Standby Generator Monitoring: Flat Batteries, Cold Starts and the Alarms That Sit Unseen Between Tests
TL;DR: A standby generator is proven only when it runs, and most of the month it sits still. NHS England’s HTM 06-01 lists flat batteries, cold engines, cold rooms and overload among the commonest reasons generator sets fail, and recommends a monthly test on the building load. Monitoring covers the rest of the month: a controller read over Modbus gives run status, alarms and battery voltage, and sensors in the enclosure show a cold room or an open door. It adds warning; it does not replace testing.
Last updated: 14 September 2026
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Key takeaways
- A standby generator is only proven when it runs. Between tests, a fault can sit unseen until the mains fails.
- NHS England’s HTM 06-01 lists flat batteries, cold engines, cold rooms and overload among the commonest reasons generator sets fail.
- The same guidance says standby plant should be tested on the building load every month, for at least one hour and preferably two.
- Controllers such as the DSE7320 MKII offer RS485 Modbus RTU. The Milesight UC300 reads up to 32 registers and sends them over LoRaWAN.
- Monitoring does not start or test the set. It tells you, between tests, that the next start is at risk.
Why do standby generators fail when they are needed?
A standby generator usually fails for ordinary reasons that were visible before the outage. NHS England’s Health Technical Memorandum 06-01, its guidance on electrical services in healthcare premises, describes a generator set as a single point of failure and lists some of the commonest reasons sets fail.
| Fault | Typical cause given in HTM 06-01 |
|---|---|
| Overload | Inadequate testing on the actual site load |
| Cold engine | Engine heater turned off or failed |
| Flat batteries | Battery charger turned off or failed, or batteries too cold |
| Cold room | Room heater off, or the room air change rate set too high while the set is on standby |
Three of those four show up in readings long before a start is attempted. A battery voltage that drifts, an enclosure that runs cold and a heater that has stopped are all measurable, which is the case for standby generator monitoring.
The guidance is just as clear on batteries: for a set to start consistently, they should be in good condition and kept fully charged while the set is both running and stationary.
How often should a standby generator be tested?
HTM 06-01 says all standby generator plant in healthcare premises should be tested online with the building load every month, for at least one hour and preferably two. It adds an annual test up to 110% of full load lasting not less than three hours, and weekly visual inspections that include battery charging, ventilation, coolant and lubricant leaks.
Those tests prove the standby generator on the day. A one-hour monthly test covers one hour in roughly 730, and weekly inspections see the set for a few minutes each. A charger that trips the day after a test, or an enclosure heater that fails in a cold week, has weeks to flatten a battery or chill an engine before anyone looks.
Standby generator monitoring does not change the test regime. It watches the hours in between, so the next test or the next outage is not the first time anyone learns about a fault.
What should standby generator monitoring read?
Standby generator monitoring should read the values that predict a failed start, plus the conditions around the set.
- Running status and common alarm. Whether the set is running, and whether the controller is showing a warning or shutdown.
- Battery voltage. A voltage that sags a little lower week by week is the early sign of a battery or charger problem.
- Charger alarms. HTM 06-01 says consideration should be given to monitoring battery charger alarms such as charger off, DC voltage out of range and charger malfunction.
- Enclosure temperature and humidity. A cold, damp generator room is one of the listed causes of failure, and it also corrodes terminals.
- Fuel level and run hours. Where the controller publishes them, these show whether the set has enough fuel for a long outage.
- The enclosure door. A record of every visit, and of a door left open in winter.
Running status, alarms, battery voltage, fuel level and run hours usually come from the controller. Enclosure climate and the door need only battery sensors.
How do you read a generator controller over Modbus?
Read a generator controller over Modbus by connecting a Modbus master to the controller’s RS485 port and polling the registers that hold the values you want. Many controllers support this. Deep Sea Electronics describes its DSE7320 MKII auto mains failure module as having integrated RS232 and RS485 ports providing Modbus RTU support, and monitoring an extensive number of engine parameters.
The Milesight UC300 acts as that master. Its RS485 port speaks Modbus RTU, and it can read 32 Modbus registers from up to 32 devices, then send the values over LoRaWAN. Pick the registers from the controller maker’s Modbus documentation: running status, common alarm, battery voltage, fuel level and run hours cover most sites.
Where a controller has no Modbus port, the UC300’s four opto-isolated digital inputs can read volt-free run and fault outputs instead. It also has two analogue inputs for 4 to 20 mA signals, such as a fuel tank level transmitter.
How should the monitoring be powered?
Power the UC300 from a 5 to 24 V DC supply inside the control panel. A 24 V starter battery on charge sits above 24 V, so do not connect the UC300 straight across it; use a DC to DC converter or a separate supply.
The UC300 is in an IP30 metal case rated from -20°C to 60°C, so it belongs inside the control panel or another enclosure, not loose in a generator room. The gateway can go outside: the UG65 is IP65, rated from -40°C to 70°C, and its 4G version sends alerts without any site network.
Wiring inside a generator panel is work for a competent engineer, with the set isolated and prevented from starting. The enclosure sensors need no wiring at all.
What alerts should a standby generator send?
Alerts should separate what needs someone now from what needs a look before the next test.
| Reading | Useful alert |
|---|---|
| Common alarm or shutdown | Immediately, day or night |
| Battery voltage | A downward trend between tests, or a drop below the level your maintainer sets |
| Enclosure temperature | Below the level your maintainer sets for the engine heater and room |
| Set running | Any start outside a planned test, which usually means the mains failed |
| Door | Open outside planned visits, or left open |
| No data | The controller or gateway has gone quiet |
Thresholds for battery voltage and temperature belong to the engine and battery manufacturers’ guidance, so agree them with whoever maintains the set.
What are the limits?
Standby generator monitoring reports; it does not maintain the set. Four limits matter.
- It is not a test. Only a run on load proves the set will carry the load. Keep the test regime.
- Modbus has to be enabled. Some controllers need a setting changed or a communications module fitted before the RS485 port answers.
- It does not start or stop the set. The UC300 has relay outputs, but using them on a generator is a design decision for the site’s engineer.
- Fuel quality is invisible. A level reading says how much fuel there is, not whether it is fit to burn.
What does a generator monitoring kit include?
A generator monitoring kit needs a gateway, a controller interface and two enclosure sensors. Our standby generator monitoring kit combines a UG65 gateway with 4G, a UC300 for the controller, an EM300-TH for enclosure temperature and humidity, and an EM300-MCS on the door, with live prices and one click to the basket.
For the battery side in more depth, see our page on standby generator battery monitoring. For getting Modbus data into other systems, read Modbus gateway to MQTT.
Frequently asked questions
How often should a standby generator be tested?
NHS England’s HTM 06-01 says standby generator plant in healthcare premises should be tested with the building load every month, for at least one hour and preferably two, with a longer annual test.
Why do standby generators fail to start?
HTM 06-01 lists flat batteries, cold engines, cold rooms and overload among the commonest causes, with typical causes such as a charger turned off or an engine heater that failed.
Can a generator controller be read over Modbus?
Often, yes. Many controllers, such as the DSE7320 MKII, support Modbus RTU over RS485. A Modbus master such as the UC300 reads the registers and sends the values on.
Does monitoring replace generator testing?
No. Monitoring shows faults between tests. Only a planned run on load proves the set can start and carry the load.
Can the UC300 be powered from the starter battery?
Not directly. It needs 5 to 24 V DC, and a 24 V battery on charge sits above that, so use a DC to DC converter or a separate supply.
Keep the next start from being a surprise
A standby generator runs for an hour or two a month and waits for the rest. Standby generator monitoring watches that wait: alarms, battery voltage, a cold room and an open door, sent as alerts while there is still time to fix them.
See the generator monitoring kit with live prices, or send us the controller model and an engineer will check its Modbus support within one working day.
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