If you already own a home battery, you have probably been told the same rule by an installer, a forum post, or the battery’s own app: charge overnight on cheap electricity, use it through the day.
That rule is a reasonable default. It is not correct all year round, and following it without thinking about your own household and the season can quietly turn a battery that should be saving you money into one that is wasting it.
This article assumes you already have a battery sized and fitted for your home. If you are still deciding how big a battery you need, read battery capacity, power rating and usable energy. If you are still deciding whether to have one at all, read is battery storage worth it. Neither of those is what this answers. This one is about a battery you already own, and when it should fill up.
The overnight trap
Most time-of-use electricity tariffs offer a cheaper overnight rate. The obvious move is to set the battery to charge to full every night on that cheap import, so it is ready to run the house the next day. For a lot of households, for a lot of the year, that is exactly right.
The trap appears on a specific combination: a household that is out for most of the day, and a solar array that is generating well.
If the battery charged to 100 percent overnight on imported electricity, it has nowhere left to put the solar generated the next morning and afternoon. The panels are still producing, but the home is empty, demand is near zero, and the battery is already full. That solar electricity, which cost nothing to generate, either exports at whatever rate your export tariff pays, which is usually lower than the rate you paid to import the electricity that filled the battery overnight, or in some setups is curtailed and not used at all.
The result is that you have spent money on imported electricity specifically to block your own free solar from being stored. The battery did its job of being full and ready. It did the wrong job. The point of a battery paired with solar is to catch the free generation you are not using in real time, and a battery that starts each solar day already full cannot do that.
This is not a reason to distrust overnight charging. It is a reason to ask, on any given day, whether the household will be home to use the overnight fill before the sun starts refilling it anyway, and whether there is likely to be more solar coming than the battery has room for.
Seasonal headroom is the missing variable
The overnight trap is really a headroom problem, and headroom changes with the season rather than with a fixed setting you pick once.
In December, days are short and generation is low. There is little risk of the panels producing more than an empty battery can absorb, because there is not much sun to absorb. Heating and lighting demand is also higher, and mornings and evenings, when the panels are doing little or nothing, need covering from stored electricity. Charging to 100 percent overnight in December is usually the right call.
In June, the position reverses. Days are long, and a well-sized array can generate well beyond what an empty house uses in real time. A battery that fills to 100 percent overnight has no headroom left to catch that generation. Charging to full every night in June is usually the wrong call, for exactly the reason above: it defends against a low-sun problem that is not the problem you have in June.
So “charge to 100 percent overnight” is not one rule. It is the right rule in low-solar, high-demand months and the wrong rule in high-solar months, particularly on days the household is out.
Some battery and inverter systems allow a seasonal or day-specific charge target rather than a single fixed overnight setting. Whether yours does, and how to set it, depends on the specific hardware you have, so that is a question for your system’s own controls or your installer rather than something a general article can answer.
All of this assumes you are the one deciding when the battery charges. Some tariffs take that decision away and pay you for the privilege, but only with certain equipment. Does your inverter talk to your energy supplier explains what that depends on.
What to check for your own household
Rather than a single number, three questions decide whether charging to full overnight makes sense on any given day.
- Is the household likely to be home enough to use a full overnight charge before much new solar arrives?
- Is the array likely to generate more than the battery has room to store, given how full it already is?
- Is the season one where generation is reliably low, so the overnight fill is not competing with much anyway?
None of these has a one-size answer, and this article deliberately quotes no saving figure, percentage or tariff rate, because those depend on your consumption, your tariff and your system rather than on a generic rule.
If your battery also stops working in cold weather, that is a different problem with a different cause: read why your battery stopped in January.
If you want help working out the right pattern for your setup and tariff, that is the sort of question Eco Connect advises on. Any physical work on your system is carried out by a vetted delivery partner, not by us.
Correct as of 27 August 2026.
