Off-Grid Solar Power Systems in the UK: Sizing, Batteries and Rules

An off-grid solar system powers a building with no connection to the electricity network, using panels, a battery, an inverter-charger and usually a generator. In the UK it has to be sized for December, when PVGIS gives a south-facing panel under two-fifths of its June output in Devon and a fifth in the Highlands. It suits cabins, barns, boats and garden offices, rarely a house that could be connected.

This guide explains when off-grid makes sense, how to size panels and batteries with real winter data, what the wiring rules say about batteries and earthing, what the parts cost in November 2026 and how a worked example for a small cabin turns out in southern England and in northern Scotland. If you only need power for a shed or a camping trip, our comparison of solar panel kits in the UK covers ready-made 12 V kits and power stations.

When off-grid makes sense, and when it does not

Off-grid means no physical connection to a distribution network at all. Nothing is notified to a network operator, nothing is exported and no supplier bill arrives. It also means every kilowatt-hour you use in January must come from panels, a battery or fuel.

It makes sense where:

• There is no grid nearby. A remote cabin, bothy, farm building or island property where a new connection would mean a long cable or overhead line.
• The load is small or seasonal. A holiday cabin used mainly from spring to autumn, a workshop, a stable block with lights and an electric fence, a garden office with a laptop and router.
• The site moves. Narrowboats, yachts, campervans and horseboxes, where off-grid is the only option.

It rarely makes sense where:

• A house already has a grid connection. Disconnecting saves the standing charge but costs a very large array, several days of battery and a generator, and gives up export payments. A grid-tied system with a battery delivers most of the benefit at a fraction of the cost; see our UK solar installation guide.
• Heat is electric. Electric heating, a heat pump or an immersion heater in winter multiplies the system size. Off-grid homes in Britain typically heat with wood, oil or LPG and keep electricity for lights, appliances and pumps.
• The connection is cheap. Electricity North West, for example, quotes new single domestic connections from £1,300, with an average of £1,790 including VAT, for standard connections. Remote sites can cost much more, so ask the network operator for a quote before deciding.

A related question is power cuts. Grid-connected inverters are built to stop when the network fails, so they do not feed a dead line that engineers are repairing; for plug-in kits the government’s specification requires disconnection within 100 milliseconds. Keeping a grid-connected home running in a power cut needs a battery system with an island mode: IET guidance describes an island mode isolator that disconnects the house from the network, a switched neutral-to-earth bond and a consumer earth electrode. That is a different design from the off-grid systems in this guide.

What an off-grid system contains

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System voltage. Small kits run at 12 V. From about 1 kW of inverter power, 24 or 48 V systems are usual, because the current at 12 V becomes very large: a 3,000 W load draws about 280 A from a 12 V battery at 90 % efficiency, but about 70 A at 48 V. Higher current needs thicker cable and bigger fuses and wastes more energy in heat.

Battery chemistry. LiFePO4 has largely replaced lead-acid in new systems because it tolerates deep discharge and many more cycles and needs no topping up. Lead-acid (AGM, gel or flooded) remains cheaper per nominal kWh and copes better with charging in freezing conditions, but should be discharged only partly to last. A UK specialist listed a 12 V 100 Ah LiFePO4 battery (1.28 kWh) at £213.49 and a 12 V AGM battery rated 104 Ah at the 10-hour rate at £178.05, both before VAT, on 4 November 2026.

Winter is the design month: PVGIS data for southern England and Scotland

A grid-connected system is sized on annual yield. An off-grid system is sized on the worst month you intend to use it, because there is no grid to fill the gap. In Britain that month is December, and the gap between summer and winter is far wider than in southern Europe.

We ran PVGIS, the European Commission’s solar model, for 1 kWp of panels facing south with 14 % system losses, at the usual roof angle of 35 degrees and at a steep 60 degrees:

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Three lessons follow.

Tilt steeply. At 60 degrees, panels lose about 4 to 6 percent of annual yield but gain about 20 to 30 percent in December. For off-grid use the winter gain is what counts. Wall-mounted panels at 90 degrees go further in the same direction.

The north is a different problem. Near Inverness, December output per kWp is less than half that near Exeter, and at Fort William, where the surrounding hills shade the low sun, it is less than a third. PVGIS includes the terrain horizon by default, which is why a sheltered glen can do worse than its latitude suggests. The Fort William figures are for one point in the town; moving it a few hundred metres changed the December result by more than half in our runs, so run PVGIS for your own site rather than borrowing a neighbour’s number.

Averages hide runs of bad days. A December average of 1.36 kWh per kWp near Exeter includes bright frosty days and weeks of thick cloud. The battery has to carry you through the cloudy runs, which is why the simulation below matters more than the monthly table.

Sizing panels and battery with PVGIS off-grid simulations

PVGIS has an off-grid tool that simulates a battery day by day over many years of satellite weather data, about 19 years for the runs below. We modelled a small cabin that uses 1.5 kWh a day all year, with panels facing south at 60 degrees and a lithium battery that stops discharging at 20 percent. The results show the share of days on which the battery ran empty before the day’s use was covered.

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These are model results from historical weather, not guarantees; a particularly dull winter will be worse. Read them like this:

• Near Exeter, 2 kWp and 10 kWh run short on about one December day in twenty. Adding battery helps more than adding panels at this point: going from 5 to 10 kWh with 2 kWp cut empty December days from 15.6 to 4.9 percent.
• Near Inverness, the same system runs short on six December days in ten. It takes about twice the panels and half as much battery again (4 kWp, 15 kWh) to match the southern result.
• Summer waste is unavoidable. The “days full” column shows that from spring to autumn the battery is full most days and surplus energy is lost. Off-grid systems trade summer waste for winter adequacy, or rely on a generator in winter.

A quick rule for panels. Divide daily use by the December yield per kWp and add a margin of 20 to 30 percent for dirt, snow, ageing and battery losses. For 1.5 kWh a day near Exeter at 60 degrees: 1.5 ÷ 1.36 × 1.25 ≈ 1.4 kWp as an absolute minimum. The simulation shows why real systems go above that: the battery needs spare energy to recharge after dull spells.

A quick rule for batteries. Usable capacity = daily use × days of autonomy ÷ usable fraction. For 1.5 kWh, three days and 80 percent usable: 1.5 × 3 ÷ 0.8 ≈ 5.6 kWh nominal. In the north, where the simulation shows long runs of empty days, more autonomy pays only if the panels can refill it; otherwise a generator is cheaper.

Batteries: chemistry, autonomy and where to put them

Days of autonomy. Two to three days is a common target in the south with a generator as backup; three to five days in the north if you want to rely on the generator rarely. Beyond that, the battery spends most of the year full and its cost per useful kWh rises steeply.

Cold. Lithium batteries should generally not be charged below freezing unless they have built-in heating; check the data sheet. Batteries in an unheated cabin in a Scottish winter need an insulated enclosure or a location that stays above freezing, which in practice often means inside the living space or a heated plant room.

Fire safety. Lithium batteries store a lot of energy in a small box. BS 7671:2018+A4:2026, published on 15 April 2026, added a chapter on stationary secondary batteries; trade summaries describe it as Chapter 57, covering isolation, overcurrent protection, thermal control, ventilation by chemistry and a risk-based approach to location, including limiting installation in roof spaces. For homes, BSI’s PAS 63100:2024 sets out fire protection for battery systems in dwellings: it treats outdoors, away from habitable rooms, as the best place for a battery, rules out bedrooms, escape routes, cupboards opening into bedrooms, roof spaces and lofts, and requires interlinked fire detection where a battery sits in a rarely visited space. It is written for dwellings; for a cabin that is lived in, its principles are a sensible benchmark. On boats, the Boat Safety Scheme publishes its own lithium battery advice.

Mixing batteries. Do not combine batteries of different ages, makes or chemistries in one bank; the bank behaves like its weakest member. Plan the capacity from the start, or choose a modular system designed to be extended.

Inverter-chargers and generator backup

The inverter-charger is the heart of an off-grid system. It converts battery DC into 230 V AC for the cabin, charges the battery from a generator or shore supply, and switches between them automatically. Many models start a generator when the battery falls below a set level.

Sizing the inverter. Add up the loads that may run at the same time and allow for start-up surges: a water pump or fridge compressor can draw several times its running power for a moment. A 3 kVA unit covers a cabin with a fridge, pump, lights and small appliances; a kettle or induction hob pushes towards 5 kVA. Check the no-load power on the data sheet, because an inverter idling all day uses energy too: 15 W for 24 hours is 0.36 kWh, a quarter of our example cabin’s daily use.

MPPT controller. It must handle the panel array’s maximum voltage on a cold morning (open-circuit voltage rises as temperature falls) and its current at the battery voltage. A 150 V, 35 A controller suits about 2 kWp on a 48 V battery; 3 to 4 kWp needs a 60 to 70 A unit or two controllers.

Generator. In northern Britain a generator is part of the design, not a fallback. A small inverter generator such as Honda’s EU22i, rated at 1,800 W continuous with a 3.6 litre tank lasting about 3.5 hours at rated load, was listed at £1,095 on Honda’s UK store on 4 November 2026. Running it a few dozen hours a year costs far less than the extra panels and batteries needed to avoid it. Site it outdoors, away from windows and vents, because of carbon monoxide.

Worked example: a small off-grid cabin

This example is illustrative. The loads are assumptions; the yields and battery behaviour come from the PVGIS runs above; prices are list prices before VAT from UK specialist sellers on 4 November 2026, except the generator.

Daily use, all year.

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Heating, hot water and cooking use wood or LPG. Including them electrically would multiply the system several times.

Southern England, near Exeter. Four 505 W panels (2.02 kWp) at 60 degrees on a ground frame, a 48 V battery of two 4 kWh lithium modules (8 kWh), a 150/35 MPPT controller and a 3 kVA inverter-charger. The PVGIS runs bracket this system between 2 kWp with 5 kWh (empty on 15.6 % of December days) and 2 kWp with 10 kWh (4.9 %); with 8 kWh, expect something under one December day in ten short, covered by an hour or two of generator.

Northern Scotland, near Inverness. Eight 505 W panels (4.04 kWp) and four battery modules (16 kWh), a 150/70 MPPT controller and the same inverter-charger. The 4 kWp, 15 kWh run left the battery empty on 5.8 % of December days.

Parts cost before VAT.

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Add a generator (about £1,100), installation labour, any trenching between the array and the cabin, and battery enclosure or ventilation work. If an installer supplies and fits the system in a home, HMRC’s zero rate for energy-saving materials may apply until 31 March 2027; for a holiday cabin, workshop or barn, check whether it counts as residential accommodation under HMRC’s notice before assuming it.

What the numbers show. The same cabin costs roughly 70 percent more in parts in the Highlands, and the battery is the largest single item in both. Halving winter use, for example by switching the fridge off when the cabin is empty, does more for the budget than any component choice.

The rules: BS 7671, earthing, Building Regulations and planning

Wiring regulations

BS 7671, the IET Wiring Regulations, applies to fixed electrical installations whether or not they connect to the grid. The current version is BS 7671:2018+A4:2026, published on 15 April 2026; the IET says the previous edition was withdrawn six months later. Relevant parts for off-grid solar are:

• Section 712, solar photovoltaic power supply systems. MCS points installers to it for every PV system. In our reading many of its provisions are framed around grid-connected systems; how each one applies to a stand-alone design is a judgement for the electrician, and the general requirements of the standard apply in any case.
• Section 551, low-voltage generating sets, which covers generators and other sources supplying an installation, including how they are switched and earthed.
• The new chapter on stationary secondary batteries, added by Amendment 4.

Alongside BS 7671, the IET Code of Practice for Electrical Energy Storage Systems, now in its third edition (2024), is the reference for battery systems. MCS’s battery installation standard, MIS 3012, sits alongside the solar standard for MCS-certified work. An off-grid system cannot earn export payments, so MCS certification brings no SEG benefit, but an MCS installer will work to these documents.

Earthing without a network connection

In a grid-connected home the distributor provides an earth terminal at the cut-out. An off-grid installation has no such terminal, so it must create its own earthing arrangement.

The IET’s guidance on island mode, written for battery systems that can disconnect from the grid, sets out the principles. Regulation 551.4.3.2.1 means a generator running as an alternative to the public supply cannot rely on the distributor’s earthing, so a consumer earth electrode is required. A neutral-to-earth bond must be provided at the source when the installation runs from the inverter. Because inverters deliver far less fault current than the grid, overcurrent devices may not disconnect a fault quickly enough, and RCDs are used to achieve automatic disconnection. For TN-S, or TT where the loop impedance does not demand a lower value, the article gives 200 ohms as the maximum electrode resistance, because above that an electrode may not present a stable resistance.

A permanently off-grid cabin applies the same physics without the grid side: the inverter is the source, its neutral is bonded to earth at one point, an earth electrode connects to the main earthing terminal, and RCDs protect the circuits. The IET article does not address installations with no grid connection at all, so the designer should follow the full Code of Practice and BS 7671; this is not a job for a first-time DIY electrician.

Building Regulations and Part P

In England, adding new circuits in a dwelling is notifiable work under Part P of the Building Regulations, covered by Approved Document P; a registered competent person self-certifies it. An off-grid cabin that is someone’s home falls within that. For outbuildings with their own off-grid supply and no connection to a dwelling, we found no official statement that settles whether Part P applies; work to BS 7671 regardless and keep the electrical installation certificate.

Scotland uses building warrants rather than Building Regulations approval, and Wales and Northern Ireland have their own regulations. If panels go on a roof that needs strengthening, councils such as West Lothian say a warrant is required in Scotland.

Planning

Planning rules apply to the panels and to the building, whether or not the grid is involved.

England. Panels on a house, or a building within its curtilage, are permitted development within limits: 0.2 metres projection on a pitched roof, 0.6 metres above a flat roof, 0.4 metres on a wall (0.2 metres where it abuts a highway). A free-standing array in the garden of a house may have no more than 9 square metres of panels, which is about four standard panels, with height limits of 1 to 4 metres depending on position. Those classes apply to domestic premises; for a barn, cabin or land with no dwelling, other permitted development classes or a planning application may apply, so ask the council.

Scotland. Solar on a dwelling is permitted development if it protrudes no more than 1 metre from a wall, roof plane, ridge or chimney (Class 6HA, since 24 May 2024), with exclusions for listed buildings and some conservation area elevations.

Wales. Panels on a wall or pitched roof may project up to 20 centimetres and not above the highest part of the roof, under the 2012 Welsh order.

Northern Ireland. Roof panels may project up to 20 centimetres on a slope facing a road, and one free-standing array of up to 14 square metres and 2 metres high is allowed if it is not nearer the road than the house.

The bigger planning question for a remote cabin is usually the cabin itself: whether it is lawful to live in, or to use as holiday accommodation, is a separate matter from the panels on its roof.

No network operator, unless you connect

A system with no physical connection to the network needs no G98 or G99 notice. That changes the moment any part of it connects: a generator changeover that can feed the grid, an inverter wired into a building that also has a grid supply, or a future connection. Any link between an off-grid system and a grid-supplied installation is work for an electrician and may need the network operator’s involvement. The same applies to plug-in solar, which is legal in England, Scotland and Wales only as a grid-connected device without a battery; our guide to plug-in solar with a battery in the UK explains why.

Step by step: planning an off-grid system

Checklist for data sheets and quotes

• Usable battery capacity, not only nominal, with the depth of discharge and cycle life the warranty assumes.
• Low-temperature charging limit of the battery and whether it has a heater.
• Inverter continuous and surge power, and its no-load consumption.
• MPPT maximum input voltage with margin for cold-weather open-circuit voltage.
• Generator interface: automatic start, charging current, and the generator’s continuous rating.
• Monitoring: remote access if the site is unattended.
• Earthing design: electrode, neutral-earth bond, RCD types.
• Fusing and isolation at the battery, the controller and the inverter.
• Warranty terms for battery, inverter and panels, and who services the system at a remote site.

Common mistakes and their consequences

• Sizing on annual yield. Consequence: a flat battery every winter. Fix: size on December with PVGIS’s off-grid tool.
• Forgetting standby loads. Consequence: an inverter or router idling all day eats a large share of a small budget. Fix: count no-load power and switch off what you can.
• Panels at roof pitch in winter-critical systems. Consequence: 20 to 30 percent less December energy than at 60 degrees. Fix: tilt steeply or use a wall or ground frame.
• 12 V for large loads. Consequence: very high currents, hot cables, voltage drop. Fix: 48 V for cabins with inverters above about 1.5 to 2 kVA.
• Lithium batteries in a freezing shed. Consequence: the battery refuses to charge, or is damaged by charging when cold. Fix: insulated or heated location, or batteries with built-in heating.
• No earth electrode. Consequence: no reliable fault protection. Fix: design the earthing arrangement as for island mode, with RCDs.
• Generator ruled out on principle. Consequence: a battery and array several times larger for a handful of winter days. Fix: plan a generator for northern sites.
• Connecting off-grid kit to a grid-supplied building. Consequence: unsafe backfeed and breach of network rules. Fix: keep systems separate or have an electrician design a compliant changeover.

Off-grid, battery backup or a small kit

Three neighbouring options are often confused:

• Off-grid system: no network connection, sized for winter, generator for backup. This guide.
• Grid-connected solar with battery backup: rooftop solar and a battery that can island the house during a power cut; normal exports and bills the rest of the time. The installer handles G98 or G99 and the island-mode earthing.
• Small kits and power stations: for sheds, allotments, camping and boats, a 12 V kit or a portable power station with panels is simpler and cheaper. Our guides to solar panel kits, portable solar panels and small solar panels cover them; Renogy UK, for example, listed 12 V and 24 V off-grid kits from £269.99 on 4 November 2026.

For panel choice, the criteria in our guide to comparing solar panels in the UK apply, though MCS certification matters less off-grid and low-light behaviour more. For ground frames and wall brackets, see our guide to solar panel brackets and mounts.

Durability and maintenance

Panels last decades; the rest of an off-grid system does not. LiFePO4 batteries are commonly rated for thousands of cycles, but in an off-grid system they cycle daily, so check the warranty in years and in energy throughput. Inverter-chargers and controllers typically carry shorter warranties than panels. Generators need oil changes and runs at load to stay reliable.

Once a season, check the battery monitor’s history for deep discharges, clean the panels if snow, leaves or bird droppings cover them, inspect cable glands and fuses, test RCDs with their test button and run the generator. Before a winter absence, leave the battery charged, switch off non-essential loads and, if the site freezes, make sure lithium batteries are protected from charging below their temperature limit.

Next steps

• Solar panel kits in the UK compared
• How to compare solar panels in the UK
• Garden and ground-mounted solar panels