Short answer: size solar and battery storage for a telecom site as three connected problems. The PV array must replace the site’s energy in the design weather period, the battery must carry the load through the required source-free interval, and the power path must support peak load and recharge current. A daily-energy calculation alone cannot answer all three.
The method below is suitable for feasibility work and supplier comparison. It is not a construction design. Final equipment ratings depend on measured load, site weather, product curves, operator requirements and local electrical rules.
Collect data before choosing equipment
The most useful starting point is a time-aligned record of load and operating conditions. Site coordinates, an hourly or 15-minute load profile, voltage interfaces, outage history, autonomy target, generator information, climate and planned radio additions should be known before a battery model is selected.
If no interval meter is available, build a schedule by subsystem. Separate the radio and transmission load from cooling, heaters, obstruction lights, security equipment and maintenance sockets. Do not multiply every nameplate rating by 24 hours: nameplate input, typical demand and short-duration peak are different quantities.
| Input | Minimum usable data | Preferred evidence |
|---|---|---|
| Load | Average watts, peak watts and DC/AC interface | 15-minute or hourly profile by subsystem in hot and cool periods |
| Solar resource | Monthly peak-sun hours for the coordinates | Multi-year hourly irradiance and ambient temperature |
| Grid and generator | Source availability and rated power | Outage duration distribution, voltage quality, fuel records and start reliability |
| Environment | Temperature range and altitude | Wind, snow, dust, salt, humidity, flooding and enclosure temperature |
| Service target | Autonomy hours | Availability target, critical-load list, reserve state of charge and recovery deadline |
| Interfaces | Nominal bus voltage and available area | Operating voltage window, connector, protection, cable and structural constraints |
For early resource screening, the European Commission’s PVGIS provides solar radiation and PV performance data for locations in Europe and many other regions. NASA POWER’s hourly service is another useful source for solar and meteorological time series. Modelled data should still be checked against site observations where service continuity is important.
Calculate daily energy and peak power separately
E_load = Σ(P_i × h_i)
Daily energy, measured in kWh, drives the first estimate of PV and battery capacity. Peak power, measured in kW, drives rectifier, inverter, controller, busbar, cable and protection ratings. A site can have modest daily energy but still impose a high current when a cooling compressor starts or radios transmit at peak traffic.
Use a load schedule that reflects operating modes. For each item, record typical power, peak power, hours per day and whether the load is critical. Cooling should be linked to ambient temperature and internal heat rather than treated as a fixed annual load. If future radios are planned, add their expected profile as a visible growth case instead of burying growth inside an unexplained factor.
Keep conversion losses separate from demand growth and reserve margin. This matters because the same 10% loss can easily be counted once in the load, again in an efficiency factor and a third time in a generic design margin.
Estimate the PV array from the design solar period
P_PV = E_source ÷ (PSH_design × K_PV)
E_source is the daily energy that must arrive at the relevant source-side boundary. PSH_design is the peak-sun-hour value for the selected design month, and K_PV is a PV-side derating factor. For a critical off-grid site, annual-average sunshine is rarely the right design input. Use the relevant low-solar period and investigate consecutive poor-weather days.
| Loss or allowance | Where it belongs | Typical design treatment |
|---|---|---|
| Module temperature, soiling, mismatch, shading, DC wiring and MPPT | PV side | Include in K_PV or in the PV simulation, not both |
| Rectifier, DC/DC, inverter and distribution losses | Downstream energy path | Apply between the PV source boundary and delivered load |
| Battery charge/discharge losses | Stored-energy path only | Apply to the share of energy that actually passes through the battery |
| Load growth, weather uncertainty and operating reserve | Design allowances | Show each allowance separately so it can be reviewed |
The one-line formula is useful for screening, but it does not reveal hourly state of charge. A time-series model should test low-solar months, high-temperature load, cloudy sequences, source outages, curtailment and recovery. It should also verify PV string open-circuit voltage at the coldest expected module temperature and operating voltage at the hottest expected cell temperature.
Size storage for usable, not nameplate, energy
C_bat = E_autonomy ÷ (DoD × η_discharge × F_EOL × F_temp)
E_autonomy is the energy consumed during the required autonomy interval. For a constant load, it is power multiplied by autonomy hours. DoD is allowable depth of discharge, η_discharge covers the defined battery-to-load path, F_EOL is the retained capacity required at end of life, and F_temp accounts for capacity available at the design temperature. Factors already included in a manufacturer’s usable-energy rating should not be applied twice.
A battery’s kWh rating does not confirm that it can serve the site. Check continuous and peak discharge power, BMS current limits, low-temperature charge restrictions, high-temperature derating, minimum state-of-charge reserve and the actual DC voltage window. For a nominal DC system, Ah = Wh ÷ V is only a screening conversion; current and capacity should be verified across the operating voltage range.
End-of-life capacity deserves an explicit line in the calculation. If a site needs the full autonomy at the end of the planned service period and the selected battery is expected to retain 80% of initial capacity, divide the beginning-of-life requirement by 0.80 before choosing modules.
Worked screening example
Consider a hypothetical telecom site with a measured average demand of 1.2 kW over 24 hours. Assume 3.5 design peak-sun hours, a 0.78 PV-side derating factor, 85% downstream energy-path efficiency and a separate 15% planning margin. The battery must provide 1.5 days of autonomy, with 80% allowable depth of discharge and 93% discharge-path efficiency.
| Step | Calculation | Screening result |
|---|---|---|
| Delivered daily load | 1.2 kW × 24 h | 28.8 kWh/day |
| Required source energy | 28.8 ÷ 0.85 × 1.15 | 39.0 kWh/day |
| PV array | 39.0 ÷ (3.5 × 0.78) | 14.3 kWp |
| Autonomy load energy | 28.8 × 1.5 | 43.2 kWh delivered |
| Battery before EOL allowance | 43.2 ÷ (0.80 × 0.93) | 58.1 kWh nominal |
| Initial battery for 80% EOL retention | 58.1 ÷ 0.80 | 72.6 kWh nominal |
The 85% downstream efficiency and 0.78 PV derating describe different parts of the system. Keeping those boundaries separate avoids double counting. The 72.6 kWh result also assumes that the project requires the stated autonomy at 80% retained capacity. If autonomy is only a beginning-of-life requirement, the procurement rule may differ.
| Changed assumption | Result before module rounding | What it shows |
|---|---|---|
| 3.0 design peak-sun hours | 16.7 kWp PV | A weaker solar month increases array size |
| 4.0 design peak-sun hours | 12.5 kWp PV | Better resource reduces the screening array |
| 1 day autonomy | 38.7 kWh battery before EOL allowance | Storage changes almost directly with autonomy |
| 2 days autonomy | 77.4 kWh battery before EOL allowance | Long autonomy can dominate cost and footprint |
Check recovery time and DC current
Autonomy answers how long the battery can support the load. Recovery answers whether the system can restore that reserve before the next outage or poor-sun interval. During recovery, the telecom load continues to run, so available source power must serve the live load and charge the battery at the same time.
In the example, restoring 43.2 kWh of delivered autonomy energy within eight hours would require about 5.8 kW of charging input if charging-path efficiency is 93%. Add the continuing 1.2 kW site load and the source must supply more than 7.0 kW before other losses and operating headroom. Solar variability may require a larger array or a longer recovery window. A generator must also remain within its approved loading range and the battery’s permitted charge current.
Current is equally important. On a confirmed nominal 48 V bus, the 1.2 kW load represents roughly 25 A before losses. A simplified 7.0 kW combined load-and-charge condition is about 146 A at 48 V. These are screening values: actual current changes with bus voltage, efficiency, charge control and equipment limits. Verify cable ampacity, voltage drop, terminals, fuses, breakers, contactors, rectifiers and BMS limits at the worst operating point.
Validate the whole system
- Run hour-by-hour state-of-charge analysis for the design months and consecutive low-sun periods.
- Test measured peak load, startup demand, traffic growth and cooling demand under high ambient temperature.
- Confirm autonomy and recovery at beginning of life and at the specified end-of-life capacity.
- Check battery charge and discharge limits across enclosure temperature and state of charge.
- Verify PV string voltage over the full module temperature range and controller operating window.
- Check generator start reliability, fuel autonomy, minimum loading and recharge runtime in hybrid designs.
- Coordinate protection and verify cable voltage drop, grounding, lightning and surge protection.
- Review mounting structure, wind, snow, corrosion, dust, drainage, access and theft protection.
- Define controller failure states, local fallback operation, alarm ownership and remote-access permissions.
- Record every data source, efficiency boundary and margin in the controlled engineering file.
For telecom power architecture and resilience principles, ITU-T Recommendation L.1210 is a useful current reference. The final design should also follow the network operator’s specification, equipment instructions and the electrical, structural and fire-safety requirements that apply at the project location.
Huijue can prepare a preliminary energy model and equipment proposal from site data. Because solar resource, operating policy and approval requirements vary by country and operator, final ratings should remain subject to project-specific engineering and local review.
Need a site-specific energy model?
Prepare the site coordinates, load profile, autonomy target, voltage interface, climate limits, available area and grid or generator history.
Send the project data through your established Huijue sales or engineering contact.