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My project to green the house, cut its carbon footprint, and reduce energy & transportation costs

By Chris Gribble

In this article, Chris explains his incredible energy-efficient home, how he exports electricity from it, and how he has saved 5471 kg of carbon, which is equivalent to planting 331 trees. Chris is a member of Dorset Humanists.

Seven years ago, I bought a detached bungalow in need of some renovation. It's brick construction, circa 1920, partially renovated in 1974, which extended the footprint to the rear, added concrete floors throughout, cavity wall insulation, and a new 'A' frame roof with mineral wool insulation. The windows and doors consisted of some old double glazing to the south and sides of the property, and single glazed windows and doors on the north side of the building. The heating system was composed of an old, inefficient non-condensing gas boiler and a 210 litre hot water tank, with microbore-fed radiators which hardly got warm unless they were close to the boiler. On a limited pay-as-you-earn renovation budget, my goal was to improve energy efficiency while reducing running costs.


In 2016, I replaced most of the old glazing. The existing roof insulation was replaced with denser 200mm insulation once renovation no longer required access to the loft. This increased the thermal efficiency of the house, and improved solar gain in the winter by trapping the Sun's heat. The net result is that even with no heating on, the house rarely drops below 12°C, and it requires less heat to raise the temperature. But I still prefer to put on a jumper and hat rather than the heating.

"I have free hot water year-round... my house is now an electricity producer and exporter"

Solar Panels

In July 2017, I added an oversized 18 panel 5kW solar PV array with a 3.6 kW inverter and a solar diverter. The capital investment was £6157 including VAT and I am on target for this to be fully paid off by July 2024. The inverter converts the direct current (DC) generated by the solar panels into alternating current (AC) which you use in the house. The solar diverter directs any surplus AC electricity directly to the immersion heater for hot water. The net result is that with two-person occupancy, I have free hot water year-round. In the worst winter period where the days are shorter and there has been limited sun I have, at worst, heated the tank manually three or four times. Even cloudy days will produce some surplus electricity and this is trickle-fed to the immersion heater.

An electric immersion heater is not the most efficient way to produce hot water, but the solar diverter cost £150 as opposed to installing solar thermal or a heat pump which would have been in the thousands.

So why did I add extra solar panels? At the time you could only export 3.6KW without buying an export license. However, if I added a DC storage battery, any extra DC generation would go straight into the battery and bypass the inverter. I plan to install a more sustainable and safer battery once prices drop and it can be cost justified. More about this later when I talk about upgrade plans.

With the installation of a solar PV system, my house is now an electricity producer and exporter. I export about 60% of what I produce, and the 40% I use myself is about 55% of my total usage. So, my electricity cost dropped dramatically. And the hot water cost shifted from gas to free electricity.

Your usage of electricity shifts when you get solar panels and install a smart meter. You become obsessed with sunshine. Any energy intensive appliance is used when the sun shines, and one appliance at a time. So, the kettle, the iron, the washing machine, the oven, the hoover, and the lawnmower are prioritised when the sun shines.

Greener Heating

In October 2017, the boiler failed, so I started looking into greener options. On the table were a range of standard and new technologies. So, what was most efficient boiler on the market? To answer that question we need to compare Coefficients of Performance (COP).

Condensing Gas boilers (Combi Boilers)

This was just a more efficient version of what I had, but I could eliminate the hot water tank if I wanted to free up space. The downside was a reliance on a gas supply, the environmental impact, and the cost of a separate standing charge and the annual boiler maintenance cost. A combi gas boiler has about 0.9 COP. In other words, for every 1KW of energy put in, you get 0.9KW of heat out.

Power & Heat Boilers

Mechanical heat and power boilers have been around for years, mainly installed in larger buildings.

But I found a new technology version that converts methane to hydrogen, drives a hydrogen fuel cell to produce about 1kW of electricity, and the heat generated provides hot water on demand and hot water for your radiators. The upside is no change to your radiator circuit and less emissions. The downside is the cost of the boiler and more costly installation to support the electrical generation. Given that I already had solar electricity production and free hot water, I didn’t pursue this option further. I also wanted to move away from gas for environmental reasons.

Heat Pumps

These are similar to fridges in reverse, that concentrate the heat, transfer it to water and can provide both heating and hot water. Air source heat pumps literally suck heat out of the air. You will need a hot water tank to store the hot water. Hot water is circulated to the radiators providing both radiated heat and convection. So, no change there to conventional gas heating. Ground source heat pumps extract heat from the earth, and the COP is more stable, reflecting more constant ground temperatures. An air source heat pump COP varies between 1.5 to 4 depending on outside air temperature. The upside is that they are very energy efficient, require little to no maintenance, and will considerably outlast a gas boiler. The downside is to get that COP efficiency you’ll usually need bigger radiators because the water circulates at a lower temperature to get the best efficiency. You’ll also need a well-insulated house, and it will take longer to get the room up to temperature from a cold start, and this solution is perhaps better suited to underfloor hydronic circuit than radiators. There is also the cost of the installation, but there are government grants. The bad press for heat pumps is usually down to a poorly-designed and specified system.

Air to Air Heat Source Pumps

This is standard air conditioning. And it was my chosen installation, as I already had hot water via my solar diverter. It was installed in November 2017 and I removed the gas supply eliminating the standing charge. It’s the same technology as an air to water heat pump, but instead of single heat exchange that transfers the heat to water and circulates, the refrigerant circulates directly to mounted fan units transferring the heat or cold to air which is then fan-circulated around the room. It utilizes a highly efficient and environmentally refrigerant, R32, with a COP above 5.

I installed a multiport system that has a single external compressor and supports four internal fan units. The external fan noise is barely audible and considerably lower than an exhaust from a combi boiler. The BTU capacity is more than required for the size of the property. The cost of the installation was £5403 including VAT, which was roughly equivalent to installing a new boiler and radiators, but should last much longer than a gas boiler with an expected lifespan of about nine years. The upside is that this is the most efficient solution, so you get a COP closer to 5 or above. It provides both heating, cooling and/or dehumidification. And the units can be set to operate at very low noise levels which are barely audible. And I could automate operation using IFTTT (an online automation platform) to leverage free solar electricity. For now, I do it manually from my phone. All internal units are controlled individually, so you can heat single or multiple rooms to specific requirements. The downside is that it provides only heat, no hot water. And hot air is not as nice as radiated heat though you get used to it pretty quickly. In terms of cost and efficiency, I can operate each internal unit on an individual efficiency rating. On the highest efficiency it takes longer to heat or cool a room. But when all four units are running on the highest efficiency in heating mode, I draw about 1.2KW of electricity. At the lowest efficiency, I draw about 2.5KW. That’s less than the demand of a kettle which draws 3KW. For a single unit, demand varies from 0.4KW to 0.8KW. Once the desired temperature is achieved, that power demand drops dramatically. My electricity consumption has increased slightly resulting from the switch from gas to electrically-driven heating, together with cooling and some EV charging.


In 2021, my car was scrapped and replaced with a second-hand Plug-in-Hybrid (PHEV). It’s got a small 9KW battery with a pure EV range of about 32 miles on a single charge. I charge this off a 13amp socket which is within my electricity generation capability. The car was a great interim solution, as all my local transport is purely electric and mostly powered by my roof, You can’t get more energy and cost efficient than that! And on a long journey of about 200 miles, at best I achieve 106MPG in hybrid mode.

Cost & Energy Efficiencies

As of March 2023, my estimated electricity usage is 2,175kWh per year at a predicted cost of £807. I produce 55% of what I use. Remember, this also includes heating, cooling, hot water and some EV charging. Since installation, I have saved 5,471kg of carbon which is equivalent to planting 331 trees. This figure does not include CO2 savings from the free hot water, heating, cooling, EV charging and not using petrol.

Future Upgrades

Environmentally Friendly Batteries

There are now safer, more sustainable, cheaper, and temperature stable battery technologies. Sodium batteries work well down to minus 20C, don’t catch fire, can be charged very quickly, will outlast any car, and will be significantly cheaper to produce and recycle. They are very suitable for home and grid storage, mopping up all that excess wind and solar power. They are less energy dense than lithium batteries so slightly bigger, but not for long. CATL, the largest battery producer, will be mass producing sodium batteries this year and interleaving them with their M3P lithium batteries to improve car range in colder climates. M3P batteries will provide almost the same energy density as the more toxic, higher fire risk ternary lithium batteries currently in use. No more cobalt or nickel will be required. And – news flash – Australian scientists have just produced a sodium battery with four times the energy density of lithium. So, we didn’t have to wait long after all. I’d like a sodium-based solar battery to store any excess electricity and move my personal consumption of what energy I produce from 55% to 100%. My target goal is to reduce the electricity bill to standing charge only, generating all the electricity I need and storing any surplus in an environmentally-friendly battery for when the sun doesn’t shine. Such a configuration would also eliminate the problem of power cuts.

Solar Film Technology

Perovskite solar cell

Currently, silicon solar panels are costly to produce, heavy, and have quite a high environmental impact. They are at the limit of their efficiency, and rely on good sunlight (the red part of the spectrum) for maximum electricity production. The new kid on the block is Solar Ink. This uses an abundant mineral called Perovskite which is made into an ink that can be sprayed onto a conducting surface or film, and even embedded in glass. Panels have now been produced that match the durability and stability requirements for current solar panels, improve on efficiency, and are much cheaper to produce and far lighter. But that’s not the best part. They use the blue part of the spectrum. So they just need light, not direct sunlight, and there's better all year-round production even in cloudy weather, with wider installation options. I’ve seen head phones which charge from the light in your home, be it natural or artificial. Applied to an EV, the car could cover the charge of your local commute just by sitting outside. An Oxford-based company has layered semi-transparent solar film onto silicon panels, dramatically improving efficiency and all-weather energy production. An Australian company is literally rolling out solar film onto commercial buildings to generate power. So my next move is to have a second solar film array on the house to generate more electricity and pump it into the car and/or a solar battery.

Intelligent Car Chargers

If you have one, your EV car has a large battery. Why not use it to power your home when the car is not in use and the cost of electricity is high, or to act as backup when the grid is down? The term applied is 'vehicle to the home' (V2H). Your car and/or solar battery can be integrated into your power supply. The technology has been around for a while and its bi-directional charging capability is being extended to other car charging standards. Today, if you had a Nissan EV and a bi-directional charger you could charge your car at 7.5p/kWh on an EV tariff and run the house on it. That would cut your electricity costs! Ideally, I want to charge the car from self-generated electricity which is free and stored directly into my car and/or solar battery, and run the house 24/7 on the electricity I generate myself. My next car will be a full EV, supporting bi-directional charging, and will have a range of about 600 miles. This will cover my longest journey, there and back, in the coldest weather. And with higher energy density sodium batteries, it's likely to be a smaller, lighter, compact EV car than a heavy SUV. Watch this space as it's not very far away!

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