In short: every time electricity is converted between AC and DC, some of it is lost as heat. A DC-DC charger takes power straight from a battery’s DC supply to the vehicle, so energy that has been stored in a battery reaches the car with fewer conversions than it would through an AC-coupled system. That can cut losses and improve returns on battery-boosted sites. But the saving depends on the whole design, so count the conversion stages before you count the savings.
Most EV fast chargers are AC-DC chargers: they take alternating current from the grid and convert it to the direct current a vehicle battery needs. On a site with battery storage, there is another option. A DC-DC charger connects to the battery’s DC side and converts DC at one voltage to DC at the voltage the vehicle needs. Here is when that saves energy and money, and when it does not.
Where do the losses come from?
Power electronics are efficient, but not perfect. Each conversion stage, AC to DC or DC to AC, loses a few per cent of the energy passing through it. Batteries also lose a little energy between charging and discharging. The more stages energy passes through between the grid and the vehicle, the more is lost on the way.
How many conversions does each design have?
Follow the energy from the grid to the car for energy that passes through a battery on the way:
| Design | Path for stored energy | Conversion stages |
|---|---|---|
| AC-coupled battery with AC-DC charger | Grid AC to battery DC, battery DC back to AC, AC to DC in the charger, then to the vehicle | 3 |
| DC-coupled battery with DC-DC charger | Grid AC to DC into the battery, then DC to DC into the vehicle | 2 |
| Charger direct from the grid, no battery | Grid AC to DC in the charger, then to the vehicle | 1 |
Removing the step where battery DC is turned back into AC, only to be turned into DC again inside the charger, is where the DC-DC design saves energy.
What could that save? A worked example
The numbers below are illustrative assumptions, not measured figures for any product. Use your suppliers’ real efficiency data at realistic loads.
- Assume each conversion stage is 97% efficient, and ignore the battery’s own losses, which are similar in both designs.
- Three stages: 0.97 × 0.97 × 0.97 = about 91.3% of the energy arrives in the vehicle.
- Two stages: 0.97 × 0.97 = about 94.1% arrives.
- If a site delivers 200,000 kWh a year through its battery, the three-stage design draws about 219,100 kWh from the grid. The two-stage design draws about 212,600 kWh.
- The difference is roughly 6,600 kWh a year. At an assumed 25p per kWh, that is about £1,600 a year in electricity, every year the site runs.
Run the same sum with your own volumes, prices and efficiency figures. The saving grows with the share of energy that passes through the battery, which is why it matters most on battery-boosted sites with small grid connections. Our article on why battery-boosted charging matters explains when those sites make sense.
Is DC-DC always more efficient?
No, and this is the part sales literature often leaves out. Efficiency depends on the whole system, not on whether a product is labelled DC-DC.
A study from the Karlsruhe Institute of Technology compared an AC-coupled and a DC-coupled EV charging station with solar and battery storage. In that example system, the DC-coupled station’s higher number of converters led to a higher total energy loss, and the AC-coupled station came out about 10% more profitable. The authors also found that more than half of the AC-coupled station’s losses came from the vehicles’ own on-board chargers.
The lesson is not that one design always wins. It is that you need to count the conversion stages in the actual system you are buying, for the way your site will actually run.
What else affects return on investment?
- Part-load efficiency: converters are often less efficient at low load. Ask for efficiency curves, not just the peak figure.
- Solar: DC-coupled systems can store solar energy without converting it to AC and back, adding to the saving where solar is significant.
- Equipment cost and complexity: fewer conversion stages can mean less equipment, but not always. Compare complete system prices.
- Maintenance and reliability: more power electronics means more to maintain. Ask about service history.
- Utilisation: savings scale with the energy delivered. A busy site recovers its investment faster.
Questions to ask suppliers
| Ask for | Why |
|---|---|
| A diagram of every conversion stage from grid to vehicle | Shows the real path, not the marketing one |
| Efficiency at 25%, 50% and 100% load | Your site will rarely run at peak |
| Battery round-trip efficiency | Losses in the battery itself |
| Expected annual losses for your usage | Turns percentages into kWh and pounds |
| Standby consumption | Systems draw power even when idle |
Frequently asked questions
What is the difference between an AC-DC and a DC-DC charger?
An AC-DC charger converts grid AC into the DC a vehicle battery needs. A DC-DC charger takes DC from a battery or DC supply and converts it to the vehicle’s required DC voltage.
Does DC-DC charging matter without a battery on site?
Usually not. Without storage, a standard AC-DC charger already converts once from grid to vehicle. The benefit of DC-DC comes from avoiding extra conversions for stored energy.
How do I compare suppliers fairly?
Ask each for expected annual energy losses for the same usage profile, then compare those alongside price and service terms.
Comparing system designs on real losses and costs is part of our solution recommendations service. Our article on battery-integrated fast charging covers sizing. Talk to us about your site.
