Monday 19 February 2024

A whole street of heat pumps - noise levels

Heat pumps make noises, of course they do. The question is, will their noise be a nuisance if every house on the street has one? The short answer is - not usually. Here I explain why we are not disturbed by ours, and why a whole street of heat pumps does not sound much louder than a couple. In urban areas, other noises are much more noticeable. In rural areas, you might be disturbed by yours but not so much your neighbour's.

Saturday 10 February 2024

Rainwater harvesting: dammed by regulation

For the whole of the East of England water supply is an increasing problem. Here in Cambridge we need to reduce the amount we take from our aquifers by about half in order to protect the chalk streams. This will be achieved with a combination of reducing leakage, reducing demand and increasing supply (by building large reservoirs). One way to reduce demand is to harvest rainwater for use where it does not have to be drinking quality, such as flushing toilets. 

Eddington in North West Cambridge has a rainwater harvesting system for 3000 homes, the largest in the country. It is often cited as an exemplar for water saving, with every home having two supplies: one from rainwater (non-potable) for use in the garden, in the washing machine and flushing the loo alongside the one from the mains for everything else. As I recall, the pipework has different colours. The rainwater is stored in a rather beautiful lake and is fairly clean but not as clean as the water we get from the chalk aquifer; there is equipment for basic treatment on site. The problem is, the rainwater harvesting supply has never been commissioned due to concerns from the Drinking Water Inspectorate (DWI). 


Brook Leys Lake at Eddington – stores rainwater for supply to homes, but not commissioned for use.

Tuesday 9 January 2024

Shifting policy costs on energy bills - revisited

As I said in a previous blog post, the current price ratio between electricity and gas means that switching to heat pumps often means bills increase - a major deterrent to many households considering taking on low carbon heating. Part of the problem is because of the way 'policy costs' are loaded mainly onto electricity bills instead of gas so when you switch to a heat pump you end up paying far more policy costs. However, if we were to change this so that policy costs were evenly distributed between gas and electricity, then heat pumps become cheaper. I have updated my previous calculation with a slightly revised method and based on the current price cap as shown in the following chart.

Chart shows the annual bill split into components: policy costs and other for each fuel. Current uses components as computed by OGGEM, 50% split means the variable policy costs are split between gas and electricity. HP costs are with the gas boiler replaced with a heat pump. Assumptions are explained below. Click on this for a larger image

Thursday 28 December 2023

A New Year wish – autonomous cars.

Driverless vehicles are in the news again. (Driverless cars: Tech possible for UK motorways by 2026, transport secretary says). 

The usual justification given for needing driverless cars is safety because most car accidents are caused by human error. I disagree because it seems unlikely driverless cars will ever be demonstrably and significantly more safe than cars driven by humans. However, that does not mean we do not need autonomous vehicles. I think the main reasons are improving equality of access and reducing GHG emissions. That may surprise you as it is often predicted autonomous vehicles will increase car use and emissions. However, driverless cars enable cheaper and more convenient public transport, and also enable policies that can discourage car use. 

Tuesday 21 November 2023

Should we shift energy taxes to make heat pumps cheaper to run?

Tariffs vary but averaged across the country, under the current OFGEM price caps, electricity costs four times gas per kWh. This means that by my estimate [1] unless you go off gas completely (hence avoiding the fixed daily charge) you need a heat pump efficiency (SCOP) of 360% to get similar costs. This is not unknown but considerably better than average. If you do go off gas completely, you need 320% which is still better than average. If you only got 300% you would be paying 6% more with the heat pump than gas. This is discouraging for people wanting to switch to low carbon heating.

However, a significant part of the electricity bill is due to environmental and social policies or 'taxes'. If these were removed or shifted, the ratio of electricity to gas price would be smaller, making heat pumps relatively cheaper. Hitherto, policy has been to keep gas cheap because so many of us rely on it for heating which is essential for health. However, doing so penalises households that make the transition to low carbon heating. What are these taxes and how much difference would this make? Here is a graph showing policy costs on gas and electricity as of September. There are more policy costs on the electricity bill than the gas bill and the home with a heat pump uses more electricity, so pays even more policy costs.


Policy costs for a typical bill of 12000 kWh gas, 3100 kWh electricity or, with a heat pump, 6650 kWh annually. The policy costs are from OFGEM [3] and the heat pump kWh used is from [1] Costs are taken from the period July/Sep 2023, the latest available. These costs vary little by region. The acronyms are explained below. 

Tuesday 31 October 2023

Understanding the costs and savings on your EPC recommendations

On your home energy performance certificate (EPC) you may find a list of recommendations for energy saving measures you can make. For each one, there is a range given for ‘typical’ installation cost and a value for ‘typical’ annual savings. However, looking at a sample of EPCs I was shocked to find that the ‘typical’ cost of each measure is the same regardless of the size or type of house. On the other hand, the savings figure does vary in a plausible fashion. 

For example, assuming you have cavity walls that are not filled it might say cavity wall insulation (CWI) will cost £500 to £1,500 to install and will save £90/year, implying a payback time of five to fifteen years. Five years is quite good but fifteen is not so good so this is not very helpful. This EPC was for a semi-detached house and looking at Checkatrade I see that the typical cost for CWI for a semi-detached house is £800 to £1,200. That gives a payback time of nine to thirteen years. That is more usefully precise – but is it accurate?

In this post I explore the meaning of these figures in more detail and compare the installation costs with on the EPC with those from Checkatrade or Energy Saving Trust.

Saturday 30 September 2023

How much can you save from solar PV and battery with a heat pump?

You can substantially reduce your electricity bills if you have solar panels and a battery. What about if you have a heat pump? It certainly does make a difference but most of your heating demand is in the winter and most of your PV generation is in the summer. You would need an enormous battery to save enough in summer to use in winter. Suppose you put in enough solar panels to match your annual use so you are net zero for electricity consumption. What would this mean for your bills and for carbon emissions? This chart shows example savings from an energy use model.

Impact on the electricity bill for the model house heated with an air source heat pump, firstly with no panels, then with solar panels (6,700 kWh/year), then adding a medium or large size battery. Assumes a flat tariff with 30p/day fixed charge, 25p/kWh for electricity supplied and 12p/kWh for electricity export.

I have used a model of a fairly typical semi-detached home. Using a combi boiler, the gas bill for this model is about 10,500 kWh/year, electricity 2,800 kWh/year. Switching to an air source heat pump for heating, electricity demand increases to 6,700 kWh/year and I have given the house solar panels to match, so net zero electricity over the year. The battery charge-discharge efficiency is 90% and the solar inverter efficiency is 95%.The weather is typical for Cambridge, with hourly resolution. 

NB. Few people have more than 4 kWp, partly for lack of suitable roof area, and partly as you need special permission from the DNO (Distribution Network Operator) to exceed this. If these were all south facing you might get 3700 kWh from them. I have also modelled the impact of an array that size. The savings are smaller but definitely worth having. Also you can increase the savings with a tariff that has lower rates for off-peak electricity - I have used the Octopus Cosy tariff as an example.