We need energy in two forms, heat and power (electricity) and it is useful to think of these separately. It is possible to use electricity for heat but it is an expensive resource, even if we are producing our own, and best used for things that only electricity can do. Fuel for heat energy tends to be far cheaper – mains gas is around 5.4p/kWh while mains electricity will be over 15p/kWh.
In case you missed the previous articles of the Eco Series with Tim, here they are:
Note: If you are not familiar with kWh it stands for kilowatt hour which is a unit of energy – a cupful, maybe. Gas and electricity companies will sell you energy at a price per kilowatt hour. Your electricity or gas bill will set out the number of kWh consumed in the period and tell you what you need to pay. Divide one by the other and you will have the actual unit cost of your energy. It is important to do this as you will need that figure to enable an accurate comparison to the cost of producing your own energy.
It is also worth bearing in mind that the energy supply company will give you a headline price per kWh – for electricity that will currently be anything between 13p and 15p. To that must be added VAT and standing charges. If you do the calculation suggested above you will get a figure different to the headline price. This is the real price of the energy you buy.
To think in terms of energy self-sufficiency we need to know how much energy we need to produce. It is possible, and not that difficult, to calculate this but the easy route is to look at the bills. For electricity you will need at least the last 12 months, preferably 2 years, to work out the average monthly consumption and how that varies winter to summer.
If you have mains gas do the same calculation. If you use oil for heating you may be buying in litres. To convert that to kWh multiply by 10.8. If you use bottled (or bulk) propane multiply by 7.1 (yes, you get far less energy from propane than you do from oil). Wood pellets give about 4.5kWh per kg, kiln dried logs about 2.6kWh per kg, home dried logs is anyone’s guess. It depends on the wood (hardwood or softwood) and how long they have been dried for, but likely to be less than 2.6kWh/kg.
ELECTRICITY
In energy self-sufficiency terms there are two truths that have to be recognised:
1. Generating exactly 100% of demand is, in practical terms, impossible. What is wanted and what is being produced will both vary with the circumstances on any given day and as a result there will always be a surplus or a shortfall. It might not be much (although it generally is) but it will be there.
2. There is no domestic scale renewable electricity generating technology that gives predictable production, day-to-day and season-to-season. We can predict with some accuracy what any technology will produce over a year but all technologies currently available at the domestic level are reliant on the weather and that changes daily.
Accepting these truths means that we have to make a decision on how much electricity we want to produce. The “default” renewable energy system has become a 4kWp solar PV array on the roof. In most of the UK that system will produce around 3,200kWh per year (if it is properly installed on a south’ish facing elevation). The average consumption for a UK household is about twice that amount.
For the typical household about 40% of the electricity produced by that PV array can be used in the house. Where people spend more time at home (retired or working from home, for example), that could rise to 60%. The reason being that 50% of the annual production will be in the 3 summer months, when we need it least.
Conversely, the bulk of a wind turbines production will be in the autumn and winter months. A 5kW wind turbine will produce around 9,000kWh of electricity per year (assuming the site is windy enough to justify installing a turbine in the first place). More than enough for the whole year but there will be days when it produces nothing at all.
So we return to the decision on how much energy we want to produce. The 5kW wind turbine will produce an excess in winter that can be sold to the grid and the funds used to meet the shortfall in production in summer. Equally, a 10kW PV array will produce around 8,500kWh across the year and summer excess can be sold to pay for the winter shortfall.
A good 1kW hydro-power scheme will produce around 8,000kWh per year. It will still be affected by the weather (more rain = more production) but is far more reliable, and predictable, than other technologies. A combination of technologies (5kW PV with a 3kW wind turbine) and a battery bank would give true self-sufficiency, but would be expensive.
Realistically battery systems do not store energy produced in summer until the winter and their cost pushes the price of the electricity generated close to that of grid electricity. Obviously a more pragmatic option might be a single technology backed-up with petrol-fuelled generator, but that can hardly be considered as sustainable or being self-sufficient.
COST/BENEFIT
A 10kWp PV array is likely to cost around £12,000 installed and connected to the grid. The PV panels are likely to last in excess of 20 years but the inverters and control systems are likely to need replacing in that period, adding perhaps £3,000 to £4,000 to the overall cost. Over 20 years the system should produce some 170,000kWh, at a cost of £15,000 or 9.4p/kWh. A wind turbine will have a higher capital cost but the unit cost over the life of the system will be broadly the same – depending on the turbine and how windy the site is.
These figures do NOT include the government’s Feed-in Tariff scheme as it ended in April 2019. It is still be possible to sell surplus electricity to the grid and the current price is around 4p/kWh.
These figures may not look very good but bear in mind that they will be the same in 20 years time. If we go back 20 years we were paying less than 4p/kWh for electricity and are now paying over 15p. It would be a brave person who predicts that the same will not happen in the next 20 years. We are, in effect, buying 20+ years of electricity on day-one and thereby fixing the price.
The solution starts with the most accurate calculation possible of the energy consumption, the funds available to spend on it and the desired level of self-sufficiency. In addition we have to consider the resources available on site.
Too many wind turbines are erected in locations with insufficient wind. Too many PV arrays on roofs with not enough south in them. Doing the calculations and realistically considering the sites potential leads to the most pragmatic and effective solution.
HEAT
The solution starts with minimising the energy demand by increasing insulation and draught-proofing – which is always the best possible investment in heating. A calculation of the energy demands, divided into space heating and hot water is needed. As a pointer, hot water demand is typically about 800kWh to 1,000kWh per person per year (adults at the lower end, teenagers at the upper!).
It has to be accepted that these (and cooking) could be dealt with using bottled gas. It is inefficient, expensive and some distance from sustainable, but it is flexible and predictable. That energy could be produced with biomass (wood pellets, harvested or bought-in logs) which would be cheaper, more efficient and sustainable. It might be that heating is to be produced using electricity – probably via a heat pump. That obviously means generating more electricity and investing more in generation technology.
If the site has the benefit of mains gas it is very difficult to advocate removing it. Supplementing with biomass, a wood-burning stove, for instance, would be a good idea if it is not already being done. It is possible to link a gas boiler to an air source heat pump in a bivalent system but financially this really only works with the benefit of the Renewable Heat Incentive.
A solar thermal array would be the obvious addition to a gas boiler. Capital cost would be £3,000 to £4,000 installed. The system will last upwards of 30 years and produce around 60,000kWh in that period, at a unit cost of 7p/kWh – compared to 5.4p/kWh for mains gas. Which may not sound like a good investment but, again, it is fixing the cost of that energy for the life of the system.
The only truly off-grid heating option is logs. Harvested from your own 4-acre coppice or bought-in; used to fuel a stove or boiler. However you look at it, off-grid heating is a tough ask.
Full self-sufficiency – going off-grid – means that there is no grid back-up. The site has to produce all it needs but not more than it needs as, without a grid connection, the surplus will just be wasted. Going off-grid is either a matter of necessity or an ideological choice. The reality is that this is the 21st century, we have a fairly robust infrastructure and living off-grid is not easy and not cheap. For those driven by necessity or ideology that won’t matter, they are going to do it anyway.
For others, the appropriate degree of self-sufficiency is cheaper, more flexible and provides a significant degree of protection from rising fuel bills. And you can always add to it later if things go really badly.
We would like to thank Tim for sharing his expertise on our blog. If you would like to ask Tim a specific question about your low energy home, then leave a comment below and we will pass it on! Tim will be back with another expert article before the end of the year.