How is an off-grid system sized?

An autonomous photovoltaic system that shuts down at night or fails to start a pump is not a "cheap solution." It's a misguided investment. How an autonomous system is sized determines whether a residence, a holiday home, an agricultural installation, or a business will have stable electricity 365 days a year or will depend on a generator and constant compromises.

Proper planning doesn't start with the wattage of photovoltaic panels or a battery that "looks big enough." It starts with actual consumption, instantaneous peaks, the project's location, winter use, and the level of autonomy you want to ensure. These data are converted into specific choices for panels, batteries, inverters, chargers, and protections.

What proper autonomous system sizing means

Sizing is the technical calculation of individual components to ensure they work together without limitations and unnecessary cost. The goal is not just to produce enough kilowatt-hours on a sunny day. The goal is to cover consumption under the most adverse operating conditions that are acceptable to the user.

For a holiday home used only from April to October, the study can be very different from that of a permanent residence in mountainous Greece. Similarly, a well pump has high motor starting requirements, while an office may have low average consumption but zero tolerance for power interruptions for computers, telecommunications, or refrigerators.

Oversizing increases initial capital and can leave equipment unused. Undersizing, however, usually costs more: deeper battery discharges, load limitations, generator use, and a shorter system lifespan.

Step 1: Recording consumption and load profiles

Daily consumption is expressed in kWh. It is calculated by the power of each appliance multiplied by its operating hours. A 120 W refrigerator operating on average 8 hours a day consumes approximately 0.96 kWh. A 100 W LED lighting for 5 hours adds 0.5 kWh.

Simply adding up appliances is only the beginning. It is necessary to consider when each load operates. Midday consumption can be directly supplied by photovoltaics, while at night it will be almost entirely covered by the battery. That's why the daily total is not enough on its own.

Special attention is required for appliances with motors and heating elements: pumps, refrigerators, pressure washers, tools, air conditioners, water heaters, and electric stoves. A motor can demand many times its nominal power for a few seconds. This peak does not significantly increase daily kWh, but it can determine the choice of inverter.

The most reliable approach is to use electricity bills, smart meter readings, or detailed recording per appliance. For new buildings or installations without consumption history, the study is based on a real usage schedule and technical load sheets, not on rough estimates.

Step 2: Choosing days of autonomy

Days of autonomy show how long the system can operate when sunlight is limited. For seasonal use, a smaller energy reserve is often sufficient. For permanent residences, critical professional applications, or areas with increased cloud cover, the reserve must be larger.

Let's assume a daily consumption of 8 kWh and a target of two days of autonomy. The useful energy that storage must have is 16 kWh. In practice, the nominal capacity must be greater because we take into account the permissible depth of discharge, losses, and desired lifespan.

LiFePO4 batteries typically utilize a higher percentage of their nominal capacity compared to older lead-acid technologies. However, this does not mean they should be designed for daily full discharge. Proper adjustment of the depth of discharge and charging protects the investment and maintains available reserve.

Step 3: Calculating batteries based on useful energy

Battery capacity is not chosen solely in kWh. The power it can deliver continuously or instantaneously is also checked. A battery may have sufficient capacity but may not support the current required for the simultaneous operation of a pump, an air conditioner, and other loads.

In the example of 16 kWh of useful energy, if an operational depth of discharge of 80% and total losses of approximately 10% are anticipated, the nominal storage must approach or exceed 22 kWh. The final choice depends on the usage profile, the voltage system, BMS limits, and the possibility of future expansion.

Modular LiFePO4 batteries are particularly practical when consumption is expected to increase. The installation can start with the correct basic capacity and be expanded with compatible modules, provided that the initial architecture has foreseen it.

Step 4: How many photovoltaic panels are needed

Photovoltaic power must cover daily consumption, conversion losses, and the energy required to recharge the batteries after days of reduced production. Efficiency is not constant throughout the year. Tilt, orientation, shading, temperature, and local solar radiation directly affect the result.

A system with a daily consumption of 8 kWh is not simply sized by dividing 8 kWh by the theoretical hours of sunshine. A loss factor is needed for inverters, wiring, MPPT, temperature, panel pollution, and battery charging. For safe operation, the study primarily examines the months with the lowest available production, provided that use is annual.

In many cases, additional photovoltaic power is more economical than excessively increasing battery capacity. The balance depends on whether the load is consumed day or night and how critical autonomy is on consecutive cloudy days.

Step 5: Choosing an inverter for continuous power and peaks

The inverter is the heart of the autonomous system. It is chosen based on the maximum simultaneous power of the loads, as well as its ability to handle starting peaks. A 5 kVA inverter is not automatically suitable for every installation with nominal loads close to 5 kW.

Continuous power, overload for a specific time, waveform quality, battery compatibility, and the possibility of parallel or three-phase operation must be considered. In demanding applications, a premium inverter with proper parameterization offers better load management, reliable charging, and detailed system monitoring.

For pumps and large motors, a soft starter, motor inverter, or different operating strategy may be required. It is not always correct to simply increase the central inverter. Reducing the starting peak often leads to a more efficient and economical overall solution.

Shading, seasonality, and the generator change the study

Two identical houses can require different autonomous systems. A small amount of shading in the morning or evening can significantly reduce production, especially when the strings and MPPTs are not properly designed. On-site assessment is essential before locking in panel power and mounting arrangement.

Seasonality is equally critical. If the holiday home is used in August, production is usually abundant. If it is used on December weekends with heating, lighting, and cooking, the requirements change drastically.

In some projects, a generator as a backup is more logical than designing for absolute coverage of every extreme winter week. This solution does not reduce the value of autonomy. It allows for a more economical balance of equipment to be chosen, provided that the generator is properly integrated into the design and activated only when needed.

Why free technical study prevents expensive mistakes

Reliable sizing combines measurements, technical calculations, and real operating conditions. It does not result from a ready-made "package" of the same size for every home or commercial installation. The quality of batteries, inverter and BMS compatibility, DC and AC fuses, grounding, and remote monitoring capabilities are part of the same decision.

At Hellenic Energy, the free technical study is based on the loads and goals of the project, so that the proposal provides measurable autonomy and not theoretical promises. When you know what you consume, when you consume it, and how much reserve you need, you can invest in a system that substantially reduces energy costs and operates reliably for years.