Which is better for irrigation, the UC511 or the UC512?

Usually the UC511. Irrigation is outdoors and in daylight, so the embedded 6V 1.7W solar panel keeps it charged, and it is the only one of the two that supports Class C for near-immediate valve commands. It also supports Milesight D2D, so a soil or pressure sensor can trigger the valve directly without going through the network server.

How long do the batteries last?

Different questions for each. The UC512 uses three replaceable 9000 mAh ER26500 lithium cells and exceeds 10 years on Class A, or around 6.2 years on Class B at SF7. The UC511 uses two 2550 mAh rechargeable cells topped up by its solar panel, quoted at around 8.4 years on Class A at SF7 with the battery full and not charging, falling to around 1.1 years on Class B and around 26 days on Class C. Both figures assume a 20 minute interval and four valve operations a day at 25 degC.

Can I get near-instant valve control?

Only with the UC511, which supports Class A, Class B, Class C and Class C to B. Class C keeps the receiver open so a downlink acts almost immediately. The UC512 supports Class A and Class B only. On Class A a command waits for the device's next uplink, which at a 20 minute interval means up to a 20 minute delay.

Are both waterproof enough for a chamber that floods?

Yes. Both are rated IP68 to one metre underwater for seven days, in the same 116 x 116 x 45.5 mm housing. The UC512 has the wider temperature range at -30 degC to +70 degC against -20 degC to +60 degC for the UC511, which is worth noting for an unheated chamber. For a dark chamber the UC512 is the better choice anyway, because a solar panel underground is no use.

What solenoids do they work with?

Both drive 2-wire DC 12V latching solenoids through two M12 A-coded male interfaces, with 1.5 m cables by default. Both also carry two digital inputs that work as digital input or pulse counter, which is how a flow meter is usually connected alongside the valve.

Milesight UC511 vs UC512 Solenoid Valve Controller

Short answer: Same body, same two M12 solenoid ports, same two digital inputs, same IP68 rating to one metre for seven days. The difference is entirely how they are powered, and that decides how quickly they can be told to close a valve. The UC511 has an embedded solar panel and rechargeable cells, and supports Class C. The UC512 runs on three replaceable 9000 mAh lithium cells for more than ten years, and stops at Class B.

The real question is how fast the valve has to respond

This is a control device, not a sensor, so latency matters in a way it would not on a monitoring product. LoRaWAN device class sets it:

Class A only accepts a downlink just after the device has transmitted. If it reports every 20 minutes, a close command can wait up to 20 minutes.
Class B opens scheduled receive windows, so the delay becomes seconds to a couple of minutes.
Class C listens continuously and acts almost immediately, at the cost of power.

Only the UC511 offers Class C, and its datasheet is honest about the price: around 8.4 years on Class A at SF7, around 1.1 years on Class B, and around 26 days on Class C on a full battery with no charging. Twenty-six days is not a deployment, it is what the unit does between sunny spells, which is exactly why the solar panel is on this model and not the other one.

The UC512 offers Class A and Class B only. On Class A it exceeds ten years; on Class B it is around 6.2 years at SF7. Those are figures you can plan a maintenance regime around.

Which one do I need?

Choose the UC511 for irrigation, and generally for anything outdoors and in daylight where someone may need to shut a valve now rather than at the next uplink. Solar plus Class C is the combination that makes on-demand control practical. It also supports Milesight D2D, so a pressure or level sensor can command the valve directly without a round trip through the network server, and it accepts both the PS02 and PS03 pressure sensors. The EA version can additionally take 5-24 V DC.

Choose the UC512 for chambers, culverts, basements, plant rooms and anywhere the sun does not reach. Ten years on Class A from replaceable cells is fit and forget, and replaceable is the operative word: when the cells do expire you change them rather than replacing the unit. It is also the wider temperature range at -30°C to +70°C against the UC511’s -20°C to +60°C, which matters for an unheated chamber in a hard winter.

Three differences that are easy to miss

D2D is UC511 only. If the design is sensor-triggers-valve with no cloud in the loop, the UC512 cannot do it.

Pressure sensor support differs. The UC511 AI version lists both PS02 and PS03; the UC512 AI version lists PS02 only.

The UC512 has no DC power connector at all. The 5-24 V input exists on the UC511 EA version only. If there is any chance of mains or DC being available later, that closes the door on the UC512.

Both drive 2-wire DC 12 V latching solenoids on 1.5 m cables as standard, and both offer local rules and a water pressure alarm, so the valve keeps working to its rules if the network drops.