When the battery hits full, the surplus does one of three things: exports to the grid, runs a diverted load, or is never generated in the first place. That third option is the one nobody expects, and on an off-grid system it's the usual answer.
TL;DR: A full battery causes the charge controller to stop drawing power from the array. Grid-tied systems push the surplus out to the utility. Systems with a diversion load dump it into a water heater or similar. Off-grid systems with neither simply leave the energy unharvested, with the array idling near open-circuit voltage. Nothing overloads, nothing overheats, and no panel is damaged by any of this.
There's a persistent mental picture of electricity backing up like water behind a closed valve. It's a bad analogy, and it's the reason this question gets asked so often.
What Actually Stops When the Battery Fills
Charging is not one continuous act. A charge controller runs through stages, and the last two are where this happens:
- Bulk. Maximum available current, battery voltage climbing.
- Absorption. Voltage held constant, current tapering as the cells fill.
- Float. Barely any current, just enough to hold charge.
By float, the controller is asking the array for almost nothing. So the array gives almost nothing. That's the whole mechanism: the load side sets the draw, and a solar panel is happy to supply anything between zero and its maximum.
Where does the rest of the sunlight go? Into heat on the panel surface, the same as it would if the panel weren't there. The array drifts toward open-circuit voltage, where current is near zero and the panel runs a few degrees warmer. Perfectly normal. Every array does this at dawn and dusk anyway.
The Three Destinations, Ranked by How Useful They Are
| Where surplus goes | Setup required | What you get back |
|---|---|---|
| Export to the grid | Grid connection, export agreement | Credit or payment at the export rate |
| Diversion load | Diverter plus a resistive load | Hot water, or heat you would have bought |
| Curtailed, not harvested | None, this is the default | Nothing |
Grid export is the best of the three wherever net metering survives at a decent rate, because you're effectively banking the kWh. Where the export rate has collapsed to a few cents, the ranking gets closer than you'd think: three cents of credit against hot water you'd otherwise heat at 18 cents is not a hard call.
Some utilities forbid export entirely and require a zero-export setting on the inverter. In that case a grid-tied system behaves exactly like the off-grid one: full battery, satisfied loads, array throttled.
Off-Grid Is Where This Bites
Grid-tied owners rarely notice any of this. Off-grid owners watch it happen every clear afternoon from about March onwards, because an off-grid array is deliberately oversized for December and December only comes once a year.
By midsummer the battery is full by eleven in the morning and the array spends the rest of the day loafing. That's not a fault, it's the design working. You sized for the worst month, and the other eleven have surplus by definition.
Which is what makes a diversion load worth the effort off-grid. Water heating runs about 12 percent of US residential electricity consumption (EIA), and a tank holds that heat for hours, so it's the natural target for energy that would otherwise evaporate. There are other options that don't involve chemistry at all, which we cover in a separate guide to storing solar energy without batteries.
Grid Operators Do Exactly the Same Thing
This isn't a quirk of small systems. When California's grid has more solar than it can absorb, the operator curtails it: in 2020 CAISO cut 1.5 million MWh of utility-scale solar, about 5 percent of its utility-scale solar production that year (EIA). The number has grown since as more capacity came online.
Same physics, four orders of magnitude apart. When there's nowhere for the energy to go, it isn't dumped, redirected or stored. It's just not generated. The DOE's own framing of storage makes the underlying point: nothing you do with surplus energy is free, because conversion and retrieval always lose something.
Should You Size Around It?
My honest view: a bit of curtailment is a sign of correct sizing, not a problem to engineer away. Chasing zero waste means buying battery capacity that sits idle eleven months a year, and batteries are the most expensive kWh in the system. A summer afternoon of unharvested sun costs you nothing. An extra 10 kWh of storage costs about $10,000.
A bit of curtailment every summer is evidence you sized the system correctly, not a fault you should spend $10,000 of battery to engineer away.
A diverter is the retrofit version of an idea that some manufacturers now ship built in. Hitachi's daytime-tariff water heater lets the owner move the heating window to noon from the remote, and under NEM 3.0 that arithmetic pays better here than it does in the market it sells into.
The exception is if you're curtailing in winter too. That means the loads have shrunk or the battery is undersized for its charge window, and it's worth reworking the numbers in our battery sizing guide.
Summary
A full battery doesn't create a hazard, it creates an idle array. Grid-tied, the surplus exports. With a diverter, it becomes hot water. With neither, it's curtailed and never harvested at all, which is exactly what grid operators do on a much larger scale. Panels are unbothered by any of it, and a few curtailed afternoons in July are usually the cheapest outcome available.