Sizing solar plus storage: the questions that decide the design
Before choosing panels or batteries, start with the load, the goal, and the grid connection. How an engineer works through a PV-and-storage design.
The most common mistake in solar design is starting with the product: "We want 100 kWp." A good design starts with questions. The answers decide the size, the layout, and whether storage earns its place at all.
1. What does the site actually use?
Everything starts with the load profile. Interval data — ideally a full year at 15-minute or hourly resolution — shows when energy is used, how high the peaks go, and how seasonal it is. A pumping station that runs at noon and a cold store that runs at night need very different systems, even at the same annual consumption.
2. What is the goal?
Different goals give different designs:
- Lower energy cost by maximizing self-consumption
- Resilience — keep critical loads running through outages
- Peak shaving — cut the highest demand peaks where tariffs penalize them
- Energy autonomy where the grid is weak or absent
- Powering a process such as irrigation, treatment, or cooling at the times it needs to run
Write the goal down, because it also defines how success is measured later.
3. What does the site allow?
Roof or land area and structure, shading from buildings, trees, and terrain, orientation and tilt, cable routes, and — critically — the grid connection: available capacity, export limits, and the distribution operator's technical requirements. A design that ignores the connection point can be perfect on paper and impossible to energize.
4. How much will it produce?
Yield depends on location, tilt, orientation, shading, temperature, and losses. As a rough order of magnitude, a well-oriented system in Albania often produces somewhere between 1,300 and 1,600 kWh per kWp per year — but rough is the key word. A site-specific simulation (for example with PVGIS or professional modelling software) should replace rules of thumb before any commitment.
Details that matter:
- Temperature. Crystalline silicon modules lose roughly 0.3–0.4% of power per °C above 25 °C cell temperature, so hot roofs under-deliver relative to the datasheet.
- DC/AC ratio. Installing more panel capacity than inverter capacity (commonly around 1.1 to 1.3) is normal and economical; the occasional midday "clipping" costs little energy and lowers the price per kWh.
- Degradation and soiling. Plan for slow annual output decline and for cleaning in dusty or agricultural settings.
5. Does storage earn its place?
Batteries are valuable when there is a clear job: shifting midday solar into the evening, riding through outages, shaving peaks, or smoothing generation. They are a poor fit when the load already follows the sun.
Sizing needs two numbers, not one:
- Energy (kWh): how much you must shift or hold. Account for the usable window — manufacturers specify how deep the battery may be cycled — and for round-trip losses, which for lithium iron phosphate (LiFePO₄) systems are typically in the low-to-mid 90% range at the battery and somewhat lower for the whole system.
- Power (kW): how fast it must deliver. A 100 kWh battery limited to 0.5C can deliver 50 kW; if the load is 80 kW, the battery alone is not enough.
We favor LiFePO₄ for its long cycle life, thermal stability, and safety profile. Whatever the chemistry, ask for evidence of conformity to relevant safety standards for stationary storage (for example IEC 62619 and IEC 63056) and transport testing (UN 38.3), and look closely at thermal management, enclosure design, and fire protection.
6. What standards frame the design?
Commonly referenced: IEC 61215 and IEC 61730 for module qualification and safety, IEC 62109 for inverter safety, IEC 62548 for array design, IEC 62446-1 for documentation, testing, and commissioning, and EN 50549 for requirements on generating plants connected to distribution networks. Local connection rules from the grid operator come on top.
7. How will you know it works?
A system is not finished at commissioning. Track the performance ratio — actual output compared with what the measured sunshine should have produced (IEC 61724-1) — plus availability, battery health, and self-consumption. A falling trend is an early warning: soiling, a failed string, a tripped inverter. Monitoring that makes the cause visible turns a lost month of production into a same-week fix.
Energy generation is easy to buy. Energy that fits the site, the grid, and the goal has to be designed.
This is why every Greecon installation connects solar, storage, and loads to one Greecon Platform — so the design assumptions can be checked against what actually happens.