Slow-Charging Your EV Wastes More Power Than You’d Think

Carefully trickle-charging your EV with surplus solar instead of buying electricity from the grid sounds like the perfect way to charge. But there’s an uncomfortable truth: slow charging can waste considerably more energy than charging at higher power with a home charger.

Spoiler alert: that doesn’t mean you should stop charging from solar. Cheap electricity still trumps charging efficiency when it comes to your wallet.

Does Slow EV Charging Really Waste More Electricity?

Yes. German motoring organisation ADAC recently put five EVs through charging tests to find out how much electricity actually reaches the battery.

The cars were charged from a household socket at 2.3 kW, from a dedicated wall charger throttled down to 4.1 kW to simulate surplus solar charging, and from a dedicated wall charger at 11 kW. The Volvo EX30 was also tested at 22 kW.

The differences were substantial.

In other words, plugging into an ordinary power point (commonly called granny charging) resulted in losses of around 13–24%. Charging at higher power from a wall charger cuts those numbers to around 5–7%. The simulated surplus-solar charging landed between the two, losing 8–13%.

The general trend was clear: the slower the AC charging, the more electricity was wasted.

Where The Missing Electricity Goes

Here’s the slightly confusing bit: much of the problem isn’t the charger hanging on your wall.

With AC charging, the real charger is inside the car. Its onboard charger converts the AC electricity supplied by your house into DC electricity that the battery can store. That conversion produces losses, but the car also needs to keep various electronics operating while it charges.

ADAC measured 100–300 watts being consumed by vehicle systems during EV battery charging.

At 11 kW, a couple of hundred watts isn’t much in percentage terms. At 2.3 kW, it becomes significant. Worse, putting the same amount of energy into the EV’s battery at 2.3 kW takes almost five times as long as at 11 kW, so those electronics stay awake much longer.

There are additional losses in cables and the battery itself, but ADAC says the onboard charger and vehicle electronics are the big ones.

And this isn’t just an ADAC curiosity. A separate 2023 study of onboard chargers across 38 EV models found that reducing charging current for smart charging could increase worldwide charging energy demand by around 1–10%.

So, Should I Stop Charging Slowly From Solar?

It depends. This is where energy efficiency and financial efficiency part company.

Imagine your EV charger is following your solar production, leaving only 2 or 3 kW to spare once your house has taken what it needs.

Throttling the EV down to match that surplus might mean you lose a higher share of the electricity during charging. But that lost electricity probably cost you next to nothing because it came from your own solar rather than the grid.

Compare that to the alternative: exporting that same solar for a small feed-in tariff instead. Losing a bit of extra energy while charging can still work out cheaper than charging more efficiently from the grid later, even at an overnight off-peak rate. That’s why charging an EV from surplus rooftop solar still makes sense.

Granny Charging From The Grid Is A Different Story

These loss percentages matter more once real money is on the line. If you’re granny charging from a standard power point and buying electricity at the grid’s full retail price, every kilowatt-hour lost to inefficiency is one you paid for and never got to use.

In ADAC’s test, the Tesla Model Y lost 12.7% of its charge at a 2.3 kW socket, compared with 6.1% at an 11 kW dedicated EV charger. The Mercedes was a more extreme case: 24.2% versus 6.9%.

That’s not an argument that an EV charger pays for itself overnight. Installation costs money, and the payback depends on how far you drive, your electricity tariff and your specific EV.

Those charging losses are a real cost worth factoring in when you’re deciding whether to keep granny charging or install a dedicated home charger.

Charging From Surplus Solar Gets Complicated

Real-world solar charging is messier than ADAC’s test, though.

ADAC measured losses at a fixed 4.1 kW, but a charger that actually tracks your solar output will ramp up and down throughout the day as generation changes. Different EVs behave differently, too, and some home chargers can run at much lower power or switch between single-phase and three-phase charging.

That makes ADAC’s 8.0-12.8% figure a snapshot, not a universal number. It confirms that lower charging power costs you some efficiency, but there’s no single percentage or perfect charging rate that applies to every EV and household.

Won’t Faster Charging Wear Out My Battery?

Not at the charging powers we’re talking about here.

Charging an EV at 7 or 11 kW AC at home is very different from plugging into a 150-, 250-, or 350-kW DC fast charger.

For perspective, even an 11 kW home charger is delivering only a fraction of the power an EV can receive from a 150–350 kW DC fast charger.

So concerns about repeated high-power DC fast charging shouldn’t be confused with running your home AC charger at its normal rated output.

The Best Way To Charge Your EV

There’s no universal answer, but there is a useful rule of thumb:

1. First, choose the cheapest sensible time to charge.
2. Then, within that window, charge as efficiently as possible.

This usually means investing in a home EV charger.

If you’re paying normal grid prices, charging painfully slowly can mean paying for considerably more electricity than actually reaches your battery.

And if you’ve got access to free or very cheap electricity at certain times of day, there may be little reason to deliberately throttle the charging rate to avoid importing from the grid.

If you’re considering moving beyond a granny charger, our EV Charger Guide explains home charging speeds, costs, solar integration and what to look for when choosing a charger.