1. Why your meter shows more than your battery gets
Charge your EV and compare two numbers: the kWh your charger or meter recorded, and the kWh your car says it added. The meter number is always higher. The difference is charging loss: energy that turned into heat or ran the car's systems instead of going into the battery.
Studies collected by Recurrent found home Level 2 charging is usually about 85% to 95% efficient, while Level 1 charging from a 120-volt outlet is often around 80% to 85%. In plain terms, to put 10 kWh into the battery, the wall may have to supply 10.5 to 12 kWh.
2. What causes the losses
The onboard charger: your car turns the AC from your charger into DC for the battery, and that conversion gives off some heat.
The car staying awake: while it charges, the car's computers, battery management system and pumps keep running. This draws a few hundred watts no matter how fast you charge.
Battery heating and cooling: in hot or freezing weather, the car may heat or cool the battery while it charges, which uses extra energy.
Heat inside the battery: a small share of the energy is lost as heat in the battery cells themselves.
| Charging type | Typical power | Typical efficiency | Why |
|---|---|---|---|
| Level 1 (120 V outlet) | About 1.4 kW | Often about 80%–85% | Slow charging keeps the car awake for many hours |
| Level 2 (240 V home charger) | About 7–11.5 kW | About 85%–95% | Faster, so the car's own systems run for fewer hours |
| DC fast charging | 50–350 kW | Usually above 90% in mild weather | The power bypasses the car's onboard charger |
What the numbers mean
Faster home charging is usually a little more efficient, because the car's computers and pumps run for fewer hours. Moving from Level 1 to Level 2 can save several percent of your charging energy. Between a 32-amp and a 48-amp Level 2 charger, the difference is small.
Cold weather is the big exception. When the battery has to be warmed before it can take a charge, efficiency can drop noticeably, which is one reason winter charging costs more.
3. Do you need extra panels for charging losses?
Usually not, as long as you start from the right number. The EPA's kWh-per-100-miles rating is measured from the wall, so it already includes the energy lost while charging. For example, the 2024 Tesla Model Y Long Range is rated at about 29 kWh per 100 miles: that's what you'll pay for at the meter, not just what reaches the battery.
Our calculator uses those EPA ratings and adds an 18% allowance for losses on the solar side (inverter, wiring, heat and dust). It does not add another charging-loss factor on top, because that would count the same losses twice and oversize your system.
The mistake to avoid is using the efficiency shown on your car's screen. That figure usually measures energy taken from the battery, not from the wall, so it looks better than the EPA rating. If you size solar from the screen number, add about 10% to 15% for charging losses.