Is using your home for energy storage really practical?

Electricity demand varies through the day, and the cost of supply depends largely on the demand peaks. Therefore reducing peak demand can bring cost savings. Most of us are going to be using electric heating in future, mostly with heat pumps, and this will be a major part of peak demand in cold weather. (Peak time currently is typically 4-7pm, including cooking time for many households.)

You can turn off your home heating for an hour or so, or at least turn it down for a bit. This could be useful to reduce demand during peaks or at other times when there is a short lull in renewable energy supply. The grid could be supplied from energy storage instead, but this is generally expensive whereas homes with masonry construction have ‘built in’ thermal storage. Turning off the heating you are using the fabric of your house as an energy storage system for free. (Well not quite free perhaps, you might need a controller like the Homely thermostat). However there are difficulties with this approach. On the whole, I do not believe it is sensible. The main reasons being:

• When you turn the heating off in your house (or down) it gets colder. This means it has to be warmer than you need to start with, which means you are likely to have bigger bills overall.

• Most homes cool quite quickly over the first hour or two. Preheating for an hour or two in advance does not help very much.

• Some people are impervious to cold - many of us are not. Even quite small changes in temperature bring discomfort. 

• Energy demand flexibility can be achieved more easily with other kinds of demand such as EV charging and especially with bi-directional vehicle to grid charging. It makes no difference to your wellbeing when your car is charged, as long as it is ready when you need it. 

In this post I explore (a) the rate at which homes cool, (b) how sensitive we are to cooling and (3) potential for demand response in buildings compared to EVs.

How big should your heat pump be?

Most people have combi-boilers and the size of the boiler required is often dictated by the need to heat hot water for baths and showers quickly rather than space heating. With heat pumps, you normally have a cylinder for hot water and it is the space heating requirement that dictates the size you need. So how big should it be? Hearsay tells me that even with boilers, sales-people love to sell you systems that are bigger than you need. My heat pump is oversized by at least 50%.

My heat pump is much bigger than it needs to be. I have tried to make it less intrusive with some stick-on leaf patterns. Still, at least I have renewable heating :-)

Oversizing does matter, both for boilers and heat pumps.

• Large systems cost more - not as much as you might expect, but still more.
• Running at small fractions of full capacity reduces efficiency (usually).
• Large systems are physically large and, in the case of heat pumps, have a greater impact on the landscape (see picture above).

In this post I discuss a very simple rule of thumb for heat pump size - thanks to Michael de Podesta - and some reasons it gives an underestimate in my case. Also I show you how to cross check your installer's estimate using the EPC certificate for your home. I wish I had done this!

Performance of my heat pump

Last year we installed a heat pump at home and we now have almost an entire heating season of data for it. Here I present some results. Our overall mean COP (efficiency) is a little better than we were told to expect. During the heating season we have made a number of changes: increasing the night time setback temperature and reducing the configured flow temperature, with impacts on heat demand, electricity consumption or both. It is tempting to focus on efficiency alone but, being practical, it is energy use that we want to minimise. For example you can expect increasing setback temperature to increase efficiency but also heat demand - which wins? 

SMETER – measuring the energy performance of a house

I think we all realise that home EPCs are not always accurate in predicting energy bills for a house. This is a shame for a lot of reasons. It makes it hard for the government to form housing policy: how do we assess potential energy savings from energy efficiency measures on the housing stock, if we don’t know how bad it is already? It is a shame for people looking to rent or buy a home who want to know what they are letting themselves in for. So why don’t we just measure the energy efficiency instead? Would that it were that simple. The government is well aware of the problem and has provided financial support for some potential solutions making use of smart meter data. The project was called SMETER (Smart Meter Enabled Thermal Efficiency Ratings). The results were promising but not brilliant. Here I explain why it is so hard, and some of the solution strategies.

Whether you are modelling or measuring, the core of the problem is the balance of heating coming in and heat going out. This chart captures the balance from a model of a typical semi detached house with filled cavity walls, double glazed windows and 100mm loft insulation, in the East of England. The model is similar to that used for calculating EPCs. The chart shows month by month heat losses (blue and purple, downwards) through conduction through walls etc and air leakage, balanced by heat gains including solar gains through windows, use of electrical appliances, heat losses from the hot water system (mainly from hot water pipes distributing heat around the house) and of course the heating. 

Estimating carbon savings for switching to EV or heat pump

Suppose you want to invest some cash in reducing your personal carbon emissions - can you save more carbon by replacing your car with an EV or by swapping your boiler with a heat pump (and how much would this cost?). Unfortunately, the answer depends on lots of variables such as how far you drive and how much is your heating bill, do you have solar panels – if so how much power are you currently exporting? So I have prepared a tool to help you estimate your savings and in this post I use it to illustrate some examples. You probably know that energy costs for an EV are lower than for a diesel or petrol car. You may be surprised that the heat pump can reduce your energy bills too, using prices from April after the cap changes.

Click here for the EV tool and here for the heat pump one. These are only estimates! If you find them useful let me know.

Energy price increases - and the impacts for heat pumps.

Most reporting on the changes to energy price caps that come into force from April are about dual fuel bills and just report the total bill, not separating gas and electricity. However, if you are considering switching to a heat pump you are also interested in the ratio between gas and electricity costs - how much will it cost you to make that switch? Currently, heat pumps typically cost a little more to run than a gas boiler, but from April the gap narrows - plausibly to nothing in many cases.

The data behind this blog comes from OFGEM.

Firstly, the typical bill - there are slightly different costs for payment methods and I have assumed a standard credit payment. I am also using Eastern prices. There are regional differences but these too are small. 

Price cap for gas and electricity, whole bill £/year. This is based on 12000 kWh/year for gas and 3100 kWh/year for electricity.

Will heating your house constantly use more energy?

We are advised when we get a heat pump to change the heating schedule to be constant, or nearly so. This is because heat pumps are efficient when supplying gentle heat but not good at heating a home from cold quickly. This is completely the opposite of what we have learned about keeping our bills low when using a gas boiler. So how much are we currently saving, and how is this different with a heat pump? I have investigated this with a model of a semi detached house (using similar models to my work for BEIS [1]). In the boiler case, savings from intermittent heating are substantial - up to 21%. In the heat pump case, the difference is much less - at most 4%.