Battery buffered EV charging may let your company expand its commercial EV charging setup straight away without having to pay for and wait for a larger grid connection.
In this article, we explain how battery buffered EV charging works, how much battery power and storage you need, and how to compare the full cost of a setup with a grid upgrade.
What is battery buffering?
Battery buffering uses a battery installed at your site to store electricity when the rest of the site is using less power than your grid connection can provide. For example, at a fleet depot, the battery might charge during the day while most vehicles are away and the rest of the site is using less electricity.
Later, if your EV chargers and other equipment need more power than you can draw from the grid, the battery supplies the extra power.
The electricity you store may come from the grid when prices are lower, from your solar panels, or both. Using commercial battery storage for EV charging this way reduces the peak power you take from the grid. This is a form of peak shaving.
Does the battery have to be built into the charger?
No. There are two main ways to set up battery buffered charging.
With battery integrated EV charging, the battery and charger are supplied as one system rather than the battery being installed separately elsewhere on the site.
You can also install a separate battery and use it to supply one or more EV chargers through your site’s electrical system.
Your installer should check which setup works with the chargers you plan to use and whether the battery, cables and other electrical equipment can supply the power they need.
A separate battery will normally use an inverter to convert the electricity from the battery into the form used by your site.
How much battery power and stored electricity do you need?
The battery needs to be large enough to support the chargers when your grid connection cannot, without paying for more battery than your site actually needs.
How much power must the battery supply?
Let’s say your site can take up to 200 kilowatts (kW) from the grid. If the rest of the site uses 80kW, that means you’ve got 120kW left to power the EV chargers. With hotel EV charging, some of that 80kW could be going to kitchens, laundry, heating and cooling while guests charge their cars.
Your EV chargers need 180kW between them to charge the vehicles at the rate you need. The grid can provide 120kW, so the battery needs to supply the remaining 60kW to make up the shortfall.
How much electricity must the battery store?
Now let’s say the chargers need that extra 60kW for 45 minutes. We’ll assume there is no solar generation during that time, so the battery has to cover the whole shortfall.
Forty-five minutes is 0.75 hours, so the battery needs to supply 60kW × 0.75 = 45 kilowatt-hours (kWh).
Kilowatt-hours measure how much electricity the battery can supply over time. In this example, it needs to deliver at least 45kWh during those 45 minutes.
A battery rated at 45kWh would only be enough if all 45kWh were available when you needed it. In practice, some charge may need to stay in reserve, and the amount the battery can store will fall as it gets older.
In this case, it would be safer to choose a battery with more than 45kWh of usable capacity. Your installer can calculate how much extra capacity you need to allow for reserves.
Can the battery recharge before the next peak?
After supplying 45kWh to the chargers, the battery needs to replace that electricity before it is needed again.
You lose some electricity when charging and using the battery. So, for every 50kWh you put into it, you might only be able to use 45kWh later.
To have another 45kWh available for the next busy period, you therefore need to put about 50kWh back into the battery.
If your site has 25kW spare for charging the battery, that will take about two hours. If the battery is needed again after only one hour, it won’t have had enough time to recharge fully.
If that keeps happening, the battery will start each busy period with less charge and eventually won’t be able to make up the full shortfall.
Ask your installer to check several busy periods and show that the battery has enough time and spare power to recharge between them.
A larger battery can delay this problem, but it cannot solve it. If you cannot put enough electricity back into the battery between busy periods, it will eventually run short whatever its size.
How should the battery and chargers work together?
Your installer should set the battery and chargers up so they respond to what is happening on the site.
The system needs to know how much power the site is using, how much the chargers need and how much charge is left in the battery.
If the chargers need more power than the grid can provide, the battery supplies the extra. If the rest of the site starts using more power too, the battery can supply more or the chargers can slow down.
You should also decide how much charge the battery must keep back for EV charging. That stops it using too much electricity earlier in the day and leaving too little for the vehicles later.
What happens if the battery cannot supply enough power?
If the battery runs low or stops working, the chargers cannot keep using more power than the grid connection can provide. The controls must then slow or pause charging to keep the site within its limit.
Your installer should set these limits before the chargers go into use. Ask what will happen if the site meter stops sending readings or the controls lose contact with the battery or chargers.
They should also confirm that the setup meets your network operator’s fail-safe requirements. The system still needs to keep the site within its grid limit if part of the control system stops working.
Should you use a battery buffer or upgrade your grid connection?
Before comparing the two, check whether you need either of them.
Ask your installer whether load management through an EV charging management system can give every vehicle enough charge using the power your site already has. If it can, you may not need a battery or a larger grid connection.
If it cannot, compare battery buffering with increasing your grid capacity. Use the same charging requirements for both options, including how much electricity the vehicles need and when they need it. Otherwise, you are not comparing like with like.
For the grid option, get a quote from your local electricity network company. It should tell you how much extra power you can get, what work is included, what you need to pay for separately and when the new connection could be ready.
What should each quote include?
Ask both suppliers to show the full cost of getting your vehicles charged, not just the headline price.
| What to compare | Battery buffering | Larger grid connection |
|---|---|---|
| Upfront cost | Include the battery, inverter, controls, installation and any other electrical or site work you need. | Include the network company’s work and any changes needed to your site’s electrical equipment. |
| Ongoing costs | Include electricity, software and maintenance. Also allow for electricity lost when charging and using the battery, and any battery replacement you expect during the period you compare. | Include the electricity and connection charges that will apply once the larger connection is in place, plus maintenance of any equipment you will own. |
| When you can start charging | Allow time for equipment delivery, installation, permissions and testing. | Check when both the network work and your own site work will be finished. |
| Future charging needs | Ask how much more charging you can add before you need a bigger battery, more battery power or a grid upgrade. | Ask how much more charging the larger connection can support before you need another upgrade. |
How should you check the claimed savings?
Use the same assumptions for both options so you are comparing like with like.
Compare them over the same number of years. If you expect to add more vehicles or chargers later, include the same expansion in both calculations.
Use the electricity prices and grid charges that would actually apply to each option.
If a supplier says the battery will save money by charging when electricity is cheaper, include the electricity lost while charging and using the battery.
If solar is part of the proposal, use the same solar installation in both calculations. Do not count electricity used directly from the solar panels as a battery saving as well.
Keep any income from battery energy trading separate. Only include income the battery could earn when that stored electricity is not needed for EV charging, and deduct any fees and other costs.
The chargers may also be able to earn income. If vehicles stay connected for longer than they need to charge, some eligible sites in Great Britain may be able to earn money from wholesale or demand flexibility markets by changing when the chargers use power. Only include this income if charging can be shifted without leaving vehicles short of the electricity they need.
If you include borrowing costs, tax or money you may get back from selling the equipment later, treat both options in the same way.
What if the battery only delays a grid upgrade?
A battery may let you start charging sooner while you wait for a larger grid connection.
If that means you avoid spending money on temporary charging or another short-term solution, include that saving.
But if you will still need the grid upgrade later, include that future cost as well. Otherwise, the battery can look cheaper simply because part of the bill has been pushed into the future.
Can your EV charging battery earn extra income?
If it has enough power and stored electricity left after you’ve put aside what the vehicles need, you may be able to earn money from it.
One way is through a flexibility service which pays businesses to change how much electricity they take from or send to the grid at a particular time. This happens because, sometimes, NESO or your local network operator wants to reduce demand on the grid.
Your business would normally take part through a specialist provider (like GridVolt). The provider deals with the service and arranges how your site will respond.
Example: using the battery during a flexibility event
Let’s say your depot normally takes 120kW from the grid between six and seven in the evening. The provider asks whether you can cut that by 30kW for the hour.
Your vans won’t need the battery until later that night. It has enough stored electricity to cover the event and still do its main job, so the battery supplies 30kW to the depot.
The depot continues to use 120kW, but only 90kW now comes from the grid:
120kW − 30kW = 90kW
Doing this means the meter on your site records a 30kW reduction in the power you’ve taken from the grid. Over one hour, that comes to:
30kW × 1 hour = 30kWh
You haven’t sent that 30kWh to the grid. The battery supplied it to your depot, which meant you didn’t need to take it from the grid.
To work out the payment, the provider compares the meter reading with the baseline. The baseline is an estimate of what your site would have taken from the grid if the event hadn’t happened. In this example, the baseline is 120kW and the meter records 90kW.
Suppose, purely to show the calculation, that the service paid £1 for each kWh of reduced grid use. Your gross payment would be:
30kWh × £1 = £30
The £1 rate is illustrative. The actual payment would depend on the service, the provider’s fees and the rules that applied to the event.
The battery may need to recharge afterwards, so your depot hasn’t necessarily used less electricity across the whole day. The provider should only use stored electricity that the vans won’t need before the battery can recharge.
Ask how much you would actually receive after the provider’s fees, recharging costs, battery losses and extra wear. Treat this as extra income as your battery setup should still make financial sense as a way to charge your vehicles without exceeding your grid limit.
What should you ask an installer to prove?
Give your installer enough information to understand when your site will need the most power. Then ask them to show that the battery and chargers will work during those periods before you order.
What information should you provide?
Share your grid connection limit and a full year of electricity readings with your installer, if you have them.
Half-hourly data analysis can show how your electricity use changes through the day and year. Your installer may also need more frequent readings if your site has short spikes in demand.
If you already have EV chargers, give them your charging records too.
For new chargers, tell them when vehicles will charge and how much power they will need. For EV fleet charging, include when the vehicles come back to base, when they need to leave the site and how much charge each one needs.
What should they show you before you order?
Ask them to show that the proposed setup can cope with the busy periods you expect.
- Vehicles get enough charge: Check that each vehicle can receive the electricity it needs before it has to leave.
- The battery can recharge: Ask them to show what happens to the battery over several busy periods. It needs enough time and spare power to recharge between them.
- The battery still works as it gets older: Ask them to repeat the calculation using the amount of power and storage the battery is expected to have later in its life.
- Charging stays within your grid limit if the battery is unavailable: Before handover, have the installer show that the chargers slow or pause if the battery runs low, shuts down or loses contact with the controls.
What site checks and permissions are needed?
Your installer also needs to check where the battery will go, whether there is enough access for maintenance and emergencies, what fire precautions are needed and whether your existing electrical equipment can support the installation.
Agree who will arrange any planning permission and network approvals before you order the equipment.
In Great Britain, they must also check whether a G99 application is required for the battery. Limiting how much electricity the site imports or exports does not remove the need for any connection application that still applies.
For sites in the Republic of Ireland, ask the installer to confirm the correct process with ESB Networks
Manage your battery and EV chargers with GridVolt
For battery buffering to work, the battery needs to have enough charge available when the EV chargers need extra power. It also needs to recharge at the right times without pushing your site over its grid limit.
GridVolt Energy Manager can control compatible batteries and EV chargers alongside solar equipment. It uses your site's electricity data, tariff and forecasts to decide when the battery should charge and when it should supply power.
Where vehicles have spare charging time, GridVolt can also help assess whether changing when eligible chargers use power could earn income from wholesale or demand flexibility markets in Great Britain with our GridTrade service. The battery and vehicle charging requirements still come first.
If you're planning a battery-buffered EV charging setup, send us the battery, inverter and charger models you are considering, along with your grid limit and electricity-use data. We can check what Energy Manager can connect to and explain how it could control the equipment together.
Your installer still designs the battery and charging installation and sets the electrical protection needed for your site.
Fill in the form on the right to find out more, or get in touch via our contact page.
FAQs
Will a battery buffer keep chargers running during a power cut?
Not automatically. Your installer must design the system for power-cut backup, with equipment that safely separates the backup supply from the grid. Check whether the backup supply includes the EV chargers, how much power they can use and for how long.
Is battery buffering the same as vehicle-to-grid charging?
No. A battery buffer uses a battery installed at the site rather than the one inside a vehicle. Vehicle-to-grid charging sends electricity from a vehicle’s battery back to the grid. It needs a vehicle and charging equipment that support this.
Can you add a battery buffer to existing EV chargers?
Yes, provided your chargers, electrical installation and controls can work with the battery. Have your installer check the charger models, wiring and grid limits, and confirm how the battery and charger controls will connect. The chargers do not need built-in batteries to use a separate battery at the site.