Guide

Power on a Private Island: Solar, Storage and Generators

Electricity on an island is a small utility that you own outright. There is no grid to lean on, no meter to read, and no supplier to blame — only a system you have sized, built and must keep running. Done well it is quiet, largely solar, and barely noticed. Done poorly it is a diesel engine that never stops. This is an orientation to sizing it well.

Begin with the load

Every credible power design starts from an honest daily energy figure, measured in kilowatt-hours, and a peak demand figure, measured in kilowatts. The two are different questions: kilowatt-hours decide how much you must generate and store over a day; kilowatts decide how much must be available in the instant a pump, an air-conditioner and an oven coincide.

A modest island residence might draw 10 to 20 kWh a day. A comfortable home with air-conditioning, full appliances and a water system runs 30 to 60 kWh a day. A larger property with pools, extensive cooling, staff quarters and — critically — a desalination plant can pass 100 kWh a day without difficulty. Two loads dominate on a tropical island and deserve naming: cooling and water. Air-conditioning is frequently the single largest consumer, and desalination, at 3 to 5.5 kWh per cubic metre of fresh water, can add tens of kilowatt-hours daily on its own, as our guide to island water supply sets out.

The cheapest kilowatt-hour is the one you never need. On an island, efficiency is not a virtue but an economy — every watt shaved is generation, storage and fuel you never have to buy.

This is why the first move in power design is reduction, not generation: efficient inverter air-conditioning, LED lighting throughout, high-efficiency pumps, good insulation and shading, and heavy loads timed to run in the solar midday. A load trimmed by a third shrinks the array, the battery and the generator together.

Solar: the primary source

On almost every island, solar is the sensible backbone. Sizing the array is a matter of dividing the daily energy need by the site's peak-sun-hours — the number of hours of full-strength equivalent sunlight, typically 4 to 6 in the tropics — and then adding 20 to 30 per cent for the real-world losses of inverters, wiring, heat, dust and salt haze.

The arithmetic is direct. A property needing 40 kWh a day at 5 peak-sun-hours requires about 8 kWp of panels before losses, and closer to 10 kWp once losses are allowed for. Because island surfaces are salt-laden and often humid, marine-grade mounting, corrosion-resistant fixings and generous ventilation behind the panels matter more than on the mainland, and they are cheap to specify at installation and costly to retrofit.

Storage: the heart of the system

Solar without storage is daylight-only power. The battery is what turns an array into a utility, and on an island it is usually the most expensive single element. It is sized from two numbers: the daily energy to be carried through the night, and the days of autonomy — the number of consecutive cloudy days the system should ride out without the generator. One to two days of autonomy is typical for a solar-plus-generator hybrid; more where the generator is a genuine last resort.

Modern lithium iron phosphate batteries tolerate deep discharge — 80 to 90 per cent usable — so a property needing 40 kWh overnight, with one day of autonomy, needs roughly 45 to 55 kWh of installed lithium capacity. The same design on older lead-acid chemistry, limited to about half its capacity, would need close to double, which is why lithium has become the island default despite its higher upfront price.

As a rough cost frame, installed off-grid systems land near USD 12,000 to 20,000 for a small cabin-scale setup, USD 15,000 to 35,000 for a mid-sized home, and USD 40,000 to 80,000 and beyond for a full-home system — before the island premium on shipping and installation labour discussed in our guide to building on a private island. Treat these as orientation; the desalination and cooling loads push guest-scale properties well past the top of the range.

The generator: backstop, not backbone

Even the best solar-and-battery system keeps a generator, and the design intent is that it runs rarely — through a long cloudy spell, an unusual peak, or a maintenance window. A diesel or, increasingly, an HVO-fuelled genset earns its place as insurance.

Its fuel appetite is worth knowing precisely, because fuel is the recurring cost and the logistical burden. A modern diesel genset consumes roughly 0.20 to 0.30 litres per kWh generated — equivalent to about 3.5 to 5 kWh per litre — but only near its efficient load band of 50 to 80 per cent of rating. Run it lightly loaded, below about 30 per cent, and consumption per kWh climbs while the engine begins to "wet-stack" and foul. The two implications for sizing are counter-intuitive: do not oversize the generator, because a large engine idling under a small load is both wasteful and self-damaging, and prefer to let the battery absorb short peaks so that when the generator does run, it runs loaded and briefly.

HVO — hydro-treated vegetable oil — is a drop-in diesel substitute that burns cleaner and stores well, and it is worth considering where supply exists. The direction of travel is unambiguous: one island resort cut its diesel use by two-thirds simply by adding a 200 kW solar array and 500 kWh of storage to an existing generator, dropping daily fuel from around 74 gallons to 20. The generator did not disappear; it moved from backbone to backstop.

The hybrid system, and fuel logistics

The mature island power system is a hybrid: solar supplies the day and charges the battery; the battery carries the night and the peaks; the generator covers the residual gap and recharges the battery in prolonged poor weather. A single hybrid inverter-charger orchestrates the three, prioritising solar, then stored energy, then fuel, and this hierarchy is what keeps annual diesel consumption — and its cost and its barge trips — low.

Fuel logistics deserve explicit design. Every litre of diesel arrives by boat or barge and must be stored safely on an island, which means bunded tanks, sound handling and a delivery schedule aligned to weather windows — the same discipline covered for materials in our guide to island access and transport. The less the generator runs, the fewer these deliveries, which is another way of saying that money spent on solar and storage buys down fuel logistics as much as fuel cost.

• Size the array from daily energy divided by peak-sun-hours, plus 20–30% for losses.
• Size the battery for overnight load plus one to two days of autonomy.
• Keep the generator right-sized to run loaded and briefly, never idling lightly.
• Design the desalination and cooling loads in from the start — they dominate.
• Store fuel in bunded tanks and align deliveries to weather windows.

Cooling, and designing the load down

Because cooling is so often the largest load, it is where efficiency pays most. High-efficiency inverter air-conditioning, ceiling fans that let thermostats sit higher, deep shading, cross-ventilation and insulation all cut the cooling kilowatt-hours that would otherwise size the whole system upward. Scheduling heavy discretionary loads — water-making, pool pumps, laundry — into the solar midday further flattens the demand the battery and generator must cover. On an island, architecture and energy are the same conversation, which is why power should be planned alongside the build rather than bolted on after, a theme our note on island insurance and resilience extends into storm survival.

Connectivity as infrastructure

Power's near-twin is connectivity, and on a modern island it is no longer a luxury but part of the infrastructure that makes the place liveable and, where relevant, lettable. Low-earth-orbit satellite service of the Starlink class has changed the calculus entirely: a maritime-grade terminal costs on the order of USD 2,500 to 5,000 as hardware, with monthly service ranging from around USD 250 for lighter use to USD 1,000 or more for high-demand offshore plans, delivering tens to a couple of hundred megabits per second in places that until recently had nothing.

Two points make it infrastructure rather than an accessory. First, it draws power, and its load — modest but continuous — belongs in the energy budget from the outset. Second, it wants redundancy: a second terminal or a backup path matters where connectivity underpins security systems, guest expectations or a working household. Treated as infrastructure and designed into the power system, it is transformative; treated as an afterthought, it is the thing that fails in bad weather when it is most wanted.

Power, water and connectivity together define whether an island functions as a home or merely a landing. When a specific island is under consideration, we work these systems through in the Acquisition Brief and record what is known of them in the Island Dossier; the broader cost picture sits in our guide to private island costs, and an island can be brought to us through Register an Island.

General orientation, not professional engineering, legal or tax advice. Enquiries: the enquiry form.