Controlling a home battery: what each brand lets you read, and what it lets you write

One line in a terminal, enter, and a relay clicks inside the inverter. The battery starts discharging while the app on the phone stays closed.

That box worth thousands of euros is called your battery. Who operates the controls appears on no product page.

The video spends fourteen minutes on why that matters. This article is the documentation underneath it: the endpoint or register number per brand, the firmware version that number applies to, the date it was checked, and the two calculations that do not fit in a video. If you never watch the video, you miss nothing here.

One thing up front, because it shapes how you read the rest. This is not about which battery is best. It is about the question almost nobody asks before buying: am I allowed to command this thing a power level, or may I only pick from a list somebody else wrote?

Key takeaways

• Open does not mean on. Victron publishes its complete register list and ships with Modbus TCP switched off. Sessy has a documented Open mode, but a setpoint you write does nothing while the unit is not in API mode.
• Reading and writing are two different rights. Almost every brand lets you read the state. The number of brands that let you write a power level in watts is considerably smaller.
• A mode is not a power level. HomeWizard is the sharpest example: four modes, no watts. And the endpoint sits on the meter, not on the battery.
• The brand tells you less than the model. Zendure, Anker SOLIX and Marstek all three split between older and newer hardware. "Does brand X support local control" therefore has no answer.
• A register number is not a constant. Victron moved the grid setpoint register from 2700 to 2716+2717 as of Venus version 3.50. Using the old number writes to a register that wears out the GX device's flash faster.
• A setpoint of −800 W rarely pulls 800 W from your battery. The measuring point is the grid connection, not the inverter. Your house load stacks on top.
• Local control survives an internet outage, not a power outage. Those are two different failures and they get confused constantly.

Control is two questions, not one

Almost every conversation on this topic stalls because "control" means two things at once. Pulled apart, they are two questions, and each brand answers them differently.

The first question is reading versus writing. You can see what the device is doing, versus: you can tell the device what to do. An API that only returns state of charge, power and voltage is a window. Useful for a dashboard, worthless for a strategy. Nearly every manufacturer gives you that window, because it costs them nothing and it looks good on the box.

The second question is a mode versus a power level. Picking a mode means choosing from a list the manufacturer wrote. Zero on the meter. Charge to full. Standby. Writing a power level means eight hundred watts, now, in that direction. The first is a choice inside somebody else's plan; the second is a plan of your own.

Those two questions are independent, which is exactly why "does this brand have an API?" is the wrong question. A brand can have beautifully documented API that only lets you read. Another can allow writing, but only a mode.

The brand column

This table is why the article exists. Four states, not three, because "works but is not supported" is a different reality from "open".

Victron is the benchmark, and its register number is version-bound

Victron is the only vendor in this research that publishes the register list itself and officially supports writing. That is why it heads the list, and why the trap underneath it is worth spelling out.

Modbus TCP is off by default. You enable it under Settings → Services → Modbus-TCP. Available devices and addresses live under Available services and on the Unit ID mapping tab of the Modbus TCP spreadsheet. The unit is watts: positive means drawing from the grid, negative means feeding back.

The grid setpoint register has moved.

Victron on why the number matters: "it is recommended that future implementations use the 32-bit setting at registers 2716 and 2717 to avoid constantly logging the new setpoint to VRM, and wearing the flash on the GX-device with repeated writes." Register 2700 still works. It is the old road, and it wears the flash.

The lesson is wider than Victron, and it is why every line in this article carries a date: a register number, an endpoint or a field name is not a constant. It belongs to a firmware version. Our own fact sheet recorded 2700 as the register without a version next to it, so there was no way to see it had expired.

What a setpoint actually measures

Here is the calculation a video cannot carry, and it is the mistake I made myself.

My assumption was: write −800 W and eight hundred watts flow into the house. You are pulling from the battery, so it goes to your appliances. That is wrong.

The measuring point is the grid connection, not the inverter. The setpoint controls the power flowing through the grid meter. Victron on ESS mode 3, verbatim: "When set to -400W, it will feed 400W back through its input. This energy will be taken from the battery. If there is also a 200W AC load connected to AC output the total energy taken from the battery will be 600W." And on mode 1: "the ESS control system tries to keep the power flowing through the grid meter at 0 Watt."

Worked out for an ordinary household:

So the battery discharges harder the more your house consumes, while the number you wrote never changes. Set a setpoint close to the maximum discharge rate, then start the dishwasher, and you hit a limit that was not in your own arithmetic.

One more detail belongs here. The default is +50 W, a deliberate dead band that keeps the system drawing slightly so it never accidentally feeds back.

The control is not where you go looking for it

Two brands show that "where does the API live" is a real question rather than a formality.

HomeWizard puts the endpoint on the meter. The manufacturer warns about it directly: "Despite its name, the /api/batteries endpoint is available on the P1 Meter and kWh Meter." No meter means no local control, whatever the battery looks like. And what you can push through it is a mode: zero (zero on the meter, the default), to_full, plus standby and predictive, both marked legacy. Four modes, no watts.

Sessy does let you write a power level, and even so a setpoint alone is not enough. Open mode is a named product feature with its own page. The documentation lives on the device's own IP address with /api appended, and there is a downloadable OpenAPI spec. Authentication is basic auth using the username and password on the sticker. Setpoint convention: negative charges, positive discharges, between the unit's minimum and maximum power.

The nuance that gets lost everywhere: a setpoint does nothing while the Sessy sits in NOM mode, or in any mode other than API. So even with the most open brand it is pick a mode first, then write a power level. Mode and watts are not opposites. They are two steps.

Zendure goes through a local REST API called zenSDK: GET /properties/report to read, POST /properties/write to write, mDNS for discovery, entirely on your own network. You enable it in the app first: add HEMS, then close the app.

The generation pattern: the model beats the brand

Of everything above, this observation ages the slowest, and that is why it carries the article.

Three brands all split at the model level rather than the brand level:

• Zendure. zenSDK runs on the newer generation (SolarFlow 800 / Plus / Pro, 1600 AC+, 2400 AC / AC+ / Pro, SmartMeter3CT). The older devices (Hyper 2000, Hub 1200, Hub 2000, Ace 1500) run over MQTT, by default to Zendure's cloud broker.
• Anker SOLIX. The official integration is published by Anker itself and runs locally over port 502, refreshing every five seconds, with both mode and power setpoint. Supported hardware includes the Solarbank 4 E5000 Pro and the Solarbank Max AC. For the Solarbank 2 and 3, local Modbus control is still an open request in Anker's own issue tracker.
• Marstek. Port 502 opens from firmware V144, plus a factory reset (hold the power button for ten seconds), and only on the Venus E v3 over an Ethernet cable. The v1 and v2 still need the RS485-to-TCP bridge.

"Does brand X support local control" is therefore not a question with an answer. The model number decides, and with Marstek the firmware version gets a vote too.

Marstek is the fourth state

Marstek deserves its own section, because it is the clearest example of something none of the three usual labels covers.

What is confirmed: several independent sources report the same thing about port 502, firmware V144 and the factory reset: German, Dutch, two forums and two GitHub repositories. Only one simultaneous Modbus TCP connection is possible, so your dashboard and your automation fight over the same socket.

What is not confirmed, and weighs more than expected: there is no public official register list. The registers were reverse-engineered by the community, among them 32100 through 32104 for voltage, current, power and state of charge. Marstek Netherlands does not list local control as officially supported in its own FAQ; the Home Assistant integration is a workaround.

That is more precise than "Marstek has Modbus" and more honest than "Marstek is closed". It works, and officially it does not exist.

What it costs, in euros and in hours

The cheapest route in euros is the most expensive in hours. That is a trade-off rather than a ratio: the euro side is solid, the hours side has never been measured.

Two caveats on that table, and both are too important for a footnote.

Bliq Powerflow is the software that comes with the Bliq battery. A commercial knowledge base presents it as an external solution that works with BYD, Huawei and SolarEdge. The manufacturer itself presents it as the software belonging to its own product, shipped with its own energy manager and SIM data plan; the compatibility claim is about inverter brands, not about other companies' batteries. I follow the manufacturer, being the heavier source here. If you want a brand-independent example: Home Battery Control, free, broad across RS485 and Modbus.

Not every P1 meter gives you local access. "Buy a P1 meter and you are in" does not follow. Some P1 meters only report to their own manufacturer's app and expose no open local endpoint. The HomeWizard P1 does expose one; that is the one we own. Check before buying whether that specific meter has a documented local API.

Two outages that are not the same thing

This was wrong in our own script until recently, so it appears here with the correction attached.

During an internet outage, local control keeps working. There was no server in the path, so your schedule runs on.

During a power outage, nothing works. The P1 meter has no power, so there is no measurement. And without a transfer switch, an ATS, your house does not run off the battery at all, however full it is. Backup power is a different device and a different topic.

Four questions to ask the documentation before you buy

And one question for yourself, which no documentation answers: how many hours are you willing to spend on this, every quarter, when a firmware update moves a register?

What is deliberately not in here

This section belongs in the article, because the omitted claims say as much as the included ones.

• Which energy suppliers offer control, and what exactly they control. There is a layer where your supplier steers your battery, and for many households that is the practical route. I do not have a per-supplier overview with public sources attached, so no supplier names carry claims here. That survey is the next research round: Zonneplan, Eneco, Essent, Vandebron, Budget Energie and the rest, recording per company whether the customer may set a power level or may only opt in.
• One brand with registers "on request only". Cut. A single source, and it was a blog rather than manufacturer documentation. Sigenergy covers the same nuance on firmer ground.
• The exact body of a write command. The endpoints above come from manufacturer documentation. The field names inside the body could not be confirmed from a primary source, so they are not printed. Run the command against your own unit before building on it.
• How many hours a self-built setup costs. No time-tracking exists for a Modbus integration. "Cheap in euros, expensive in hours" is a direction, not a ratio, and that is how it is written.
• Re-checking Sigenergy. The brand column entry comes from our own knowledge base, recorded for the SigenStor, Sigen PV and Sigen Hybrid, and not for the SigenMate. It was not re-verified against a source this round.

Frequently asked questions

The video

The accessible version of this story is on Jay's Desk: who actually gets to control your battery, in fourteen minutes, with the terminal on screen and the relay clicking.

This article is the complete version: every register with its firmware threshold, every endpoint with its condition, the arithmetic behind the setpoint, and the verification date per brand.

Transparency

No sponsorship, no affiliate links, no brand preference. Nothing was lent, given or paid for this video or this article. The Zendure units used as an example come from a closed collaboration dating to the ThuisbatterijNederland period; there is no payment and no arrangement in place.

Where a figure comes from a manufacturer, that is stated. Where two sources contradict each other, both appear. Where a number needs a firmware version or a measurement date to be true, it is attached.

Sources