What an off-grid solar system is
A off-grid or off-grid solar system is a complete, closed electrical installation. It produces, stores and distributes energy with no connection to the public grid. There is no grid as backup, no offsetting, no electricity bill – and precisely for that reason the design has to be exactly right.
The system consists of four main subsystems: the solar panels that generate the energy, the charge controllers (MPPT) or the hybrid inverter that manage charging, the battery bank that stores it, and the inverter which converts the stored direct current into 230V alternating current for household or commercial appliances. On many projects we also add backup generator as a fifth subsystem.
The fundamental difference from a grid-connected system is the sizing philosophy. In a system Net Billing an undersized solar system simply means smaller savings. In an off-grid system it means a power cut. This is why the study for an off-grid project is far more demanding: it has to cover even the worst period of the year.
A well-designed off-grid system ultimately gives you something no grid-connected system can: full independence. No dependence on tariffs, on grid faults, on bureaucracy or on when the line extension will arrive.
The five subsystems
Where off-grid solar is used
Each application category has a different load profile and different reliability requirements – and therefore a different design.
Off-grid homes
Permanent homes off the grid, where extending a line would be prohibitively expensive. They require design for year-round use, with sufficient winter autonomy and usually a backup generator.
Villas & holiday homes
Luxury homes with air conditioning, a pool, pumps and high-power appliances. Often seasonal use with summer peaks – an ideal case for solar autonomy.
Islands
Small islands and remote coastal locations with a weak, saturated or non-existent grid. Excellent solar potential, but they require corrosion protection for the equipment.
Farms
Pumping and irrigation, cold rooms, milking equipment, greenhouses, electric fencing. Consumption coincides with daylight hours, reducing storage requirements.
Telecommunications
Base stations, repeaters, meteorological and hydrometric stations. A steady 24/7 load and zero tolerance for interruption – they need extra autonomy and backup circuits.
Cabins & site huts
Guard posts, mountain refuges, site cabins, pumping stations, monitoring stations. Small, standardisable systems with an emphasis on simplicity and durability.
Batteries: the heart of an off-grid system
In an off-grid system the battery is not an accessory – it is the component that decides whether you have power at three in the morning. It is also, typically, the most expensive subsystem and the one with the shortest service life. Choosing the technology and capacity is the most critical decision in the project.
LiFePO4 lithium
The leading technology for off-grid systems. A very high cycle count, deep discharge, high charging efficiency, small volume and weight, integrated BMS. Higher initial cost, but significantly lower cost per stored kilowatt-hour over time.
GEL / AGM (VRLA)
Proven deep-discharge lead technologies, with a lower initial cost. They require a more conservative depth of discharge and have fewer life cycles. They remain a sensible choice for small, seasonal-use or tightly budgeted systems.
What we look at when sizing the array
- ▸Usable capacity – not the nominal figure. We take into account the permitted depth of discharge of the technology.
- ▸Charge & discharge current – the bank must be able to handle the momentary peaks of the loads.
- ▸Temperature environment – temperature has a decisive effect on capacity and ageing.
- ▸Array voltage – 24V or 48V depending on size, to reduce losses and cable cross-sections.
- ▸Expandability – the ability to add modules later, without replacing the whole bank.
Load calculation: the first and most important step
Every off-grid system starts from a load schedule. We record each appliance, its power, its hours of operation per day and its seasonality. The total gives the daily energy requirement in kWh – the foundation of every calculation that follows.
| Load | Capacity | Hours / day | Energy / day |
|---|---|---|---|
| Fridge (A+++) | 150 W | 8 (equivalent) | 1,2 kWh |
| Inverter air conditioner | 900 W | 6 | 5,4 kWh |
| LED lighting | 120 W | 5 | 0,6 kWh |
| Water pump | 750 W | 1,5 | 1,1 kWh |
| Other appliances & standby | — | — | 1,7 kWh |
| Total daily consumption | ≈ 10 kWh |
An indicative example to illustrate the method. Beyond daily energy, we also record the maximum simultaneous power (for sizing the inverter) as well as the inrush currents inductive loads such as pumps, compressors and power tools – loads that can momentarily demand several times their rated power.
Autonomy calculation & system sizing
Autonomy is the number of days the system can cover the loads with no sunshine at all. It is the figure that separates a serious study from a rough estimate.
Daily energy
From the load schedule, by season. Separate calculation for winter and summer.
Days of autonomy
Typically 1-2 days for seasonal use with a generator, 2-4 for a permanent home, more for critical installations.
Battery capacity
Daily energy multiplied by days of autonomy, divided by the permitted depth of discharge and the efficiency of the system.
Solar capacity
Calculated to cover daily consumption and to recharge the bank in the worst season.
Inverter capacity
Based on maximum simultaneous power and inrush currents – with sufficient headroom for peaks.
Checked by simulation
An annual hourly simulation to confirm there is no period with an energy deficit.
The winter rule: an off-grid system is designed for the December, not for July. In Greece, winter daily production can be less than half the summer figure. If the system is intended for year-round use and is sized on the summer months, it will fail in January. Alternatively, a smaller system is combined with a backup generator for the few unfavourable days – often more economical than heavy oversizing.
Generator backup & hybrid operation
Backup generator
A generator is not an admission of failure – it is a cost optimisation tool. Rather than sizing panels and batteries for the 10 worst days of the year, we design the solar system for realistic conditions and cover the extreme days with a few hours of generator running time.
On modern systems the generator works together automatically with the inverter: when the battery level drops below a threshold, the inverter starts the generator, charges the bank at the optimum current, supplies the loads at the same time and shuts the generator down as soon as the desired level is reached. No manual intervention is required.
- ✓Automatic start/stop based on the state of charge
- ✓Time windows of operation to avoid noise at night
- ✓Supporting load peaks alongside the inverter
- ✓A sharp reduction in the required array size
Hybrid operation
The term hybrid is used in two senses. The first concerns the hybrid inverter: a single unit that manages solar, batteries, loads and – optionally – the grid or a generator, under one priority logic.
The second concerns the hybrid system topology: an installation that is essentially off-grid, but keeps the grid or a generator as a second source. In practice this is the most resilient option for permanent homes: you get the autonomy of off-grid and the security of an alternative source.
When a grid connection is available, the hybrid solution is often combined with a zero export, so you get autonomy and backup without going through the offsetting process.
- Solar production to the loads
- Surplus to charging the batteries
- Battery to the loads when there is no sun
- Grid or generator when the battery is exhausted
Study, design & installation
Needs analysis
Load schedule, seasonality of use, critical loads that cannot tolerate interruption, future expansion. Setting the autonomy target and the acceptable risk.
Technical solution
Selection of topology and array voltage, sizing of panels, MPPT, inverter and batteries, single-line diagrams, study of cable cross-sections, protection devices and earthing.
Layout
Shading and horizon analysis, optimal orientation and tilt for winter output, battery room location with ventilation and thermal protection.
Construction
Mounting frames resistant to wind loading, DC cabling with correct cross-sections, DC/AC protection panels, lightning protection, installation of the array and inverter.
Settings & testing
Configuration of charging curves, generator thresholds and load priorities. Peak testing, measurements, Installer Declaration and user training.
Monitoring
Remote supervision of production, consumption and battery levels, with alerts. Critical on remote installations where physical access is difficult.
Off-grid system maintenance
An off-grid system needs a little but consistent maintenance. Because there is no grid as a safety net, one neglected detail can end in an outage. The Fotergeia maintenance programme includes:
Panel cleaning
Dust, salt and organic residues noticeably reduce production – critical when every kilowatt-hour counts.
Array inspection
Cell voltages, balance, temperatures, terminal tightness, cleanliness and ventilation of the space.
Electrical inspection
Insulation and earthing measurements, checks of fuses and surge arresters, and thermal imaging of connections.
Generator maintenance
Oil, filters, fuel, a test start. A generator that is not maintained will not start when you need it.
Firmware check
Inverter and BMS updates, re-checking of charging parameters and thresholds.
Data analysis
Comparison of actual against predicted performance, spotting ageing trends before they become faults.
Frequently asked questions about off-grid solar
Can an off-grid system cover air conditioning and a pool?
Yes, provided it is designed for them. Air conditioning and pool pumps are large loads, but they run mainly during hours of strong sunlight – which makes them well suited to solar supply. What matters is correct sizing of the inverter and the array for the peaks.
Do you need a permit for an off-grid system?
Because there is no grid connection, no connection application or offsetting contract is required. What is required is a technical study, compliance with electrical installation regulations and an Installer Declaration, while depending on the location and mounting method there may be planning or zoning requirements. We check these before design.
What happens after several consecutive cloudy days?
This is where the designed autonomy comes in. If the number of days exceeds the design, the backup generator takes over – or, in systems without a generator, the system prioritises critical loads according to the settings we have defined.
How long does an off-grid system last?
Panels typically 25+ years with a performance warranty, the inverter typically 10-15 years, lithium batteries usually more than a decade with correct management. In practice the battery bank is the subsystem that will be replaced first – and we design so that replacement is straightforward.
Can I start small and expand?
Yes, provided the original architecture allows for it: array voltage, inverter capacity and cable cross-sections are chosen from the outset with headroom. A cheap initial system with no provision for expansion usually costs more overall.
Planning an off-grid installation?
Send us a list of the appliances you want to supply and the location. We will send back a load schedule, an autonomy proposal and a detailed quotation.
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