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WattCurious

What Happens to Excess Solar Energy?

Updated 8 October 20267 min readBatteries & Storage

An electrical system has no buffer. Generation and consumption are equal at every instant, whether anyone intends it or not, so surplus is never a quantity sitting somewhere waiting to be dealt with. It is a control problem, resolved within milliseconds by storing the energy, sending it somewhere else, giving it to a load that would not otherwise run, or simply declining to generate it.

Key takeaways

  • Generation equals consumption at every instant, so surplus is resolved in real time rather than accumulated and handled later.
  • Curtailment is not waste in the physical sense: the inverter moves off the maximum power point and the energy is never converted at all.
  • Exporting raises voltage at the point of connection, which is the physical reason a grid can refuse more.
  • Diverting surplus into a heat store is the cheapest storage most homes already own, and it is one-way — heat does not come back as electricity.
  • Off-grid, there is no grid to absorb anything, so the charge controller curtails every sunny afternoon as a matter of routine.
On this page
  1. There is no waiting room
  2. Curtailment: declining to convert it
  3. Export, and why the grid pushes back
  4. Storing it, and the limits of that
  5. Diversion: the storage most houses already have
  6. Off-grid: the same problem with nowhere to send it

There is no waiting room

The instinct behind the question is that surplus energy is a substance — that it piles up somewhere and something must be done with it. It does not. An electrical system carries no buffer at all: at every instant, what is generated equals what is consumed, plus whatever is flowing into storage, plus losses.

A single array feeding a junction where four paths leave. One goes to the house loads. One goes into a battery while it has room. One goes out to the grid. One goes to a diverted load such as a water heater. A fifth arrow points back at the array itself, labelled not generated, showing that when the other four are full or unavailable the inverter simply declines to convert the available light.A single array feeding a junction where four paths leave. One goes to the house loads. One goes into a battery while it has room. One goes out to the grid. One goes to a diverted load such as a water heater. A fifth arrow points back at the array itself, labelled not generated, showing that when the other four are full or unavailable the inverter simply declines to convert the available light.
Four destinations and one refusal. When the first four are unavailable, the fifth is automatic — and it happens in milliseconds, every time.

So the surplus is resolved continuously, by the inverter, faster than anything could observe it. The interesting question is not where it goes but which of the five outcomes the system is currently choosing, and why.

Curtailment: declining to convert it

The most counterintuitive destination is the one where nothing travels anywhere. To understand it, look at what the inverter is actually doing to the array.

A module current-voltage curve with the power curve beneath it. In normal operation the inverter holds the array at the maximum power point, near the knee of the curve. Under curtailment the operating point is moved to the right, towards open-circuit voltage, where current falls sharply and the power extracted is a fraction of what is available. A note explains that the light not converted becomes a small amount of extra heat in the module rather than energy lost from the system.A module current-voltage curve with the power curve beneath it. In normal operation the inverter holds the array at the maximum power point, near the knee of the curve. Under curtailment the operating point is moved to the right, towards open-circuit voltage, where current falls sharply and the power extracted is a fraction of what is available. A note explains that the light not converted becomes a small amount of extra heat in the module rather than energy lost from the system.
Curtailment moves the operating point off the knee of the curve. The array is not throttled and nothing is dumped — less current is drawn, so less power is produced.

A photovoltaic array does not push a fixed amount of power at you. It presents a current-voltage curve, and whoever is connected chooses a point on it. Normally the inverter tracks the point where current times voltage is greatest — the maximum power point. To curtail, it simply moves that point towards open-circuit voltage, where the current collapses.

The consequence is worth stating plainly: the unconverted sunlight is not stored, transmitted or destroyed. It warms the module slightly and that is all. Which is why an array that spends part of the day curtailed suffers nothing for it — unlike a battery, there is no cycle and no wear in producing less. A wind turbine above its rated speed does the same thing mechanically, pitching its blades to spill power it will not convert — though there the reason is the loads on the machine, not a full battery.

Export, and why the grid pushes back

Sending surplus to the grid looks like the obvious answer, and physically it is the one with the most capacity behind it. But there is a limit that has nothing to do with anyone's rules.

A distribution feeder from a substation to three houses at increasing distance. With no export, voltage falls gently along the feeder. With one house exporting, voltage rises at its connection point, and the rise is larger for the house furthest from the substation because the cable impedance between them is greater. A curve underneath shows an inverter's volt-watt response: output held constant up to a threshold voltage, then reduced progressively as local voltage climbs, and disconnection beyond an upper limit.A distribution feeder from a substation to three houses at increasing distance. With no export, voltage falls gently along the feeder. With one house exporting, voltage rises at its connection point, and the rise is larger for the house furthest from the substation because the cable impedance between them is greater. A curve underneath shows an inverter's volt-watt response: output held constant up to a threshold voltage, then reduced progressively as local voltage climbs, and disconnection beyond an upper limit.
Pushing power into a cable raises the voltage where you push. The further from the substation, the more it rises — so the same array can export freely at one address and be curtailed at another.

Current flowing into an impedance raises the voltage at the point of injection. Export enough, far enough down a feeder, and the local voltage climbs towards the top of its permitted band. Inverters respond to this with a volt-watt characteristic: hold output while voltage is normal, reduce it progressively as voltage rises, disconnect if it rises too far.

That is why the answer to "why can't I export more" is often a length of cable rather than a decision. Two identical systems can behave completely differently on the same afternoon because one sits near a substation and one sits at the end of a long rural feeder.

Storing it, and the limits of that

A battery is the destination most people picture, and it works exactly as expected until it is full.

Where surplus can go, and what each destination is good at
DestinationAbsorbsGives backLimit
House loadsWhatever is runningImmediately, as useOnly what the house actually needs
BatteryUp to its charge-power limitAs electricity, at high efficiencyFills, then stops; every cycle is wear
Grid exportVery large amountsNothing to you directlyVoltage rise at your connection
Diverted loadWhatever it is sized forAs heat, never as electricityOne-way, and only if the heat is wanted
CurtailmentNothing — it is not convertedNothingAlways available; costs the system nothing

Physical characteristics, not a recommendation. Which destinations exist at all depends on the system: an off-grid installation has no export path, and a system with no battery and no diversion has only two options.

Two limits matter. A battery has a charge-power ceiling as well as a capacity: a large pack that can only accept a modest charge rate will not absorb a sharp midday peak no matter how empty it is, a constraint enforced by its battery management system. And charging is not free of consequence — depth and frequency of cycling decide how long the pack lasts, which is the trade examined in depth of discharge.

At grid scale the same problem is solved by moving energy in time on a far larger scale, which is what pumped hydro storage exists to do.

Diversion: the storage most houses already have

If surplus cannot be stored as electricity, it can often be stored as heat — and a hot water cylinder is a substantial thermal store that most buildings already contain.

A priority ladder for surplus. At the top, house loads take what they need. Next, the battery charges while it has room and while it can accept the available power. Next, a diversion controller modulates a water heater element so that consumption tracks the remaining surplus exactly. Next, whatever is left is exported. At the bottom, if none of these can absorb it, the inverter curtails. A small chart alongside shows a diverter tracking a fluctuating surplus minute by minute.A priority ladder for surplus. At the top, house loads take what they need. Next, the battery charges while it has room and while it can accept the available power. Next, a diversion controller modulates a water heater element so that consumption tracks the remaining surplus exactly. Next, whatever is left is exported. At the bottom, if none of these can absorb it, the inverter curtails. A small chart alongside shows a diverter tracking a fluctuating surplus minute by minute.
A priority order, not a set of alternatives. A diverter modulates continuously so consumption matches surplus, which is what keeps the system balanced without exporting.

A diversion controller measures the surplus and modulates a resistive element to match it, continuously. The attraction is that it absorbs whatever is going spare without needing to be told, and the honest limitation is that it is one-way: heat is not recoverable as electricity, so a diverted kilowatt-hour is only worth having if the heat was wanted anyway.

The same logic applies to any load that can be moved rather than merely absorbed — running a heat pump, a hot water cycle or machinery during the surplus rather than after it. A heat pump is the most valuable of these, because it moves several units of heat for each unit of electricity instead of converting one into one as a resistive element does, and the building's own thermal mass lets the heating run somewhat earlier than strictly needed. That is not storage at all. It is scheduling, and it is usually the most effective of the options.

Off-grid: the same problem with nowhere to send it

Take away the grid — the defining difference between grid-tied, off-grid and hybrid systems — and the picture simplifies brutally. There are three destinations instead of four, and the last one is used constantly.

A day's traces for an off-grid system. Array output rises through the morning while the battery accepts everything it can. As the battery approaches full, the charge controller tapers the current, holding voltage steady while the cells finish. From then until evening, the shaded gap between what the array could produce and what the system takes grows wide, labelled curtailed. The load trace beneath stays modest throughout.A day's traces for an off-grid system. Array output rises through the morning while the battery accepts everything it can. As the battery approaches full, the charge controller tapers the current, holding voltage steady while the cells finish. From then until evening, the shaded gap between what the array could produce and what the system takes grows wide, labelled curtailed. The load trace beneath stays modest throughout.
The gap after the taper is the array being asked for less than it could give. On a good day in an off-grid system, most of the afternoon looks like this.

An off-grid system spends much of a sunny afternoon curtailed, because the battery is full and the loads are modest. The charge controller tapers as the pack approaches full — reducing current to hold voltage steady while the cells finish charging — and from then until evening the array is deliberately underworked. That is not a design failure; it is what happens when a system is sized for the worst month, as off-grid load calculation requires, and then experiences one of the good ones.

Which returns to the point the whole question rests on. Surplus solar is not a substance to be disposed of. It is an array being asked for less than it could give — and an array asked for less is simply an array producing less, quietly, at no cost to itself.

Frequently asked questions

Is curtailed energy wasted?

Nothing is discarded, because nothing is converted. The inverter shifts the array's operating point away from maximum power, so the cells simply deliver less current and the surplus light becomes a small amount of extra heat in the modules. There is no energy sitting anywhere unused — it was never turned into electricity in the first place.

Why can't I just export everything I generate?

Physics sets a limit before anything else does. Pushing current into the grid at your connection point raises the voltage there, and the further you are from the substation, the more it rises for a given amount of power. Inverters are required to reduce output as local voltage climbs, and to disconnect if it goes too far — which means a strong midday surplus can be curtailed by the wiring rather than by any rule.

What is a diversion or dump load?

A load the system switches on specifically to absorb surplus, most often a water heater element. The controller modulates it so that the amount consumed tracks the amount available, keeping the system balanced without exporting. It suits anything that stores what it is given — hot water, thermal mass, a heat pump run early — and suits nothing that has to run at a particular time.

Does a battery solve the problem?

It moves it. A battery absorbs surplus until it is full, and on a sunny day with a light load it fills by the early afternoon, after which the system is back to exporting, diverting or curtailing. Storage changes when the surplus appears, not whether it appears.

Is clipping the same as curtailment?

Related but not identical. Clipping is the inverter holding its output at its own rated limit because the array is briefly offering more than it can convert — a sizing decision made at design time. Curtailment is the inverter deliberately taking less than it could, because the battery is full, the grid is pushing back, or there is nowhere for the energy to go.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

  • National Renewable Energy Laboratory (NREL)Research on photovoltaic inverter behaviour, curtailment and distribution-system integration.
  • International Energy Agency (IEA)Background on variable renewable integration and system flexibility.
  • IEEE 1547 and IEC 61727 interconnection standardsDefine the voltage and frequency response an inverter must provide when connected to a distribution network; the specific settings are set per network.

Editorial Team

Research, drafting and review

Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 8 October 2026. How we research and review