![[Itsuki Village (Kuma District, Kumamoto)] Off-grid power for a disaster-prevention radio relay station (Always-on communications)](/images/cases/case-003/hero_011.jpg)
Overview: What must not stop is “communications”
Deep in southern Kumamoto, Itsuki Village (Kuma District) is a small mountain community with a population under 1,000. Surrounded by steep terrain, it is vulnerable to isolation when disasters cut roads and access routes—geography itself becomes a risk factor.
In such places, a disaster-prevention radio relay station becomes the “last mile” of emergency communications. It stays quiet in normal times, but if communications fail during outages and disruption, local safety deteriorates immediately. What we need to protect is not electricity alone, but the ability to keep communications available.
In this project, alongside the digital upgrade of the disaster-prevention radio system, we moved away from dependence on the power grid. We implemented an off-grid power system using Personal Energy® JIZAI, designed around autonomous, distributed, and asynchronous control. Solar power is the primary source, and the system continuously manages charge/discharge while supplying power to the relay equipment and HVAC.
Context: Conditions a mountain relay site faces
- Grid restoration can take a long time—or be unpredictable—after an outage
- Road damage, heavy rain, and rockfalls make maintenance and transport difficult
- Sites are usually unmanned; you cannot rely on onsite work during emergencies
- Quiet in normal times, but cannot stop “when it matters”
- Smaller populations and economic bases tend to feel the risk of centralized dependence more sharply
Timeline: Installed just before the 2020 heavy-rain disaster
The system installation was completed on June 20, 2020. Soon after initial commissioning, the July 2020 Heavy Rain Disaster struck Kyushu. The Kuma River basin suffered widespread infrastructure disruption and severe damage.
The maintenance road leading to the site was also cut off by landslides. Under these conditions, the requirement was simple: communications must keep operating even when nobody can reach the site. Off-grid power matters precisely in this “real world” scenario.
System: Personal Energy (autonomous / distributed / asynchronous)
This is not merely a system “disconnected from the grid.” As Personal Energy® JIZAI, it implements autonomous, distributed, and asynchronous control: solar input continuously drives charge/discharge management while supplying power to the relay equipment and HVAC.
In electrical terms, the AC output supports continuous operation at rated 6000W (within 100–120VAC). The battery uses lithium iron phosphate (LFP, nominal 51.2V), with capacity in the range of 19.2kWh (rated) / 17.6kWh (effective), enabling “always-on” operation matched to load requirements.
Excerpted specs (system configuration & design requirements)
For public-sector planning and future projects, we share only the minimum specification items that can be disclosed. Final specifications are always engineered for each site based on operating conditions (loads, operation, maintenance, and expected outage scenarios).
| Application | Off-grid power system for a digital disaster-prevention radio relay station |
|---|---|
| System | JIZAI-PV2.5-4,INV6.4,DC4.8,LIB19.6 (configuration codes) |
| Enclosure | SUS304 outdoor cabinet, IP55 |
| External AC input | AC85–265V (continuous) / 47–63Hz |
| AC input current / power | 14.2 / 8.2A (input) / Max input power 1497W |
| PV input (DC) | Nominal 320VDC (input range: 150–400VDC ±3%) / Max input power: 10kW (aggregated) |
| AC output | Rated: 6000W / 6000VA, 120VAC (output range: 100–120VAC ±3%), 50/60Hz |
| Battery | Nominal 51.2V / Rated 19.2kWh (per module 1.2kWh 24Ah typ.) / Effective 17.6kWh (per module 1.1kWh 22Ah) |
| Charge / discharge (example) | Max charge 100.0A / Max discharge 130A / Charge time approx. 4h |
| Operation | Unmanned operation / Remote monitoring (details not disclosed) |
In unmanned off-grid systems, “never stop” must be paired with “stop safely.” Protective devices such as ELBs are implemented in multiple stages.
Battery storage sustains operation at night and during poor weather. Personal Energy has over 15 years of field experience, including many deployments for public-sector use.
A BMS with real-time OS “JIZAI” controls the overall off-grid system for stable operation.
CIGS modules are used to help secure generation under low-light conditions.
Disaster-prevention radio systems are used by prefectures and municipalities for emergency response and recovery in accordance with local disaster plans. After large-scale disasters in recent years, digital systems have been promoted to support diverse needs such as data and image transmission.
Deployment: From installation to proof of operation
This project’s value was proven immediately: right after completion (June 20, 2020), heavy rain caused outages and road isolation. The fact that the system continued operating under these constraints directly validates the “always-on” design.
- Transport (logistics & preparation)
- Placement (positioning & fixing)
- Wiring (connections, waterproofing, grounding)
- Testing (insulation, energizing, measurement, operational checks)
- Operation (status indication & recording)
Results: Resilience and local independence
With solar as the primary source, ongoing electricity charges for the site can be eliminated. The cost structure changes, improving long-term sustainability.
Even when roads and power lines are disrupted, the relay station continues operating without interruption.
Instead of stopping the moment a central system fails, resilience is built by adding redundancy in smaller autonomous units— a concrete implementation of distributed infrastructure.
- Limits of centralization: Convenience can also mean “stop everything when cut off.”
- Restoring agency through technology: Not just cost-cutting—enabling communities to sustain their own safety
- Stronger local capacity: Keeping communications available directly supports safety checks and rescue operations
Related reading & contact
“Inconvenience benefits” does not mean simply accepting inconvenience. It means restoring agency that can be lost inside convenience. Off-grid is one way to implement that agency in real infrastructure.
For relay stations, monitoring, and communications loads that must not stop, we propose a minimum-necessary design to keep operations running. Let’s start by organizing your site conditions (loads, operation, maintenance, expected outages).
Even with the same “always-on” goal, outage conditions, loads, and operations differ by site. Compare cases with similar constraints.
Off-grid is a practical way to make freedom work under constraints.