Off-grid solar system specification for Cyprus

An off-grid system in Cyprus is specified against the worst week of the year, not the average one. Every other decision follows from that: the array is sized on December daily yield, the battery on how many consecutive overcast days the property has to ride through without a generator, and the inverter on the largest simultaneous load rather than on the annual consumption figure. A system sized against annual averages looks correct on the quotation and runs flat in January.

The specification in plain terms

Four numbers define an off-grid system. Everything else on the quotation is a consequence of them, and a proposal that does not state all four has not been designed.

Daily energy demand, in kWh, for the worst month. Not the annual bill divided by 365. Winter demand on an off-grid property is usually higher than summer demand once lighting hours, water heating and any resistive heating are counted, and it arrives in the month when generation is lowest.

Peak simultaneous load, in kW, with the surge behind it. A borehole pump, a compressor and an oven starting within the same second is the number the inverter has to survive. Continuous rating and surge rating are separate figures on the datasheet and both belong in the specification.

Autonomy, in days. How long the property runs on stored energy alone with no useful generation. Two to three days is the usual design target for a permanently occupied property. A weekend house can be specified lower; a property running refrigeration for stock or livestock water cannot.

Worst-month yield per kWp. This is where Cyprus differs sharply from northern Europe in one direction and behaves less generously than people assume in the other. Annual irradiance is excellent. Seasonal spread is still real: December daily yield per kWp runs at roughly 40 per cent of the June figure, and low winter sun angles put the array behind obstructions that clear it without trouble in summer. Size the array on December and the system carries the rest of the year comfortably. Size it on the annual mean and it spends the winter leaning on a generator.

The DC bus and the battery

48 V nominal DC is the standard bus for a whole-property system. 12 V and 24 V remain reasonable for cabins, pump sets and instrumentation, but at whole-property loads they force conductor sizes that cost more than the voltage step would have.

Battery capacity has to be quoted as usable, not nameplate. Lithium iron phosphate is normally specified at 80 to 90 per cent depth of discharge; lead-acid at 50 per cent if it is to reach its rated cycle life. A quotation comparing a lithium nameplate figure against a lead-acid nameplate figure is comparing two different things, and the gap between them is where most off-grid disappointment originates.

The charge controller and the inverter-charger

MPPT rather than PWM is standard practice at any meaningful array size, and the controller's maximum PV input voltage has to be checked against the array's open-circuit voltage at the coldest expected temperature — a mountain site above Polis Chrysochous can see a winter morning cold enough to push string voltage above what the summer calculation predicted.

The inverter-charger specification needs continuous output, surge output and duration, charger current, and the generator input rating with its transfer behaviour. If a generator forms part of the design, whether it auto-starts on a state-of-charge threshold or has to be started by hand is a specification decision, not an afterthought.

Standards that apply in Cyprus

An off-grid property has no meter, and that leads people to assume it sits outside the rules. It does not. The fixed electrical installation downstream of the inverter is a fixed installation like any other: it has to be carried out and certified by a registered electrical contractor, and where a design certificate is required the engineer signing it should be on the ETEK register. That register is public and worth checking before signature rather than after.

Where a grid connection exists or could later be applied for, the Electricity Authority of Cyprus governs what may be connected and under what conditions. A system built as off-grid today that might be tied in later should be specified with that transition in mind, because retrofitting a compliant connection to equipment that was never intended for it is the expensive way round.

Three further standards matter more here than the datasheets suggest:

• Structural and wind loading on the mounting. Ground-mounted arrays on open agricultural land take wind loads that a rooftop array partly avoids. The ballast or foundation calculation belongs in the documentation set.
• Corrosion class of the fixings. On the coast, and Kato Paphos is the clearest case, salt-laden air makes galvanised and marine-grade fixings standard rather than optional. Stainless of the wrong grade fails at the bolt before the frame shows anything.
• Battery room construction and ventilation. Separation, fire behaviour and temperature control are part of the specification, not site decisions taken on the day.

Common specification errors

Sizing on the annual average. The single most common error, and the reason a January generator run appears in the second winter of ownership.

Quoting nameplate battery capacity. Usable capacity at the specified depth of discharge is the only figure that means anything operationally.

Ignoring plant-room temperature. Lithium iron phosphate loses cycle life when held above 35 °C. A battery cabinet in an uninsulated store on the western wall of an inland property is above that for weeks at a time. Inland Nicosia sees the widest temperature range on the island, which drives insulation and shading specifications, and inland agricultural sites inherit the same conditions. Ventilation, shading or active cooling of the battery space is a line item.

Sizing the inverter on running load. Motor starting current is several times running current. Borehole and irrigation pumps are the usual culprits, and the symptom is a system that works all summer and trips the first time two things start together.

Long DC runs left uncalculated. Agricultural sites put the array where the land allows and the loads where the buildings are. Voltage drop across that distance is a real loss with a real cost, and the fix — larger conductor, higher string voltage, or moving the conversion closer to the array — is much cheaper at design stage.

No winter shading survey. A survey walked in June sees a different sun path from the one that governs December output.

Access not checked before ordering. Battery cabinets and inverter racks are heavy. On the hillside plots around Peyia, access roads limit crane and delivery vehicle size, and an item that cannot reach the plant room is not a specification problem that gets solved on delivery day.

How to check what was installed

Ask for the document set before the final payment, not after. A properly executed off-grid installation hands over:

• A single-line diagram showing array strings, isolators, controller, battery, inverter and the distribution board, with conductor sizes and protection ratings marked.
• The electrical installation certificate, signed by the registered contractor who carried out the work.
• A commissioning record: measured string voltages and currents, battery state of charge at handover, inverter firmware version, and the BMS parameter set as configured.
• Battery module serial numbers and the warranty registration confirming they are logged with the manufacturer.
• The generator interface settings, if one is fitted, including the state-of-charge thresholds for auto-start and stop.

Then check three physical things. Every DC isolator should be labelled and reachable without moving equipment. The battery space should have the ventilation path the design assumed, actually open, not blocked by stored goods. And on a coastal site, the fixings on the array frame should match the corrosion class the specification called for — that one is visible from the ground with binoculars a year later, when the galvanising either still looks like galvanising or does not.

Finally, log a week of operation in December. Compare measured daily yield per kWp and end-of-day state of charge against the design assumptions. If the installer cannot tell you what those assumptions were, the system was sold rather than designed. Our off-grid solar section covers system types in more depth, the services index lists the other technical guides on this site, and the area pages set out which contractors cover which districts — worth checking on remote sites, since distance from the main contractor base in Paphos adds travel time to most quoted day rates and to every service call afterwards.

Common questions

How much battery capacity does an off-grid property in Cyprus need?

It follows from daily demand in the worst month multiplied by the autonomy target, then divided by the usable fraction of nameplate capacity. Two to three days of autonomy is the usual target for permanent occupancy. A property with a generator and an owner content to run it can be specified lower; a property with no generator and no tolerance for a dark evening cannot.

Is a generator still needed if the array is oversized?

Usually, yes, as insurance rather than as a routine energy source. Oversizing the array shortens the annual hours a generator runs but does not remove the consecutive-overcast-days case that sets the battery size. The design question is how much array is worth buying to avoid how many generator hours.

Does an off-grid installation need certification if there is no meter?

The fixed electrical installation does. Absence of a supply connection does not remove the requirement for the work to be carried out and certified by a registered electrical contractor, and where a design certificate applies the signing engineer should be on the ETEK register.

What temperature does the battery space need to be held at?

Below the point where cycle life degrades. Lithium iron phosphate suffers when held above 35 °C, which an unventilated store on an inland site exceeds for long stretches of summer. Ventilation and shading are usually enough; active cooling is specified where the room cannot be kept below that by passive means.

Can an off-grid system be converted to grid-tied later?

Sometimes, and it is far cheaper when the possibility was in the original specification. The inverter has to be one the Electricity Authority of Cyprus will accept for connection, the metering and protection arrangement has to be compatible, and the application process runs on its own timetable. Specifying for the possibility costs little at the outset and a great deal retrospectively.

What is the most useful single question to put to an installer?

Ask for the modelled December daily yield per kWp and the assumptions behind it — tilt, azimuth, shading and derating. A number without those assumptions is a marketing figure, and an installer who cannot produce them has not modelled the site.

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