single charging day

Thrown away per year

₹25–35k

and nothing looks broken

Zero Export to LOAD. The inverter is blind to the main panel. It matches its generation to its own LOAD port and stops there — so the EV charger, the lift and the servant AC buy every unit they use from the grid, at noon in June, with 10 kW of sun on the roof. There is no net metering on a society single-point supply, so the surplus is not sold either. It is simply thrown away. Switch to Zero Export to CT and watch the solar go backwards out of the Grid port.

The metaphor that makes it obvious Think of the house as one water pipe feeding one junction, every tap drawing from that junction. Your solar is your own pump, plumbed into the same junction. The CT is the meter at the gate. Its only job is to tell the pump "society water is still coming in — push harder", until the gate reads zero. At that moment your solar is feeding the whole house.

Load on the raw-grid side	Covered by solar once the CT / meter is in?
EV charger, 7.4 kW	YES — the biggest single gain. Charged at midday, ~22 units a day off the bill
Servant AC	YES — runs hardest on summer afternoons, exactly when the sun is strongest
Lift	YES — all day, every day
UGF kitchen instant, 3 kW	No — it fires at 07:00 and 20:00 when there is no sun. Unaffected either way.

Deye manual, section 5.7 — "Zero Export To CT: Hybrid inverter will not only provide power to the backup load connected but also give power to the home load connected."

The blocker — and five ways out of it

The CTs must clip around the incomer in the stilt main DB. The inverter is on the terrace. The supplied lead is 4 metres. The run is about 15. That gap has more than one answer, and they are not all expensive.

The problem:

🔴 The one measurement that decides this whole section Get someone to run a tape along the actual cable route — stilt main DB, up the shaft, across the terrace, into the inverter — not the straight-line height. Earlier working put it at 13–16 m, which straddles the limit. At or under 15 m, Options 1 and 2 below are legitimate and cost almost nothing. Over 15 m, the meter is not a preference — it is the requirement. One tape measure decides a ₹7,000 question.

The five ways out, side by side

Three of them sense the current and cost between ₹500 and ₹9,000. One skips sensing altogether. One is doing nothing. Click each to see what physically changes.

All-in cost

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Backed by the maker?

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Risk

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What it captures

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Option	All-in	What you are betting on	Where it fails
1 · Extend the supplied CTs on shielded CAT6	₹500–1,500	That your real route measures ≤ 15 m, and that one twisted pair per CT keeps the noise out. Copper, same gauge or thicker, screen earthed at the inverter end only.	Over 15 m, or a lazy twist-and-tape joint. Fails silently.
2 · Buy CTs with a longer factory lead	₹1,500–3,000	Same idea as Option 1 with no joint anywhere — one continuous manufactured lead. The ratio must match exactly what the inverter expects (100 A : 33.3 mA on this unit).	Wrong ratio = every reading wrong by a fixed factor. Check before buying.
3 · RS485 energy meter (the original proposal)	₹5,000–9,000	Nothing. Distance stops being a question — the meter sits next to the cables it measures and sends a digital message. Factory-supported, wiring diagrams in the manual.	Only real risk: the Meter-485 port is dead on some hardware versions. Check your serial first.
4 · Move the EV charger onto the inverter bus	₹2,000–4,000	No sensing at all. Re-terminate the EV way from the raw-grid busbar onto the inverter busbar in the same panel, behind a shed contactor that drops it in an outage.	7.4 kW lands on one phase. Needs the electrical owner's sign-off, not mine.
5 · Do nothing	₹0 today	Nothing, and it works perfectly — the house runs, the app looks healthy, the solar still covers everything on the inverter side.	₹25,000–35,000 a year, permanently, invisibly.

✅ The decision you can take today without deciding anything Options 1, 2 and 3 all need the same physical cable pulled up the same shaft. A 20 m roll of shielded CAT6 (F/UTP) costs roughly ₹600–900 and carries four twisted pairs — three for the CTs if you take Option 1 or 2, or one pair for RS485 A/B if you take Option 3, with spares either way. Pull it now, while the shafts are open, and you have bought yourself the right to decide later — after the route is measured, after the installer confirms which meter his firmware supports, after the inverter serial is checked. It is the only item on this page with a deadline set by the building rather than by you.

Also considered, and not worth your time Move the inverter down to the stilt — no. The solar DC runs, the battery's 0–35 °C window and the terrace plant room are all settled around it. Sense somewhere closer to the terrace — no. Nothing within 4 m of the inverter carries whole-house current; that only exists at the stilt incomer. Wireless CT link — no. Not a thing this inverter supports. Move everything onto the inverter load bus — no. That is the whole-home rewire the CT exists to avoid, and 7.4 kW of EV plus the lift would swamp the island in an outage.

The fix — send a digital message instead of an analogue one

A small DIN-rail energy meter sits inside the main DB, right next to the cables it measures. Its own short leads do the sensing; RS485 carries the answer 15 m up to the terrace.

Meter:

Both are named in the Deye manual, section 3.7.1

CHINT DTSU666 — approx ₹4,000–6,000. It must be the 3-phase DTSU version. DDSU666 is the single-phase part and is the wrong thing entirely — the names are one letter apart and this is the mistake that actually gets made. Check the part number on the box, not the invoice line.

Evidence in the Deye manual	What it says
Parts list, section 3.1	"Meter (optional) ×1" and "Sensor Clamp ×3" — the meter is a factory option, not a workaround
Section 3.7.1 Meter Connection	Full wiring diagrams for a CHINT meter and an Eastron meter over RS485 A/B
Port 7 on the inverter	"Meter_485: for energy meter communication"
Setting, section 5.10	"Ex_Meter For CT: when using zero-export to CT mode, the hybrid inverter can select EX_Meter For CT function and use the different meters, e.g. CHNT and Eastron."

Four things must be told to the panel vendor NOW, not later 1. 4–6 spare DIN module widths in the main DB for the meter. 2. Access at the incomer for three sensor rings. 3. 2-core shielded twisted-pair RS485, stilt to terrace, pulled on the same route as the solar DC cables while the shafts are open. 4. The ferrite ring supplied with the inverter, four turns of the meter cable through it.

Power-flow simulator — drag the sliders and watch where the solar goes

The same arithmetic the inverter runs, millisecond by millisecond. Order of priority: backup load first, then home load, then the battery, then whatever is left.

Work mode: Solar Sell:

Guide scenarios:

☀ PV generation7.0 kW 🔋 Battery demand2.0 kW 🏠 Backup load2.0 kW ⚡ Home / raw-grid load2.0 kW

Solar actually used

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of what the roof could make

Net flow at the gate

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at the utility boundary

Solar thrown away

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curtailed, or bought back later

Home load on solar

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the whole point of the CT

Scenario	PV	Backup	Home	Battery	Net at the gate
A — High PV, Sell ON	7.0 kW	2.0	2.0	2.0	+1.0 kW exported
B — High PV, Sell OFF	6.0 kW (curtailed from 7.0)	2.0	2.0	2.0	0.0 kW — strict zero export
C — Low PV	3.0 kW	2.0	2.0	1.0	−2.0 kW imported

The five-minute test that proves it actually works

Run this in front of the installer, before he leaves site. Settings can look perfectly right on the screen and still be wrong.

Pretend the install is:

Solarman app · live

PV output3.00 kW

Grid import at the gate0 W

Raw-grid load switched on— none —

Waiting — switch on the load

How to read it

Pick a sunny afternoon with the battery full. That is the only condition where the answer is unambiguous.
Note grid import in the app — it should sit near 0 W.
Switch on a genuine raw-grid load: the servant AC, or a 1 kW heater on a stilt socket.
PV rises by about 1 kW and grid import stays near 0 → working. The inverter saw the load through the CT and pushed more solar out to cover it.
Grid import jumps to about 1 kW and PV does not move → not working. Wrong work mode, or the meter/CT is misplaced or reversed.

Why a reversed CT is so dangerous. Flip the ring and the inverter reads import as export and export as import — the control loop runs backwards. Deye manual, section 3.7: "when the reading of the load power on the LCD is not correct, please reverse the CT arrow." The arrow on the casing must point towards the house and the inverter, away from the utility meter. And it must clip around the Live conductor only — Live and Neutral together inside one ring cancel out and read a flat 0 A.

The settings, and the checklist to tick before the installer leaves

Six settings and seven actions. None of them can be added cheaply once the shafts are closed and the panel is built.

Inverter setting	Value for this house
System Work Mode	ZERO EXPORT TO CT
Solar Sell	OFF — there is no net metering on a society single-point supply, so there is nothing to sell
Zero-export Power offset	20–50 W, never 0 — a micro-buffer of import so a motor switching off cannot momentarily push power past the gate
Time Of Use	UNTICKED — with the grid present the battery discharges only if TOU is ticked. Leaving it off keeps the battery for outages while PV still offsets the whole house.
Energy Pattern	Batt First
Advanced Function ▸ EX_Meter For CT	Meter Select = CHNT or Eastron, matching the meter actually bought

Why the offset is 20–50 W and not zero Many digital prepaid revenue meters treat any detected export — even a millisecond transient when a heavy motor switches off — as an anti-tamper event, and trip. A small permanent import buffer means the flow at the gate never actually reverses polarity.

Action checklist — tick every box before the installer leaves site 0 of 7 done

✓Confirm which meter model the installer's firmware supports — CHINT DTSU666 or Eastron SDM630MCT — and get it in writing before buying.
✓Order the 3-phase meter. Check the part number reads DTSU (3-phase), not DDSU (1-phase).
✓Pull the shielded RS485 cable from the stilt main DB up to the terrace inverter NOW, with the solar cables.
✓Instruct the DB vendor: 4–6 spare DIN modules, plus incomer access for three sensor rings.
✓Connect the meter BEFORE the inverter is first powered on — otherwise the menu option does not appear.
✓Set the six values above on the inverter, and pass the meter cable four turns through the ferrite ring.
✓Run the five-minute handover test in section B5, in front of the installer, before signing off.

Risk to close before spending money	What to do
The meter port may not work on every unit	Manual, Appendix I, on the Meter-485 port: "for some hardware versions, this port is useless." Confirm with your supplier or Deye India that the meter port on your serial number is functional, before buying the meter.
Direction matters	A reversed sensor reads backwards and the system regulates the wrong way — runaway feed-in, or aggressive importing. Check at commissioning with the B5 test.
Phase matching, on a 3-phase unit	CT1, CT2 and CT3 must correspond strictly to R, Y and B. Mismatched phases cause large calculation errors and erratic hunting.

If only ONE thing is done from this whole section Pull the shielded RS485 cable from the stilt main DB to the terrace inverter while the shafts are still open. The meter can be bought and fitted later — next month, next year. The cable route cannot be recreated once the building is finished.

A+B

Two different jobs, one inverter

They are often confused with each other. They share almost nothing.

	Section A — load shedding	Section B — the CT / meter
When it matters	The grid has failed. Night, usually.	The grid is fine. Midday, sun out.
What it protects	The battery — so lights, fans and the fridge outlast the cut and nobody wakes up in the dark.	The bill — so the EV, the lift and the servant AC run on the roof instead of on the meter.
The hardware	GEN port signal → pilot relay → one 4-pole contactor per floor board.	Three sensor rings at the incomer → energy meter in the main DB → RS485 to the Meter-485 port.
What happens if it is missing	The battery flattens at 2 a.m. and the whole house goes dark, ACs and all.	Nothing visible. The system looks like it is working. ₹25–35k of solar a year quietly evaporates.
Status	Settled. Contactors bought and installed.	OPEN — Action 1 in the master electrical docket. Cable route is the deadline, not the meter.

The one sentence that links them. Section A decides what stays on when there is no grid; Section B decides what gets fed by the sun when there is. Neither can do the other's job, and the CT work has a deadline that the shedding work does not — a contactor can be added to a finished house, a 15 m cable through closed shafts cannot.

Schematic simulation for the pilot — single-line, timings compressed (real override = 4 hours). One AC pole shown for clarity.