An electrifying journey: renewable PV energy, a greener heating system and an electric vehicle

09 February, 2022

An electrifying journey: renewable PV energy, a greener heating system and an electric vehicle

An electrifying journey: renewable PV energy, a greener heating system and an electric vehicle

In this week's blog, our colleague Felix shares more about his family's personal 'electrifying' journey to be more green and sharing advice and tips on what to look out for if you want to do the same.

Early in 2017, we decided it was the right time to step up our game to help counter the climate crisis, starting at our own household. At the time we had a power shower with a dedicated 10kW electric line, other warm water and radiators were heated using a conventional gas boiler. After booking a survey by a green energy company, we had been convinced to install a solar photovoltaic (PV) system and overhaul our heating system.

It all sounded very sensible, but to be perfectly honest at the time we didn’t have a great idea of how much energy a typical household was consuming, how much energy solar panels generate, or how solar energy production would translate into savings on energy bills. It was also not exactly straight forward to find all relevant information; rather than being comprehensive guide, in the following I’ll try to share our personal journey and what we have learned along the way - but I can already reveal that it was a great decision!

A few words on energy consumption

In our experience, not many people know exactly (ourselves included) how much energy different electrical equipment requires, or how much a typical household consumes every day of the year. We were quite surprised to see the difference in consumption of various devices, e.g. charging a mobile phone vs. an electric car, and how this relates to an average household consumption.

Here are some general usage statistics (rough estimates) for different activities carried out at home:

Dishwasher

2 kW/cycle

Washing machine/tumble dryer

1.5 kW/cycle

Fridge/freezer/standby devices

<0.5 kW/h

Oven

3 kW/h

Kettle

2-3 kW/h

Lighting (LED/halogen)

<0.1/ <0.5 kW/h

Desktop PC

0.3kW/h

Charging mobile phone

2kW/year (not all that much…)

EV car charger

up to 7.2 kW/h

Use cases

A single-person household in a well-lit flat with lights, TV and internet streaming devices running is probably not consuming more than 2-4kWh of electricity per day. If we assume an hourly electricity rate of 15p per kWh, this equates to ~50p per day (15p/kWh* for 100% renewable energy is probably not the cheapest tariff, but let’s see what happens to prices this year…

As a family of two adults and three children, you may find yourself in a situation where you have to run one washing machine/tumble dryer as well as several loads of the dishwasher every single day, which brings you more into a 10-15kWh/day territory.

Solar photovoltaic (PV) system

General Considerations

The idea of using idle roof space to generate your very own renewable energy is a very appealing one. A solar PV installation is a medium to long term investment, where you invest initially and expect to break even after around a 10 year mark; from then onwards you will generate pure profit (of several hundred GBP per year, depending on your tariff) until the system’s end of life (at least 25-30 years). I won’t go into specifics about costs or environmental aspects at this point, you can find out further details with a green energy company of your choice.

Solar panels are very modular. Depending on several factors, one panel generates between 200-400 Watts per hour, so a typical 12-16 panel household installation may produce a peak energy of 2.5 to 4 kW/hour. As a general rule, the more roof area you have available, the more the energy and profit you can produce (at an initially higher cost).

The roof orientation is very important, the roof angle less so. South facing roofs are ideal to make the most out of the intense British mid-day sun, whereas East or West facing roofs may produce electricity for longer, albeit at a reduced maximum output.

Shade is generally bad for solar PV systems, whether it is cast by clouds, nearby buildings or tall trees. I would estimate that fully shaded panels only produce a small fraction of their output in direct sunlight (perhaps 10-20%). 

Example solar panel set up
Our solar panel setup. House Roof (1st floor);
Garage Roof (ground floor extension roof)

Our installation

Our own installation consists of 25 panels spread over two roofs (main roof plus extension roof) and has a peak production of 5.6 kW (a high peak production is important if you want to charge an electric vehicle (EV) at zero cost).

We also replaced our heating system with two independent circuits; one for domestic hot water (300 litre storage cylinder) and another one for the central heating (120 litre tank). Both storage tanks are heated by immersion elements using excess solar energy, or using a highly efficient condensation gas boiler in case not enough sun is available.

Personal observations

A solar system generates a lot of data which can be very addictive to monitor if you love crunching numbers. Some facts are fairly obvious if you think about them, but here are some additional thoughts on how this affected us:

  • power generation varies greatly over the course of a day, peaking between 11am and 3pm
  • there is a slight difference in overall production between main and extension roofs due to earlier shade in the afternoon, so do take vegetation into consideration before considering investing in solar panels
  • much more important though is the difference in light intensity caused by the angle of the Sun. In winter, when the northern hemisphere is tilted away from the Sun, fixed-angle (i.e. roof) solar panels not only have a much reduced peak production (~2.5kW compared to 5.6kW from around April onwards), but also during summer the production time ranges from ~6:30am to 5pm, while in winter it is 9:30am to 4pm only. So even though it might be a friendly, sunny morning in January, chances are there isn’t any solar production just yet.
  • power generation varies considerably depending on the time of year. Our overall energy production amounts to ~8,000 kWh/year, following a seasonally fluctuating profile:

graph showing recent energy production
Total energy production in Wh over the past five years (the gap in Aug 2020 was a technical outage caused by a lightning storm)

The good news: from around March to October, we produce a decent amount of energy (typically 10-45kWh per day), much in excess of our personal requirements. Surplus energy heats the warm water and radiator circuits, allowing us to switch off the gas heating completely during this period.

The sad news: during the winter months (November to February) we often struggle to generate enough electricity to power a household even on sunny days.

Screenshot of Immersun monitoring app
Screen capture of Immersun monitoring app. Top panel:
Solar production (green, on the right), household consumption
(middle), water immersion heating (blue, bottom right), export
to the grid (left). Bottom panel: energy profile on a day with
adaptive EV charging.

For the first days/weeks/months, we spent a scary amount of time monitoring various events, but by now maximising using up our home-produced energy has become second nature. Examples include programming dishwasher/washing machine cycles to run during the day or charging the EV on sunny days only (if possible at all).

On most solar PV systems, energy production and consumption can be monitored in real time. Using programmable EV car charging allows us to make best use of solar production. Only unused excess energy is exported to the grid, generating some additional income.

Storing PV-generated electricity using a battery pack – a reality check

Solar energy of course has one serious limitation: it is dependent on the Sun shining. To make matters worse, the typical energy consumption pattern is diametrically opposed to the solar generation profile: while solar production follows a bell curve peaking around noon, we typically consume electricity in the mornings (getting ready for work/school) or evenings when everyone gets back home (this general rule might have to be adapted if more and more people adopt a hybrid or full working-from-home lifestyle as a result of the pandemic). An obvious solution to this disparity would be to store the solar energy produced during the day in a battery storage system, allowing you to use entirely home-produced renewable energy also outside of sun-working hours.

When we considered the battery storage topic we made the following observations:

Lithium ion batteries (especially lithium iron phosphate) currently appear the best choice as home storage solutions. They have a high number of life cycles as well as a high depth of discharge. The downside is that they are initially rather costly, and have lifetime of around 10 years (so they might have to be replaced several times during the lifetime of a PV system).

Here is my back-of-an-envelope calculation when I looked at this in late 2020:

Depending on brand, an 8kWh battery storage system would set you back between ~£4,500 and £8,500. Just going with the cheapest possible solution and its guaranteed life time of 10 years, the battery would cost roughly £450/year.

If we assume the absolute best case scenario in which we could charge the battery up to full every single day, and also use up our own energy every single evening/night/morning, we could save a maximum £409 over one year. Looking at the actual solar production at our location in Cambridge, a more realistic number of times we could probably enjoy a full charge/discharge cycle would probably be in the region of 20% of the time, reducing the possible savings to ~£150 per year (being generous here).

We came to the conclusion that – while the idea of living on 100% home-produced electricity is very desirable – it was just not economically viable at the time. Factors that might swing this calculation in favour of a battery storage system would be greatly reduced initial purchasing costs, dramatically increased energy costs, or a combination thereof (who knows what the future holds?). The situation might also have changed somewhat since, so here is a more up-to-date comparison as an example.

Alternative energy storage solutions

Typical solar energy production and energy usage patterns
Typical solar energy production and energy usage patterns

Since battery storage didn’t seem to be an economically viable option at the time, we opted for two alternative sinks for surplus electricity:

  1. Our warm water and heating circuits are heated with immersion elements powered by solar energy, roughly storing 10-15 kWh of electricity per day – but in the form of hot water. This is not only a larger amount of energy compared to most battery installations, it also come at a fraction of their cost. As a result, as long as the Sun is shining, we do not need to use gas (a fossil fuel) for the majority of the year!
  2. We made the switch to an electric car in 2017, as it seemed to be the logical next step to swap using fossil fuel burning internal combustion engines for an electric engine, mostly using electricity generated by the solar PV system (a zero cost as well as emission model). Electric cars seem to stay with us for the foreseeable future, so our arrangement is set to serve us well for years to come.

Final thoughts

I am still excited about the idea of becoming as independent of electricity from the grid as possible. As soon as battery storage becomes financially viable I am sure we will look again. This will almost certainly be accelerated greatly when the first generation of electric vehicle car batteries get taken out of service (they might still be great as home storage solutions for many years). As another option, promising solutions are in development that may allow us in the future to use the huge battery packs in electric cars as storage systems to power our homes during night time in a process called bi-directional charging. For comparison, while most domestic battery storage systems come sizes of 2-10 kWh, car batteries already boast capacities of 30-110 kWh. As an example, a particularly promising bi-directional charging system has been introduced very recently by a company called Wallbox, which will even allow you to become fully grid-independent during complete black-outs.

I am convinced our electric journey is only just beginning …

Solar Together

As a final note, the Green Labs Steering Group also have experience of Solar Together, a local group buying scheme for solar and/or batteries, which facilitates the process, provides guarantees and warranties on the products and workmanship, and achieves competitive rates as a collective. You can register for free and without obligation to find out more.