IThis guide introduces the options for Domestic Scale Renewable Energy. I provide an analysis of the costs and benefits of Photovoltaics, Solar Thermal, Ground Source Heat Pump, and Air Source Heat Pump. Other renewable technologies are also given a brief overview.
Summary of Options
|Technology type||Energy type||Typical System Cost||Annual Cost Saving *||Est. Annual Carbon Saving (kg carbon dioxide )||Payback Periods (estimated)|
|Photovoltaics (4 kWp)||Electricity||£6200||£375||1500 kg||16-17years|
|Solar Thermal||Heat||£4500||£285||260 kg||16 years|
|Ground Source Heat Pump||Heat||£14000||£1685||1800 kg||8 years|
|Air Source Heat Pump||Heat||£7000||£790||1800 kg||9 years|
- Systems installed after March 2019
- *All Costs and savings are estimates based on a typical 3 bedroom house.
- All cost savings include future incentive payments
Photovoltaics / Solar panels for electrical generation
Photovoltaic (PV) panels are by far the most common method of domestic scale renewable electricity generation. They are relatively simple to install however the position and angle of the panels to achieve optimum sunlight is a key consideration.
PV panels are rated in terms of kWp (kilowatts peak), however, most of the time they will produce less than the peak output depending on the sunlight intensity. PV panels will generate most of their energy in summer, but will still generate in winter and will also generate on cloudy days at a lower output.
Costs and Savings
- A typical PV array would be between 2 and 4 kWp.
- A 4kWp system can generate around 3800 kilowatt hours of electricity a year (UK average) and it will save around 1500 kg of carbon dioxide per year.
- Savings on the electricity bill by producing your own power (rather than buying grid power) is roughly £175 per annum. This figure will vary according to location and how much energy you actually use at home during daylight hours.
- Since the closure of the FIT (feed in tariff) a new export mechanism will come into force in Jan 2020 called the SEG (Smart Export Guarantee). This will likely be around 5p per kWh. Thus a 4 kW system is likely to generate in the region of £200 per year SEG.
- Hence overall energy saving using a 4 kWp PV system (installed after April 2019) could be approximately £375 per year.
- A 4 kWp system costs around £6,200 to install (including VAT at 5 per cent).
- On a £6200 installation cost this represents a payback of around 16-17 years.
Note that systems installed before April 2019 will benefit from the UK FIT (feed in tariff) and may have payback times of less than 10 years (the FIT previously had much more attractive payments for energy generated.)
In future with falling battery costs, many homes may have the facility to save all the energy they generate by PV using a domestic battery pack – this would improve annual energy savings but would further increase the initial system cost.
Solar water heating systems use panels connected to a domestic hot water system to reduce or eliminate the need for the boiler to run in warmer or sunnier weather. As with PV the vast majority of energy in a UK location will be generated in the summer months.
In most cases, solar panels cannot be used with combi-boilers, as they require a special hot water tank with a second heat exchanger loop for the solar circuit.
There are two main types of solar water heating collector: flat plate and evacuated tubes. Evacuated tubes generally have a slightly higher output than flat plate systems, offsetting a higher purchase price.
The cost of installing an approved solar hot water system ranges from approximately £4,000 to £5,000. These prices, however, are dependent on the size of the system and the quantity of water to be heated / stored.
Costs and Savings
- A typical domestic solar thermal system will save approximately 260 kg of carbon dioxide per year based on an average home currently using gas hot water heating.
- Hot water savings on a typical UK home will be in the region of £55 per annum
- Additional savings can be made via the Governments RHI (Renewable Heat Incentive): For a typical terraced house currently using gas water heater, a tariff of approximately £230 per year would be payable.
- Hence for a typical solar thermal system on an average sized house in UK, the annual cost saving would be approximately £285 per year.
- One a typical £4500 installation cost this represents a payback of approximately 16 years
Ground Source Heat Pump
A ground source heat pump (GSHP) uses pipes buried in the earth to extract heat from the ground. This heat can then be used to heat radiators, underfloor or warm air heating systems and hot water in your home.
The heat pump circulates a mixture of water and antifreeze and heat from the ground is absorbed into the fluid and then passes through a heat exchanger into the heat pump. The heat pump requires electricity to run but will typically produce heat at over 3x the electrical energy required to run it. This is known as a COP (Coefficient of performance) of 3.
The ground at a depth of one metre stays at a relatively constant temperature throughout the year, so the heat pump can be used even in mid-winter. The length of the ground loop depends on the size of your home and the amount of heat you need. Longer loops can draw more heat from the ground, but need more space to be buried in.
If space is limited, a vertical borehole can be drilled instead where the heat is drawn from a greater depth but using less ground area. Boreholes tend to be more costly.
Costs and savings
- Installing a typical system costs around £10,000-£18,000.
- Using a GSHP will save typically 1800 kg of carbon dioxide (if replacing a modern gas boiler)
- Fuel savings will depend on what type of heating you previously used. In the best case (replacing modern electrical heating) this can be up to £900. In the worst case (replacing a modern gas boiler) the saving will be around £185 per annum.
- The RHI (renewable heat incentive) grant can also be claimed. This will typically pay in the region of £1500 per year.
- So total savings if replacing a modern gas boiler will be in the region of £1685 per year
For an average installation cost of £14,000 this would represent a payback of 8 years.
Air Source Heat Pump
Air source heat pumps (ASHP) are very similar to ground source heat pumps – however they absorb heat from the outside air. This heat can then be used to heat radiators, underfloor heating systems, or warm air convectors and hot water in your home. The ASHP will operate better when the ambient temperature is higher – however it can pull heat from the air even when the temperature is as low as -15° C.
You can think of an ASHP a little like a domestic fridge running in reverse. It pulls heat from the outside air, and feeds this into the home.
ASHP are much less costly to install than GSHP as no groundworks are required, however their potential for heat supply is reduced
Costs and savings
- Installing a typical system costs around £6000 to £8000
- Using a ASHP will save typically 1800 kg of carbon dioxide (if replacing a modern gas boiler)
- Fuel savings will depend on what type of heating you previously used. In the best case (replacing modern electrical heating) this can be up to £600. In the worst case (replacing a modern gas boiler) the saving will be around £110 per annum.
- The RHI (Renewable Heat Incentive) grant can also be claimed. This will typically pay in the region of £680 per year.
- So total savings if replacing a modern gas boiler will be in the region of £790 per year
- For an average installation cost of £7000 this would represent a payback of 9 years
Other renewable technologies possible at domestic scale
- Biomass Heating
Note that wind is problematic at a domestic scale due to noise and disturbance issues. This may be a decent option if you live in a remote location however.
Biomass is only a truly sustainable option if using wood fired boilers (as opposed to log burners which don’t generate heat as effectively). Wood fired boiler are usually used in bigger applications (schools etc.) and are not particularly suited to domestic properties.
Notes on the author:
Graham Baker is a freelance energy engineering consultant based in Bristol, UK.
Energy Savings Trust, BEIS RHI calculator, NEF (National Energy Foundation)
If you have any comments on this piece please respond to this thread!