Using a battery to save carbon emissions

This post was updated on 1st March with more recent data.

The Transition Cambridge Energy Group is a great place to kick around ideas and share experiences. One topic that keeps coming up again and again is batteries – are they worthwhile? One of our members, Ian, has taken the plunge and would like to share his reasoning and experience with you. 

Ian likes to be an early adopter of low carbon technology though like the rest of us he does not have infinite money to spend! He installed a heat pump and PV panels in 2011. He also has an electric car, and since mid January, he has a 15 kWh battery and is delighted with the carbon savings. At this time of year there is usually a considerable difference in the carbon intensity of electricity between night time, when demand is low, and day time when demand is high so by charging at night for use in the day he makes a considerable saving. 

How do you get carbon savings from using a battery? 

This chart from grid watch shows where we have been getting our electricity from over the last month. You can see how demand fluctuates sharply through each 24 cycle between daytime and night time. The overall emissions are mainly to do with how much orange there is – from gas power stations. Wind is blue. During the first few days in January the wind was very strong but since then we have used a lot of gas, especially during the day when demand is high. 

Chart from https://gridwatch.co.uk/ showing much less use of gas power stations overnight than in the day. Grey is nuclear, orange is CCGT, red is biomass blue is wind, solar is yellow. The others include coal, hydro electric power, and interchanges with other countries. 

In the summer the effect is less, partly because demand is lower and partly because there is more solar during the day. This means the peak emissions are actually much later in the evening – around 10pm as you can see in this chart from my personal blog. 

Chart from Savings on bills can carbon with TOU tariffs. Green represents cheap times and red are expensive times using the TIDE tariff from Green Energy.

Carbon savings in February were around 15%

Ian’s battery system allows him to program it a bit like a heating controller. He currently has it set to charge up overnight at cheap rate (on the Octopus Go Faster tariff) and during the day from the PV panels when there is spare power. He uses the stored power during the morning peak (6.30-10.30am) and after 4pm. In the first month after installation gross carbon savings were only about 8% but during February the weather has been more changeable and the emissions savings were about 20%.  However there are electrical losses associated with the battery – probably about 4% in his case due to an AC coupled set up. This means that he is actually importing a little more electricity than he would have done without the battery. Also, Ian already makes some savings by using a Homely thermostat to manage his heat pump. We estimate net savings in February were about 15%, not including savings from using more of the power from his solar panels, which is also stored in the battery when possible.

The carbon savings are highly weather dependent. Looking to the future, as the grid gets more and more clean energy, the differences due to the weather will increase, with emissions in windy or sunny periods much less than they are now. Also, his current control system is fairly dumb with a fixed schedule. A smarter system could make more savings by adapting to real time conditions (both price and carbon short term forecasts). 

How does this make cost savings? Will the battery pay for itself? 

The cost savings from using the battery are different in summer and winter. In the winter, Ian’s demand is high (because of the heat pump - he is currently using up to 45 kWh/day all told including EV charging) and the PV power is low. So the main cost savings are from charging the battery at cheap times. Ian is currently on the Octopus Go Faster tariff which is currently designed for people with EVs. It offers very cheap electricity for a few hours each night – currently between 1.30am and 6.30am so he charges the battery then, as well as his EV. The battery is not big enough to store a whole day’s demand so, he cannot get all he needs at the cheap rate. However he has made savings: before the battery was installed, in the first week in Jan, Ian paid an average of 12.25p/kWh (including VAT at 5%). In the third week, with the battery installed, overall use was up a bit, but he paid on average 8.9p/kWh. 

In the summer, Ian has much lower demand – less than 10 kWh/day - and much more solar power. With the battery he will be able to get all or nearly all his electricity from his own panels for free. Also, he plans to switch to the Octopus Agile tariff which allows him to take advantage of better prices for exporting his PV power. The export tariff is called Octopus Agile Outgoing and there is a price premium in the mid to late afternoon 4-7pm due to transmission system charges. (There is a similar cost premium for the corresponding import tariff.) So in the summer, Ian will make a profit by using the battery to store his own solar power for his own use, and by exporting any spare during the peak times. 

Ian is hoping to improve savings and ease of operation with more intelligent control systems. He is looking at Tribe and Homely, to optimise interaction between the battery and the heat pump. 

In summer, cost savings and carbon savings conflict. 

Unfortunately there is a conflict between carbon savings and cost savings in the summer, because the peak emissions are later in the evening than the peak pricing period with Agile Outgoing. So there is a choice to make there – do you optimise for cost or emissions saving or a blend of both? Whichever you choose you are still saving carbon by using your own solar power which is effectively zero emissions. 

A Lithium iron phosphate battery - heavier but safer and faster than a Li-ion.

 

Ian’s battery -Huawei Luna Smart PV battery with 3 modules each 5kWh.

Ian’s battery is a lithium iron phosphate battery. It is a bit heavier than a lithium ion but this is not a problem as it is not going anywhere. This type of battery is also considered safer and can deliver higher charge and discharge rates. The cost was £500/kWh usable storage (including 20% VAT). Smaller batteries will cost more, because a lot of the cost is fixed or nearly so. However, all prices have come down a lot over the last four years when Ian first looked at the possibilities. 

The system he installed is a Huawei Solar Fusion system, with three modules of 5 kWh each so 15 kWh usable storage. You can have one to three modules in this system. He chose the size to be enough to cover daily demand for most of the year. Bigger than this and you will be paying for storage that is rarely used. 

The electrician dealt with getting approval for higher than normal export power.

The battery was installed by a local electrician who was competent to do the installation and also to fill in the forms for G99 approval - permission to export more than the standard 4 kW. He managed to get permission for 8.68 kW which means Ian can output all his PV power and discharge the battery at the same time. This will useful for getting the maximum return in summer, by exporting stored power and sunshine in the premium period.

Ian found the team at Midsummer Energy very helpful in providing advice, recommending an electrician and supplier of the equipment. 

Get the battery last so you can calculate the best size. 

Ian points out that you need to complete all your energy saving measures and your low carbon heating/transport plans before you consider installing a battery. This is because the battery needs to be the right size for your energy demand – too small and you will miss out on carbon savings, too big and you have wasted a lot of money. Ian’s home is energy efficient – he bought the house off-plan on the Accordia development which meant he could influence the design to some extent. He opted to maximise solar gain through windows, especially into the living area. Also Ian’s home, like many in Accordia, has underfloor heating which is great for heat pumps and he got one installed right from the start. The house is EPC rated B - or would have been with a gas boiler. With the heat pump it rates lower because electric heating is massively penalised in the EPC ratings. Ian also has PV panels and an electric car. With all this considered, he calculates the carbon savings were 60-70% down compared to a gas boiler and conventional car, even before adding the battery. 

Batteries give control as well as carbon savings

As well as carbon savings, the battery gives Ian more control on when he draws power for the heat pump and other devices. This flexibility is itself valuable for balancing the grid and we are going to need more of this as more people switch to electricity for heating and transport.