Off-Grid Renewables for Cornwall Farmhouses and Rural Properties

The 'off-grid' designation primarily refers to absence of mains gas connection (the principal heating fuel for 78% of UK homes). In Cornwall this affects approximately 41,000 dwellings (18% of total housing stock), concentrated in:

• Bodmin Moor and surrounds: Blisland, St Breward, St Neot, Altarnun, Lanivet, Cardinham — granite-built farmhouses 1700-1900, typically 100-180m², oil-heated, large land plots
• North Coast hinterland: Trebarwith, Tintagel rural, Crackington, Morwenstow, Hartland border — slate-and-cob construction, exposed coastal weather, often single-phase electric
• Lizard peninsula: Coverack, Cadgwith, Cury, Mullion, Lizard village, Goonhilly — granite and serpentine stone farmhouses, often on private water supply (well or borehole) without mains water either
• West Penwith hinterland: Sancreed, St Buryan, Sennen, Pendeen, Morvah, Zennor — typical hill farms 80-200m², granite walls, often oil-heated with secondary wood stove
• Rural Roseland: Veryan, Gerrans area, Portscatho hinterland, St Just-in-Roseland — mixed property types but consistent off-gas

Typical heating fuels currently: oil 62%, LPG 18%, electric storage heaters 12%, wood/biomass primary 5%, coal/anthracite 3%. Replacement of all primary heating with renewable + electric backup is the standard pathway, but the design depends heavily on the existing electrical supply and any DNO upgrade implications.

Most Cornish rural farmhouses have single-phase electrical supplies at 230V. The supply capacity varies:

• 60A single-phase: 14.4kVA peak capacity. Older 1960s-1980s rural connections. Allows ~9kW continuous load (kettles, fridges, lighting, basic heat pump) but no margin for EV charging or large heat pump.
• 80A single-phase: 19.2kVA. More common 1990s-2010s rural connections. Allows 12-14kW continuous load — sufficient for a 9-11kW ASHP plus reasonable other loads.
• 100A single-phase: 24kVA. Modern rural connections. Comfortable for 12-14kW ASHP plus EV charger plus solar export.
• 3-phase 100A per phase (where available): 72kVA capacity. Industrial or commercial-spec connections, rare in rural Cornwall — typically only at converted farms where a tractor/feed shed was historically commercial.

The relevant standard for export and load is the G98/G99 process operated by NGED (National Grid Electricity Distribution). G98 covers installations under 16A per phase (typically 3.68 kW solar inverter and small heat pumps), processed automatically. G99 covers larger installations, requiring a 10-day acknowledgement and full DNO review, typically 6-10 weeks. For a typical farmhouse install (10kW heat pump + 5kWp solar + EV charger), G99 notification is required.

Rural Cornish farmhouses have a real choice between ASHP and GSHP, unlike urban/suburban properties where land constraint forces ASHP. The decision tree:

Choose ASHP when: (1) the property has only 1,500m² or less of land excluding the main garden, (2) the existing oil heating system has reasonable pipework that can be reused with upgrades, (3) electrical supply is 80A or more, (4) budget is the primary driver (ASHP typically £14,000-£22,000 installed vs £28,000-£48,000 for GSHP).

Choose GSHP when: (1) the property has 3,000m² or more of land suitable for horizontal collector loops (or pony paddock that can accommodate boreholes), (2) noise is a concern — GSHP has no outdoor unit so essentially silent, (3) the property is at altitude (Bodmin Moor at 250m+, Goss Moor) where ambient winter air temperatures are colder, hurting ASHP SCOP, (4) the owner intends to remain in the property 20+ years and wants premium efficiency.

Typical GSHP specifications: 8-12kW NIBE F1255 or Vaillant flexoTHERM Exclusive 8-15kW, with either a horizontal collector loop (typically 600-1,200m of slinky pipe in trenches at 1.5m depth across 800-1,500m²) or 2-3 vertical boreholes at 80-130m depth (£3,500-£5,500 per borehole drilling cost). SCOP typically 4.8-5.3 in Cornwall (vs 3.8-4.4 for ASHP at altitude/exposed sites). Acoustic profile: zero outdoor unit, internal compressor mounted in plant room at typical 40-45 dB at 1m inside.

True off-grid (not just off-gas) Cornish properties — typically remote farmhouses on Bodmin Moor or the Lizard not connected to the mains electricity grid, of which there are around 350-500 in the county — require complete self-sufficiency. These systems combine:

• Solar PV array: Typically 8-15kWp on south-facing roof, ground-mount, or barn roof. Sized to cover typical day-time loads plus charge battery for evening/morning
• Battery bank: Typically 25-50kWh of lithium iron phosphate (LFP). Common products: Givenergy Giv-Bat stacked (5kWh per module), Pylontech US5000 in 19-inch rack, Victron Energy systems with Lynx distributor. 1-3 day autonomy typical.
• Diesel or petrol generator backup: 8-15kVA inverter generator (Honda EU22i, Pramac S6500, Kohler SDMO) for emergency backup and winter weeks of low solar generation. Runs 200-400 hours/year typical.
• Inverter/charger: Victron Quattro 8000-15000 or SMA Sunny Island. Manages solar, battery, generator and grid (if intermittently connected) priority logic.
• Energy management system: Victron VRM or similar — monitors all sources, optimises charging, controls generator start/stop.

For partially-off-grid properties (mains electricity available but unreliable due to overhead lines and storm damage), a similar architecture but with grid as primary input and battery+generator as backup is typical. Cornwall sees significantly more storm-related outages than UK average due to exposed coastal infrastructure.

The typical Cornish farmhouse is currently oil-heated: a 1,200-2,500 litre oil tank in the yard, a Boulter/Worcester/Grant oil boiler in the utility, oil consumption typically 2,200-4,000 litres/year. Annual oil cost in 2026 (oil at 78p/L average): £1,700-£3,100. Heat pump retrofit converts this to electric heating, typically reducing total energy cost by 50-65% (depending on tariff and SCOP).

The retrofit process:

• Survey and design: Heat loss calculation to MCS 3005-D, radiator audit, pipe-route assessment, electrical supply check, oil tank decommissioning plan
• Heat pump install: Outdoor unit typically sited in the yard or against the utility wall, on a concrete pad with acoustic mounts. Cornwall's rural setting usually allows generous siting (no neighbour noise concerns)
• Cylinder replacement: Old vented or pressurised oil-tank-style cylinder replaced with 250-300L unvented (Mixergy, Telford). Often gains airing cupboard space.
• Radiator upgrades: Typically 50-70% of radiators replaced for low-temperature operation. Cast-iron radiators in Victorian farmhouse parts often retained.
• Oil tank decommissioning: Tank emptied (used oil typically given to neighbouring farms for AGA fuel or kerosene heater backup), tank professionally cleaned, removed and recycled. Cost £450-£900.
• Pipework upgrades: Often the existing oil boiler used 22mm primary flow/return which is adequate for heat pump up to ~12kW. Larger systems require upgrade to 28mm.

Typical project cost: £18,500-£26,500 gross, £11,000-£19,000 after £7,500 BUS grant.

Many rural Cornish farmhouses have private water supplies — borehole, well or spring — without mains water connection. Approximately 4,500 Cornish properties (most in the Lizard, West Penwith and parts of Bodmin Moor) are on private supply. This affects renewable design in two ways:

Hot water cylinder considerations: Private supplies often have higher mineral content (calcium, magnesium, iron) and variable pressure. We specify cylinders rated for the actual water chemistry: stainless steel cylinders (Mixergy, Telford Tempest) for high-iron supplies, twin-coil designs to allow rapid recovery on heavy use days. Standard maintenance recommendation: cylinder anode inspection every 3 years (vs 5 for mains supply).

System volume and demand profile: Farms often have higher hot water demand (livestock washing, milking parlour cleaning if dairy, large family bathroom loads). Standard 250L cylinders may be inadequate — we typically specify 300-400L on dairy farms, often with thermal store integration for very large demand. Where stable pressure is critical (e.g. for power showers across a 4-5 bed farmhouse), we add booster pumps and accumulators.

Thermal store option: For very large or complex hot water demands, a thermal store (e.g. 500-1000L unvented thermal store from Akvaterm, Tempress or McDonald Engineers) accepts heat input from the heat pump, wood stove back-boiler, and even solar thermal, distributing as needed via secondary plate heat exchangers. Premium spec but flexible for working farms.

EV adoption in rural Cornwall is increasing, but the single-phase electrical supplies that dominate rural connections create technical constraints. The relevant G98 limit is 16A per phase, which equates to 3.68kW continuous load — much less than the typical 7kW domestic EV charger draws.

Practical approaches for rural EV charging on single-phase supplies:

• 3.6 kW slow charger: Within G98 limits, automatic notification only. Provides around 16-22 miles/hour of charge — fine for overnight charging covering typical 35-45 mile rural daily mileage. £700-£1,000 installed.
• 7 kW charger with G99 notification: Standard residential EV charger speed, requires 10-day DNO notification. Charges typical EV in 7-9 hours overnight. Where the supply is 80A+, the heat pump and EV charger can coexist with appropriate load management (Myenergi Zappi has built-in current monitoring to prevent overload). £1,200-£1,800 installed.
• Solar-following Zappi: Myenergi Zappi v2.1 in 'eco' or 'eco+' modes uses excess solar generation by priority, minimising grid import. For farms with 6-10 kWp solar this means substantial 'free' EV miles during sunny periods.
• Supply upgrade where economically justified: If multiple EVs, large heat pump, and growing demand, upgrade to 3-phase. NGED quotes typically £8,000-£35,000 depending on distance from nearest 3-phase pole. We assess feasibility at survey.

Two backup energy sources are common in Cornish rural farmhouses: small diesel or petrol generators for grid outage backup, and wood stoves or AGAs for secondary heat (and emotional/cultural reasons — the Cornish farmhouse kitchen with a working stove is irreducible).

Generator integration: Where a property has a standby generator (typically 8-15kVA single-phase or 15-25kVA three-phase), it can be integrated with battery storage via an automatic transfer switch (ATS) and inverter charger. The setup: when grid fails, ATS isolates the property, battery powers loads for several hours, generator auto-starts to recharge battery and supply loads simultaneously. This delivers seamless backup with minimal generator runtime. Cost £3,500-£7,500 for ATS + control integration where generator already exists.

Wood stove and AGA compatibility: Wood stoves with back-boilers and AGAs running on oil or LPG present compatibility considerations:

• Wood stoves with back-boiler: Can supply hot water to a thermal store in parallel with the heat pump. Requires careful design to manage flow temperatures and prevent boiler overheat. Heat pump remains primary; stove supplements during use.
• Oil-fired AGA: Cornish farmhouse staple. New Cornwall DPD applies to primary heating, not cooking — but for existing AGAs, owners often want to retain. We design around AGA, treating it as a heat input to the kitchen (typically 2-3kW thermal output 24/7) and accounting for this in the heat pump sizing.
• LPG AGA conversion to electric AGA: Increasingly common option. Electric AGAs (eAGA) consume around 3.5-5 kWh/day, can be powered from solar + battery during day, retain the look-and-feel of traditional AGA. Conversion cost £4,800-£7,500.