Wind, Hydro and Micro-CHP

Microgeneration Beyond Solar: Micro Wind, Micro Hydro, Micro-CHP and Fuel Cells

Microgeneration is the production of heat or electricity at home from a low carbon source, and the law draws the boundary at 50 kilowatts of electricity or 45 kilowatts of heat¹. Anything at or below those limits counts as microgeneration; anything above does not. Those thresholds have stood since the Energy Act 2004 first put the term into statute, when Parliament set microgeneration at "in relation to the generation of electricity, 50 kilowatts" and "in relation to the production of heat, 45 kilowatts thermal"³, and they still govern how the sector is defined across the UK today⁴.

Solar panels are the best known form of microgeneration, but they are only one of the technologies the law recognises. The statutory definition covers equipment that "relies wholly or mainly on biomass, biofuels, fuel cells, photovoltaics, water (including waves and tides), wind, solar power, geothermal sources, combined heat and power systems and other sources of energy or technologies for the generation of electricity or the production of heat"¹. This page covers the four that sit beyond solar: small wind turbines, micro hydro, micro combined heat and power, and domestic fuel cells. For each one it sets out what the technology is, what site and permissions it needs, how it is treated under the connection and export rules, and what it does, and does not do, for a household's energy independence.

Understanding microgeneration: the 50kW and 45kW thresholds

The two numbers that define microgeneration come from section 82(6) of the Energy Act 2004, and every later piece of guidance repeats them. In relation to the generation of electricity, the limit is 50 kilowatts; in relation to the production of heat, it is 45 kilowatts thermal³. Scottish guidance states the same pair of figures as "equipment with an output of up to 50 kilowatts of electricity or 45 kilowatts of thermal (heat) energy"⁹, and a Scottish consultation on skills and microgeneration describes the sector as "micro-renewable technologies used for zero/low carbon generation of heat (up to 45kW) and electricity (up to 50kW)"¹¹.

The limits matter in practice because they decide which certification scheme, which planning rules and which connection process apply. The Microgeneration Certification Scheme describes itself as "the leading quality assurance organisation for microgeneration (small scale renewable 50kW or smaller) technologies" and gives accreditation "for generating units with an installed capacity of 50kw or less"¹². A Scottish quality assurance statement likewise frames the sector as electricity "up to 50kW" and heat "up to 45kW"¹⁴. Cross the line and the installation leaves the household regime altogether.

The definition also covers what the equipment is for. Scottish guidance defines domestic microgeneration as "the production of electricity or heat for domestic consumption using microgeneration equipment"¹⁰, and the wider statutory definition lists the energy sources it includes: biomass, biofuels, fuel cells, photovoltaics, water including waves and tides, wind, solar power, geothermal sources and combined heat and power¹. A VAT relief definition adds a further angle: a "microgeneration system" means equipment that generates electricity and is itself an energy-saving material¹⁵.

Separate from the statutory limits, the electricity industry classifies installations by connection size⁸:

Typical applications reach up to 50kW for solar PV, up to 250kW for wind generation and up to 500kW for anaerobic digestion⁸. So a household turbine can be "micro" in the statutory sense while still being a small scale generation application to the network operator.

Domestic wind power: compact turbines for houses and modest plots

Small-scale turbines generate electricity from wind, and their output depends on wind speed and location¹⁶. That single sentence carries most of what a householder needs to weigh: the technology works, but the wind resource at the specific site decides whether it works well. Northern Ireland's official guidance notes that wind turbines can have a life of up to 22.5 years, but need service checks every few years to make sure they work efficiently¹⁷.

For homes with no grid connection, wind is often paired with another source. It is common to run a wind system with a diesel generator for use during periods of low wind speeds¹⁷. Any unused or excess electricity from a grid-connected system can be exported to the grid and sold to the local electricity supply company¹⁷. The detailed questions, how much a turbine costs, how much electricity it produces, what wind speed a site needs and how to measure it, are covered on the dedicated pages for domestic wind turbines, wind turbine output and sizing and wind speed and siting.

Planning rules treat domestic wind as householder development. Scottish guidance states that the installation of microgeneration equipment can help generate energy and reduce costs, and that a free-standing wind turbine is permitted development where it "is used only for the purposes of domestic microgeneration"¹⁸. Non-domestic scale microgeneration equipment will generally require planning permission in the meantime¹⁸. The rules differ between the nations, and each has its own page: wind turbine planning permission in England, Scotland, Wales and Northern Ireland.

At the smallest end, the law now recognises plug-in equipment. A plug-in microgenerator is one "intended for connection to a low voltage consumer's installation by means of a standard plug and socket" and with "a maximum rated alternating current output not exceeding 800 watts"¹⁹. That category sits below even the micro renewable connection class and is covered in more detail on the off-grid wind power page.

Micro hydro: electricity from moving water

Hydroelectric power systems use running water turning a turbine to produce electricity, and useful power may be produced from even a small stream⁷. A micro hydro plant is one that generates less than 100 kilowatts⁷. The two physical quantities that size a scheme are head, the height the water falls, and flow, how much water passes through; both are covered on the head and flow page, and the turbine types that suit different combinations on the hydro turbine types page.

Micro hydro has a distinctive strength among the household renewables: steadiness. For houses with no mains connection but with access to a micro hydro site, a good hydro system can generate a steady, more reliable electricity supply than other renewable technologies at a lower cost⁷. In off-grid applications the power is used for lighting and electrical appliances, and space and water heating can be supplied when available power exceeds demand⁷. That profile is the opposite of wind and solar, which follow weather and daylight. Micro-hydro systems are described as ideal for off-grid homes, offering long-term savings despite high initial costs¹⁶.

The trade-offs are the site requirements and the paperwork. A hydro scheme needs a suitable watercourse, and abstractions generally need a licence, covered on the water abstraction licences and permits page, along with environmental duties such as fish passage and screening, covered on the environmental requirements page. Costs are installer-quoted for a specific site; there is no published price that applies across schemes, and the micro hydro cost page explains what drives them. The main pages for the technology are micro hydro power for homes and installing and maintaining a micro hydro system.

Micro-CHP: a boiler producing electricity as a bonus

A micro combined heat and power unit heats the building just like a gas boiler would and also generates electricity, and most of the electricity generated is used within the building¹³. Its primary function is to run central heating for the home⁵. The Feed-in Tariff scheme's definition put it plainly:

A CHP unit is gas-fired⁵.

The idea is not new at scale. CHP systems have been in use for some time in Northern Ireland in large scale installations such as hospitals and leisure centres⁵. The domestic version shrinks the same principle to a single household appliance: these systems generate both heat and electricity, offering a highly efficient alternative to traditional boilers¹⁶. Because the electricity is a by-product of heat production, the unit only generates when the heating is on, which shapes everything about how it performs in practice.

The different engine types inside these units, Stirling engine, internal combustion and fuel cell, are compared on the micro-CHP types page, and the choice between a micro-CHP unit and a conventional condensing gas boiler on the micro-CHP vs a gas boiler page. How a unit performs against a heat pump is covered at micro-CHP vs an air source heat pump.

Micro-CHP output: heat led, with up to 1kW of power

The headline figure is about 1kW of electricity while the unit is running⁵. That is a by-product figure, not a rating: the unit is sized to heat the home, and the electricity follows the heating demand. Most of the electricity generated is used within the building¹³, which is what makes the technology attractive, since it displaces electricity a household would otherwise buy.

The connection rules cap the category. Fossil fuel-derived Combined Heat and Power up to 2kW is treated as "microCHP" for connection purposes⁶, and micro-combined heat and power generators count among the technologies with a maximum total installed capacity of up to 50kW in the wider microgeneration class²². So a domestic unit sits well inside the statutory 50kW electricity limit, and the practical output is smaller still.

What a household actually generates over a year depends on how long the heating runs. A home with a long heating season produces more by-product electricity than one heated only briefly in winter, because the generator only turns when there is heat demand. The performance estimate process, and how it must be carried out, is covered on the micro-CHP performance estimate page, with the testing and certification background on the performance and certification page. For context on what households were replacing when they moved to renewable heat, the Domestic Renewable Heat Incentive annual report records the fuels that constituted the clear majority of replaced boilers²³:

Fuel cells and other low-carbon heating and power options

Fuel cells appear in the statutory list of microgeneration energy sources alongside biomass, biofuels, photovoltaics, water, wind, solar power, geothermal sources and combined heat and power¹. A fuel cell unit is, in effect, a micro-CHP system whose engine is a fuel cell rather than a burning one: it produces heat and electricity together from a fuel, and the micro-CHP types page covers it as the third type. The dedicated pages are domestic fuel cells, whether a fuel cell heating system can run on natural gas, and the comparison between fuel cell heating and micro-CHP.

Fuel cells sit within a wider set of low carbon heat routes that the building rules now recognise. A Welsh consultation on Approved Document L lists the eligible low carbon heat sources for sleeving capacity as²⁴:

• electrically powered heat pumps
• waste heat, including from power stations
• geothermal heat
• electric boiler
• solar thermal
• biofuels

That list is about heat networks rather than individual homes, but it shows the direction of travel in the standards: heat is expected to come increasingly from electricity and renewable sources rather than gas combustion, which is the strategic pressure on gas-fired micro-CHP and the opportunity for fuel cells.

The other routes to household generation and heat include the heat pump family, which is generation-adjacent rather than generation: a heat pump moves heat rather than making electricity, but it changes the household's dependence on gas. Air source heat pumps have their own permitted development regime, and the guidance states that within the curtilage of or on a dwellinghouse or block of flats, you can install, alter or replace a microgeneration air source heat pump under permitted development if criteria are met²⁵. Which technology suits which home and site is set out on the which microgeneration suits your home and site page.

Planning rules and flue positioning

For micro-CHP, planning permission is not normally needed when installing the system in a house if the work is all internal²⁷. The flue is where planning comes in. If the installation requires a flue outside, it will normally be permitted development if the stated conditions are met²⁸. Flues on the rear or side elevation of the building are allowed to a maximum of one metre above the highest part of the roof²⁷.

The conditions bite in protected areas. In a conservation area or in a World Heritage site the flue should not be fitted on the principal or side elevation if it would be visible from a highway²⁷. Scottish guidance states the same rule for flues forming part of a combined heat and power system: planning permission is required if the flue is to be attached to the principal elevation of a dwellinghouse or flat within a conservation area or World Heritage Site³⁰. The same document adds that planning permission is needed for flues for dwellinghouses or flats within an Air Quality Management Area³⁰.

The permitted development classes in the legislation cover flues for the whole microgeneration family, not just micro-CHP³¹:

For dwellinghouses, Scottish guidance confirms permitted development rights allow the installation, alteration or replacement of a flue forming part of a biomass system³⁰.

Two further rules complete the picture. If a project also requires an outside building to store fuel or related equipment, the same rules apply to that building as to other extensions and garden outbuildings²⁷. And for heat pumps, the air source heat pump must not be installed on a site designated as a scheduled monument or on a building or land within the curtilage of a listed building²⁵. The listed building question for all microgeneration is covered on the listed building consent page.

MCS accreditation, fitting and exporting to the grid

The Microgeneration Certification Scheme is the quality mark that ties the sector together. MCS describes itself as the leading quality assurance organisation for microgeneration technologies of 50kW or smaller¹², and scheme rules state that all MCS installers are required to be a member of a UKAS approved certification body, which assesses their competence¹². Certification matters for more than reassurance: it is a condition of many permitted development rights and of eligibility for export arrangements. The detail is on the MCS certification for wind and hydro page and the micro-CHP certification page.

Export is the other half of the deal for any generating household. Any unused or excess electricity can be exported to the grid and sold to the local electricity supply company¹⁷. The Feed-in Tariff scheme, which paid generators including micro-CHP up to 50kW, is closed to new applications; Ofgem's guidance for FIT generators and its annual reports now cover the existing fleet⁶. Ofgem's quarterly statistics record the number of solar photovoltaic, wind and micro combined heat and power installations registered on the MCS database³². How selling electricity works today is covered on the exporting electricity page, the closed scheme on the Feed-in Tariff page, and Northern Ireland's separate arrangements on the Northern Ireland export payments page.

Connection itself runs through the network operator's G98 and G99 processes, covered on the G98 and G99 page, with Northern Ireland's variant on its own page. Electrical safety sits alongside: small-scale generators such as micro CHP units fall within the special installation and location rules of the wiring regulations³³, and the electrical safety and wiring page covers what that means for a household installation.

Buying, running and maintaining micro-CHP: the costs

There is no published national price for a micro-CHP unit: prices are installer-quoted, and the costs, outputs and running expenses are set out on the micro-CHP cost, output and running costs page. What can be stated here is the shape of the economics. The unit is gas-fired⁵, so the household keeps a fuel bill and a dependence on the gas supply; the by-product electricity displaces bought electricity while the heating runs¹³; and the annual electricity generated depends on heating hours rather than on a fixed output figure.

Maintenance follows the same pattern as a boiler, with the addition of the generating components. The installer's certification under MCS, through a UKAS approved certification body, is the quality assurance on both the installation and the ongoing servicing arrangements¹². Ofgem's quarterly statistics track how many micro-CHP installations are registered on the MCS database, alongside solar and wind³², which is the best available indicator of how widely the technology is actually being installed in the UK.

For households weighing micro-CHP against other options, the relevant comparison points are on the micro-CHP vs a gas boiler and micro-CHP vs an air source heat pump pages, and the funding landscape on the grants and funding page. One definition worth knowing when reading supplier material: Ofgem's energy terms define a microbusiness as one that uses less than 100,000 kWh of electricity a year³⁴, a threshold that sometimes appears in tariff eligibility rules.

UK manufacturers and their product lines

The UK market for these technologies is small and its history is cautionary: several of the best known names have failed or withdrawn their products. That matters directly to a buyer, because a withdrawn product's warranty and spare parts supply cannot be taken for granted.

In micro-CHP, the Baxi Ecogen, a Stirling engine micro-CHP boiler, was withdrawn, and the Flow Energy boiler's company collapsed; both are documented on the Baxi Ecogen and Flow Energy boiler pages. In fuel cells, the Viessmann Vitovalor range represents the fuel cell route to home heat and power, covered on the Viessmann Vitovalor page.

In micro wind, the failures are just as significant: Proven Energy and Evance both ceased trading, Windsave's roof turbine was withdrawn from sale, and Ampair continued only in marine and off-grid machines after the company failed. The makers still serving homes and off-grid sites include Marlec Engineering, the UK maker behind the Rutland windchargers, Leading Edge with its micro wind turbines for homes and off-grid sites, and FuturEnergy. In hydro, Gilkes is the long established UK hydro turbine manufacturer. Each has a dedicated page: Rutland windchargers, Marlec Engineering, Leading Edge, Proven Energy, Evance, Windsave, Ampair, FuturEnergy and Gilkes.

How each technology boosts self-sufficiency

Each of these technologies buys a different kind of independence, and each leaves a different dependence behind. Micro wind generates electricity on site and can export the surplus¹⁷, but it depends on the wind resource at the site, and off-grid systems commonly keep a diesel generator for periods of low wind speeds¹⁷. Micro hydro is the steadiest of the renewables: for houses with no mains connection but with access to a good site, it can generate a more reliable electricity supply than other renewable technologies at a lower cost⁷, and it can carry space and water heating when available power exceeds demand⁷. Its dependence is on the watercourse: the flow varies with the seasons, and the scheme needs licences to abstract the water.

Micro-CHP generates electricity at home, and most of that electricity is used within the building¹³, which cuts what a household buys from a supplier. But it is gas-fired⁵, so it swaps one dependence for another: the household keeps its connection to the gas network and its exposure to gas prices, and the electricity only flows while the heating runs. Fuel cells share that fuel dependence while offering a different conversion route, and the wider low carbon heat list, heat pumps, waste heat, geothermal, electric boilers, solar thermal and biofuels²⁴, points to the longer term shift away from combustion in the home.

The honest summary is that microgeneration reduces dependence on suppliers and the grid without removing it, except where a site supports a full off-grid system. The statutory limits of 50kW of electricity and 45kW of heat¹ are generous for a single home, and the connection classes, 3.68kW or less single phase for micro renewable generation⁸, set the practical frame for what most households install. The fuller treatment of what these technologies do, and do not do, for a household's self-sufficiency is on the microgeneration and energy independence page.