More and more people are asking about the best way to monitor household energy use. There are policy pressures from the drive to reduce carbon emissions and peak power demand from homes generally, as well as specific drivers, like Scotland’s designation of energy efficiency as a national infrastructure priority since 2015. However, reliable monitoring of energy use and comfort conditions in homes is difficult, and there are avoidable pitfalls. Here is our advice.
- Be clear about your objectives before you start recruiting households to participate and well before you start thinking about what to monitor. Many projects start with a wish-list of things (temperature, humidity, CO2, electricity, gas…) they would like to track and assume they’ll work out how to use the data later. At best, this leads to over-investment in monitoring equipment. At worst, it can result in a huge database that cannot be analysed because key parameters are missing.
- Accept that what you are trying to do is HARD. Recruiting households to participate is commonly perceived to be the most difficult step. It is – until you come to installing monitoring and communications equipment and trying to get that to work. And even this is surpassed by the difficulty of maintaining a monitoring system once it has been installed. (We have not encountered a single monitoring and comms system that can be installed and then forgotten about. More commonly, if you look away for more than a few days you will find that part of the system has tripped up and you face a gap in all or part of the data.) Depending on your objectives, analysing the data can be at least as hard again.
- It is much harder and more expensive to monitor gas or oil use than electricity use. You cannot assume that gas meters will have a pulse output that can be connected to a logger. Sometimes, manually recording gas or oil use is the best solution. Bear in mind that if householders are to record readings they need a regular reminder, and they need to phone in or record readings online so you know it has been done.
- Current clamps are economical and easy to install, but they measure current and not the power factor or voltage. The power factor describes the link between ‘apparent power’, which the clamp can measure, and not net power, which it cannot. The power factor varies for different loads and appliances, but it can easily average 0.8 (which means a clamp would overestimate the real power use by 25%) and UK voltage can vary from 216 V to 254V, which means that relying on current clamps alone to assess power use can bring an inaccuracy of up to a further 10%. If all your electricity meters are quite new, they should have flashing LED lights to record net power, and you can use this for more accurate monitoring using an optical sensor like Current Cost’s OptiSmart.
- You will need external temperature data as well as the energy use data to make sense of any heating energy data. It is possible to use existing data from a local weather station (typically the nearest airport), but it may be better to collect your own temperature data close to the site. If you install your own, though, the location is critical: it must be out of direct sunlight and exposed equally from all directions.
- Resolution matters, but the monitoring frequency should link to your objectives. Unless you aim to ‘disaggregate’ electric appliances (identifying individual appliances from their energy signatures) you probably don’t need very high-resolution data. Daily readings for heating fuel, hourly readings for indoor and outdoor temperature, and 10-minute data for electricity are a reasonable starting point for most projects. This is also more economical in terms of file size and passing data between devices – which affects battery life and how often you have to replace batteries.
- Most people assume they will need real-time data, and a ‘wide area’ communication link to transmit data to a central data centre. Wrong. It depends on the objectives again, and communications links are fragile and prone to failure. We sometimes recommend simple, unconnected monitoring devices with built-in loggers. These can be collected or posted to a data centre for analysis, with replacement devices provided if you need to continue monitoring over a long period.
- Make the most of existing data. Few households keep a record of their energy bills, but some do, and it is worth asking households if they have their bills for the two years preceding any monitoring. Even if they don’t have bills, many households can view their account details online and use this to assess past energy use. You should only use figures between actual (not estimated) readings. This can be combined with weather data (see Point 5 above) to see whether energy use has changed – possibly because of energy efficiency upgrades.
- Consider using smart-meter data. Getting on for 12% of homes already have smart meters, which offer the opportunity of collecting half-hourly electricity and gas data. In theory, all homes will have smart meters by 2020. It makes sense to use this – if you can get householders’ consent to access their smart meter data.
- If you need to separate energy use for hot-water from space heating energy, you can often estimate this without additional monitoring equipment using data from the warmest months, when no space heating is needed. (This method does not work for homes with solar hot water panels, and is more difficult if a household has gas cooking and water heating.)
We have worked on five separate large-scale monitoring projects, as well as advising DECC (now absorbed into BEIS) on monitoring household energy use. We also wrote this report and prepared these costing tables for a potential large-scale study of energy use in homes, for up to 10,000 homes. We are currently carrying out impact assessment work for a large insulation company, examining the effect on internal temperature, fuel use, relative humidity and overall comfort of energy interventions on 70 homes.