Solar Battery Draining Overnight: Why and How to Fix It

Is this actually a problem?

Before assuming something is wrong, it helps to know what normal looks like.

Your home never truly switches off. Even at 3am, the fridge compressor cycles, the broadband router hums away, smoke alarms draw a trickle, and every device left on standby takes its share. This is your base load — the constant background electricity consumption that never drops to zero.

For a typical UK household, the overnight base load sits somewhere between 150W and 500W. At 300W, running for eight hours overnight, that's 2.4kWh of energy drawn from the battery. On a 10kWh battery, that's a 24% drop before sunrise — and that's perfectly normal.

So if your battery has gone from 80% to 55% overnight and you have a fridge, a router, and a few devices on standby, nothing is wrong. Where it becomes a problem is when the drain is far higher than your base load can account for, or when the battery is hitting 0% and pulling from the grid before your solar panels get going in the morning.

Cause 1: Higher base load than expected

The most straightforward explanation is that your home simply uses more overnight than you realised.

Common culprits:

• Immersion heater on a timer — if it's set to run overnight, a 3kW element draws 3kW. An hour of that is 3kWh gone in one hit.
• Underfloor heating — especially electric mats in bathrooms or kitchens left on 24/7 thermostats. These cycle throughout the night and can add 0.5–2kWh.
• Gaming consoles and TVs in standby — newer consoles draw surprisingly high standby loads (up to 20–30W each).
• Tumble dryer finishing a late cycle — a forgotten load running at midnight is easy to overlook.
• Electric blankets, heated towel rails, dehumidifiers — all add up.

The fix here is behavioural. Work through your home and identify what's drawing power between 10pm and 6am. A cheap plug-in energy monitor on individual devices, or a whole-home clamp meter, will show you exactly where the drain is coming from.

Cause 2: Minimum state of charge set too low

Most battery systems let you set a minimum state of charge (SOC) — the floor below which the battery will not discharge. This is commonly set to 10% or 20% as a default.

If your minimum SOC has been set to 0% (or left unconfigured on some systems), the battery will discharge fully overnight. Repeatedly running a lithium battery down to 0% accelerates degradation and shortens its usable life.

There is also a practical problem: a battery that empties at 4am leaves you drawing from the grid for the rest of the night and the early morning, before solar generation gets going. On a standard tariff at 24p/kWh, those early morning imports add up.

The fix: Open your inverter app or web portal and find the minimum SOC setting. Most people set this to 10–20%. If you are on a time-of-use tariff like Octopus Go and want to reserve battery space for cheap overnight charging, you may want to set a higher floor — sometimes 20–30% — so the battery is ready to absorb the cheap-rate charge window.

Cause 3: CT clamp issue

The CT (current transformer) clamp is the sensor that wraps around your incoming mains cable and tells the inverter how much electricity the house is importing or exporting. If this clamp is reading incorrectly, the inverter gets a distorted picture of what the house needs — and the battery may discharge far more than necessary.

Two specific failure modes cause overnight drain:

Clamp reading too high: If the CT clamp is partially displaced, damaged, or picking up interference, it may report that the house is drawing 1kW when it is actually drawing 200W. The inverter responds by drawing more from the battery than the house needs, and the excess may be wasted as heat in the inverter or dumped to grid depending on settings.

Reversed CT clamp causing phantom export: If the CT clamp was fitted facing the wrong direction, import and export readings are swapped. The inverter thinks you are exporting when you are actually importing. In some configurations this causes the battery to discharge aggressively overnight — because the inverter believes it needs to balance a large import that isn't actually happening, or because it's discharging the battery and sending that energy to the grid rather than the home.

Signs of a CT clamp problem:

• Your app shows the home drawing much less than your base load should be
• Your smart meter and inverter readings don't match
• Your grid import overnight is higher than the battery discharge shown in the app (energy is unaccounted for)
• The battery discharges quickly but your home's lighting and appliances seem fine

See CT clamp wrong readings for a detailed diagnostic guide. This is a job for your installer to fix — do not adjust CT clamps yourself if you are not a qualified electrician.

Cause 4: Battery discharging to grid overnight

Some inverter configurations — particularly after a firmware update, settings reset, or installer misconfiguration — can cause the battery to export to the grid overnight rather than power the home.

This is more common with hybrid inverters that support SEG export and have grid export enabled. If the priority order is set incorrectly, the inverter may discharge the battery to grid (earning a small export payment) instead of using that energy in the home.

How to check: Look at your smart meter's export reading overnight. If your meter is showing export during the hours of darkness, when solar generation is zero, something is either discharging the battery intentionally to grid, or there is a CT clamp polarity fault (see above).

On most inverter apps, you can see overnight power flows — look for an arrow or flow indicator pointing from the battery to the grid rather than from the battery to the home.

If you confirm the battery is exporting to grid overnight and this is not intentional, check the inverter's grid export settings and discharge priority order. Your installer can correct this remotely in many cases.

Cause 5: Tariff discharge schedule set incorrectly

If you are on a time-of-use tariff such as Octopus Flux or Agile, your inverter should have a discharge schedule that matches the tariff's peak export windows.

Octopus Flux, for example, pays the highest export rate between 16:00–19:00. The correct behaviour is for the battery to discharge during that peak window and then hold charge overnight (or charge from the cheap rate window at 02:00–05:00).

If the discharge schedule is configured incorrectly — for example, set to discharge between midnight and 6am — the battery empties during a window when export rates are low or when you could otherwise be using that stored energy for morning consumption.

The fix: Check that the discharge windows in your inverter settings match the cheap/peak windows on your tariff. See best tariffs for battery storage for a breakdown of which windows apply to each tariff. If the schedules were pre-configured by your installer but you have since switched tariffs, the schedules will need updating.

Cause 6: Battery degradation

Lithium battery cells degrade gradually over time. A battery that was installed with a 10kWh usable capacity several years ago may now only hold 8.5kWh — and the apps often still display state of charge as a percentage of the original rated capacity rather than actual current capacity.

The result: the battery appears to discharge faster than before, because the same base load now represents a larger percentage of the available energy.

Most modern battery systems display a state of health (SOH) figure in the app or web portal — typically expressed as a percentage of original capacity. A battery at 85% SOH has lost 15% of its original capacity. This is normal degradation after several years of daily cycling; most manufacturers warrant 70–80% SOH at 10 years.

If your battery's SOH has dropped significantly and you are finding it empties overnight where it previously did not, the options are:

• Accept the reduced capacity and adjust minimum SOC settings accordingly
• Contact your installer to assess whether a cell replacement or system upgrade is warranted

Cause 7: Inverter standby consumption

This one is often overlooked. The inverter itself is an electrical device, and it draws power continuously — even overnight, even when no solar is being generated and the house load is being served from the battery.

Typical inverter standby draw is 20–80W depending on model and age. Over eight hours, that is 0.16–0.64kWh drawn directly from the battery just to keep the inverter running.

On a 5kWh battery, 0.64kWh is nearly 13% of capacity consumed before you account for any household load. This is normal and expected — every inverter datasheet will list the standby consumption figure — but it does add to the overnight drain.

There is no fix required here; this is simply the cost of having an always-on inverter. It is worth factoring into your sizing calculations if you are planning a new system. Some inverters have a sleep mode that reduces standby consumption when battery SOC drops below a certain threshold, which can help.

How to diagnose your overnight drain

The fastest route to an answer is your inverter or battery app's power flow history. Most apps — GivEnergy, Solis Cloud, SolarEdge, Victron, SunSynk, EcoFlow — include a graph or timeline showing minute-by-minute power flows throughout the previous night.

Look for:

1. The base load line overnight. This should be a roughly flat line representing your home's consumption. A 300–500W flat line is normal. Spikes to 2–3kW indicate a large appliance running.
2. The battery discharge rate. This should closely match the home consumption line. If battery discharge is significantly higher than home consumption, energy is going somewhere — most likely the grid.
3. Any export shown overnight. If your app shows export (arrows pointing to grid) during darkness hours, you have a configuration or CT clamp issue.
4. Grid import alongside battery discharge. If both the battery and the grid are supplying the home simultaneously overnight, the inverter may not be able to cover the full load, or there is a metering error.

Compare your app's overnight home consumption figure with your smart meter's overnight import reading. They should roughly add up — overnight battery discharge should cover most home consumption, with a small amount of grid import if the battery reaches minimum SOC. If the numbers don't reconcile, there is a metering or configuration fault.

Fixes summary

When overnight drain is a real problem

Most overnight drain has a mundane explanation — high base load, a misconfigured setting, or normal battery degradation. But there are situations that warrant a call to your installer:

For issues related to the battery not charging at all rather than over-discharging, see battery not charging from solar. For smart meter and monitoring discrepancies, solar monitoring apps in the UK covers how to cross-reference your data sources.

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