How big a home battery do I need to charge an EV overnight from solar?

For the reference case here—7,000 W average for 6 hours—you need about 55 kWh of nominal battery capacity once you account for inverter losses, usable depth of discharge, and a safety margin. That is much larger than a typical single residential battery module.

Can one standard home battery do a full overnight EV charge?

Usually no, not for a sustained 7 kW charge over 6 hours. Most single home battery units in our database are in the 3 to 10 kWh class, so this use case usually needs a multi-battery stack or a larger whole-home storage system.

What if I lower the EV charging rate?

Lowering charge power can make the system much more realistic because it cuts both the power requirement and, if you shorten the energy target, the total battery size. For example, many smaller batteries can support low-power overnight top-ups, but not a full 42 kWh usable transfer.

Overnight EV Charging From Solar: Battery Size Math

Short answer

Yes, a home battery can cover overnight EV charging from solar, but for the reference load of 7,000 W for 6 hours you should size for about 55 kWh of nominal battery capacity. The reason is simple: your EV needs 42 kWh delivered, and once you add inverter losses, battery operating limits, and a sensible reserve, the real battery bank needed is far larger than the raw 42 kWh figure.

The math

Start with the energy your car actually needs overnight:

Load energy = power × time

7,000 W × 6 h = 42,000 Wh

That is:

42,000 Wh = 42 kWh usable at the charger

If you want to sanity-check other scenarios, SolarWorld’s the calculator behind these numbers uses this same framework.

Now add the real-world factors that turn a usable load target into a battery size.

1) Inverter and conversion losses

Most home battery systems do not deliver every stored watt-hour to the car. You lose some energy in the inverter, battery electronics, and EVSE/charging path. A reasonable planning assumption is around 10% total conversion loss, which means you divide by 0.90 to find the battery-side energy needed. That is broadly consistent with published battery and inverter efficiency ranges from sources such as the U.S. Department of Energy and NREL, which show storage and power-conversion losses are real and material in system sizing (DOE Energy Saver, NREL).

Battery-side energy needed = 42 kWh ÷ 0.90 = 46.7 kWh

2) Depth of discharge

Not every battery should be planned around draining to absolute zero every night. Some products allow very high usable capacity, but sizing right at the edge leaves no buffer for battery aging, cold weather, or a night when charging runs longer than expected. For planning, I use an effective 90% depth-of-discharge factor here.

Required nominal capacity before reserve = 46.7 kWh ÷ 0.90 = 51.9 kWh

3) Safety margin

You also want reserve. If your solar harvest was a bit lower than expected, or the battery is no longer brand-new, a zero-margin design becomes annoying fast. A 5% safety margin is modest and realistic.

Final recommended nominal capacity = 51.9 kWh × 1.05 = 54.5 kWh

Rounded:

Recommended battery size = 55 kWh nominal

That is the clean answer for this use case.

Formula summary

You can write it as:

Battery size = (device wattage × hours) ÷ inverter efficiency ÷ usable DoD × safety margin

Plugging in the numbers:

Battery size = (7,000 W × 6 h) ÷ 0.90 ÷ 0.90 × 1.05

Battery size = 54,444 Wh ≈ 55 kWh

One more check: power matters too

Energy is only half the job. Your battery also has to sustain the charging power.

Here, the EV charger is drawing:

7,000 W = 7.0 kW continuous

So any battery or battery stack also needs at least 7 kW continuous output, preferably with some headroom. That is why some smaller batteries fail this use case even if you stacked enough energy modules: the power electronics may still bottleneck the charge rate.

Real examples from our database

None of the single batteries below can cover the full reference scenario by themselves on energy capacity alone. A few can handle the power; most cannot handle both power and energy. That is the key takeaway from the full database: for a 42 kWh overnight EV load, you are usually looking at a stacked system, not one battery.

Product	Image	Key spec	Runtime in this 7 kW scenario	Price
BYD Battery-Box Premium HVS 10.2	Image not yet available.	10.24 kWh usable; 7.68 kW continuous	10.24 kWh ÷ 7 kW = 1.46 h (~1 h 28 min)	$7,200 MSRP
BYD Battery-Box Premium HVM 8.3	Image not yet available.	8.28 kWh usable; 9.2 kW continuous	8.28 kWh ÷ 7 kW = 1.18 h (~1 h 11 min)	$6,300 MSRP
BYD Battery-Box Premium HVS 7.7	Image not yet available.	7.68 kWh usable; 7.68 kW continuous	7.68 kWh ÷ 7 kW = 1.10 h (~1 h 6 min)	$5,800 MSRP
BYD Battery-Box Premium HVS 5.1	Image not yet available.	5.12 kWh usable; 7.68 kW continuous	5.12 kWh ÷ 7 kW = 0.73 h (~44 min)	$4,200 MSRP
Pylontech US5000	Image not yet available.	4.32 kWh usable; 3.0 kW continuous	Not suitable at 7 kW; power output is below load	$1,500 MSRP
Pylontech US3000C	Image not yet available.	3.2 kWh usable; 1.8 kW continuous	Not suitable at 7 kW; power output is below load	$1,100 MSRP

What these examples mean in practice

If you want a few concrete starting points from the list above:

Best single-unit fit on power + strongest energy of this group: BYD Battery-Box Premium HVS 10.2. It can sustain the 7 kW load, but only for about 1.46 hours.
Highest output power in this set: BYD Battery-Box Premium HVM 8.3. Good power headroom at 9.2 kW, but still far short on energy.
Power-capable but smaller: BYD Battery-Box Premium HVS 7.7 and BYD Battery-Box Premium HVS 5.1. They can technically support the charger’s power, but not for long.
Budget modules, but underpowered for this exact load: Pylontech US5000 and Pylontech US3000C. These make more sense in lower-power charging setups or larger parallel banks, subject to inverter and system design limits.

If you compare these numbers to the 55 kWh nominal target, you can see the gap right away. Even the largest single unit here is only about one-fifth of the recommended system size for the reference case.

What goes wrong

1) Undersizing the battery bank

The most common mistake is sizing to the raw 42 kWh load and forgetting losses and reserve. That usually leaves you short before morning, especially once the battery ages or the car pulls a bit more than planned.

2) Power mismatch at the EV charger

A battery can have enough total kWh and still fail if its continuous output is below the charger draw. In this dataset, the Pylontech US5000 and Pylontech US3000C are clear examples: their 3.0 kW and 1.8 kW outputs are below a 7 kW charging load.

3) Cold-weather performance drop

Lithium iron phosphate batteries are durable and common here, but cold temperatures can reduce available power and usable energy, and some systems restrict charging when cells are cold. That matters if your battery is installed in an unconditioned garage or outside enclosure.

4) Port and system compatibility issues

A battery module alone does not charge an EV. You still need a compatible inverter, battery management setup, and EV charging equipment that can actually use the stored solar energy the way you intend. If the battery, inverter, and EVSE are not designed to work together, the system may default to pulling from the grid at night.

When to step up a tier

Step up to the next-bigger battery size when your math is even slightly borderline.

For this use case, “borderline” means any of these:

Your calculation lands within about 10% of the battery’s usable capacity
Your EV sometimes charges above the planned 7 kW
You want to preserve backup reserve for other overnight loads
Your local climate is cold enough to reduce effective battery performance
You expect battery aging to matter over years of daily cycling

On the numbers here, that means a 10 kWh-class battery is not close; it is simply too small for a 42 kWh overnight EV transfer. Even five 10.24 kWh modules only get you to 51.2 kWh usable, and that is still below the 55 kWh nominal recommendation once you include planning margin. Six gets you to 61.44 kWh usable-equivalent nominal in module terms, which is much more realistic for this reference scenario.

There is also a practical design question: if you can lower the EV charging rate or split charging across more hours, you may be able to choose a smaller system. For example, a lower-power overnight top-up can ease both the continuous power requirement and the stress on the inverter. If your goal is not a full 42 kWh nightly refill, the battery bank can shrink fast.

So the trigger to move up a tier is simple: if your chosen setup cannot comfortably clear both 7 kW continuous output and 55 kWh nominal planned capacity, it is the wrong size for this exact use case.

How we picked the products above

We filtered our full database for home battery products with published usable capacity, continuous power output, chemistry, warranty, and MSRP, then compared them against the reference load of 7 kW for 6 hours. Runtime was calculated from stated usable kWh ÷ 7 kW, and we flagged products that fail the power requirement outright. We did not invent missing values; if a field is absent, we say so plainly. You can read our scoring methodology for the broader framework we use across storage products, including spec verification, normalization, and comparison rules.