How Many Home Batteries Would You Need to Survive a Power Cut?
How Many Home Batteries Would You Need to Survive a Power Cut?
A 10 kWh battery sounds enormous—until you start deciding what you want it to power.
In my previous article, I looked at one of the great misconceptions surrounding domestic solar power: having solar panels and a battery does not automatically mean that your house will continue running during a power cut.
The system has to have been designed and configured to operate in backup or EPS mode. The Energy Saving Trust makes the same point: solar panels and batteries normally need additional equipment and appropriate configuration if they are to supply the home when the grid goes down. Energy Saving Trust
But suppose you do have proper backup.
The next question is much more interesting:
How much battery do you actually need?
I have around 50 kWh of battery storage at home. Compared with the roughly 10 kWh that the Energy Saving Trust describes as a typical domestic battery system, that sounds enormous. Energy Saving Trust
So, presumably, a 50 kWh battery means I could simply carry on as normal during a power cut?
Not necessarily.
Because the most important question isn't:
"How big is my battery?"
It is:
"What am I trying to keep running, for how long—and how much battery is actually available when the power fails?"
That last part could make an extraordinary difference.
A 50 kWh Battery Might Not Contain 50 kWh When You Need It
Battery capacities tend to be discussed as though the battery were permanently sitting there fully charged, patiently waiting for a power cut.
Real life is rather different.
My batteries are working every day. They may be charging from solar, supplying the house, charging overnight from a cheap tariff or discharging at an expensive time of day.
Imagine a power failure occurring when my nominal 50 kWh battery bank is at only 20%.
There is then approximately:
50 kWh x 0.20 = 10 kWh
stored in the battery.
But the system may not allow me to discharge it all the way to zero.
The Energy Saving Trust points out that battery systems normally stop discharging at a predetermined level rather than allowing the battery to reach zero. Energy Saving Trust
LuxPower equipment, for example, provides separate settings relating to on-grid and off-grid discharge cut-off, while its documentation also includes settings for off-grid output and seamless transfer to backup. The precise behaviour therefore depends on the particular installation and its configuration. LuxpowerTek
Suppose, purely as an illustration, that I begin the power cut at 20% and want to preserve 10%.
I don't really have 10 kWh available.
I have:
50 kWh x (0.20 - 0.10) = 5 kWh
And after inverter and conversion losses, the electricity actually available to the house would be a little lower again.
Suddenly my enormous 50 kWh battery installation could behave more like a small domestic battery.
That changes the discussion completely.
The First Rule of Backup: Stop Trying to Live Normally
There are really two completely different questions.
Question one: How much battery do I need to keep the important things running?
Question two: How much battery do I need to pretend there isn't a power cut?
They produce dramatically different answers.
If there were an extended outage, I wouldn't consider running everything exactly as I normally would.
The washing machine can wait.
The tumble dryer can certainly wait.
The dishwasher can wait.
An electric car doesn't desperately need charging while the electricity network is down.
I don't need every television, computer and piece of studio equipment operating.
And running large electrical heating loads without thinking about them could consume a remarkable amount of stored energy.
Instead, I would switch from normal-house mode to resilience mode.
That is where a battery starts becoming extremely valuable.
What Actually Needs Electricity?
Imagine walking around the house during a power cut.
What would genuinely matter?
The fridge and freezer would be high on my list. I have food stored in them and would rather not lose it.
Then there is lighting.
Modern LED lighting requires comparatively little electricity, so keeping a few important rooms illuminated is not particularly demanding.
Then there is the internet router and networking equipment.
A house can become remarkably inconvenient without connectivity. Mobile networks may remain available, but broadband can be useful for obtaining information, communicating and continuing some work.
It is worth remembering that ordinary broadband equipment needs electricity. UK Power Networks specifically advises households that broadband equipment, heat pumps and cordless telephones normally will not operate when their electricity supply disappears. UK Power Networks
Then there are phones, tablets and perhaps one or two computers.
Again, their electricity requirements are modest compared with space heating or cooking.
There may also be pumps, control systems, alarms and other small essential loads.
And for some households there is an altogether more important consideration:
medical equipment.
Where somebody depends upon electrically powered medical equipment, this becomes much more than an energy-saving calculation. UK Power Networks advises people to establish how long their backup supply will operate and to discuss contingency arrangements with their healthcare or equipment provider. UK Power Networks
That should be planned separately rather than simply assuming the household battery will always be available.
The Difference Between kWh and kW
This is perhaps the most important technical distinction in the entire article.
A battery rated at 10 kWh tells us how much energy it can store.
It does not tell us how much power it can deliver at one moment.
Think of it rather like a water tank.
kWh is roughly equivalent to the size of the tank.
kW is roughly equivalent to how quickly water can come through the pipe.
You could have an enormous tank connected to a very small pipe.
Likewise, I could have 50 kWh of batteries but an inverter capable of delivering only a certain maximum continuous output.
That means having plenty of stored energy doesn't necessarily mean I can simultaneously run an electric shower, oven, kettle, heat pump and everything else.
The inverter may reach its maximum output long before the batteries are remotely empty.
So a genuine backup assessment needs two numbers:
How many kWh can I store?
and
How many kW can my system deliver?
Ignoring either can lead to disappointment.
Why a Kettle Is Not Necessarily the Enemy
High-power appliances often get blamed for destroying battery reserves.
But power alone isn't the whole story.
Suppose a kettle uses 3 kW.
That sounds enormous compared with a 10 W LED lamp.
But perhaps the kettle operates for only three minutes.
Its energy consumption is approximately:
3 kW x 3/60 hours = 0.15 kWh
That isn't very much.
If I boiled only the water actually required, making a cup of tea during a power cut would hardly destroy a 50 kWh battery bank.
The problem is whether the inverter can supply that 3 kW at the same time as everything else that is running.
This is why both kW and kWh matter.
An Electric Oven Is a Different Proposition
Now imagine using a 3 kW oven for an hour.
That could consume roughly 3 kWh, depending on thermostat cycling.
Do that repeatedly and the battery begins disappearing rather quickly.
A microwave could therefore make much more sense during an outage.
So might an air fryer, depending on what is being cooked and for how long.
This is precisely the sort of situation where resilience requires a change of behaviour rather than simply buying an even larger battery.
And Then There Is Heating
This is where my own house becomes particularly interesting because it is heavily electrified.
A heat pump can be extremely efficient compared with direct resistance heating, but it still requires electricity.
During an extended winter power cut, space heating could easily become one of the largest demands on the battery.
The question then changes from:
"Can my battery run my heat pump?"
to:
"For how many hours should I run the heat pump, to what temperature, and which areas genuinely need heating?"
There might be a considerable difference between trying to keep every room at the normal temperature and maintaining a smaller number of occupied rooms at a safe, comfortable level.
That is resilience thinking.
My battery might technically be capable of running the heating.
That doesn't mean doing so continuously would be the best use of stored electricity during a long outage.
Three Very Different Ways to Use a 50 kWh Battery
Here is a deliberately simplified illustration.
Assume my nominal battery capacity is 50 kWh, I preserve the final 10% as a reserve, and there are some conversion losses. These are illustrative calculations, not figures for a particular installation.
| Battery state when outage begins | Approximate AC energy potentially available in this example |
|---|---|
| 100% | about 40.5 kWh |
| 75% | about 29.3 kWh |
| 50% | about 18 kWh |
| 20% | about 4.5 kWh |
Now consider three fictional households using that available energy in very different ways.
| Emergency strategy | Illustrative daily electricity use | 50 kWh system starting full could last roughly |
|---|---|---|
| Essentials only | 3 kWh/day | 13 days |
| Comfortable but careful | 10 kWh/day | 4 days |
| Largely normal all-electric living | 35 kWh/day | Just over 1 day |
Those numbers aren't predictions for my house.
That isn't the point.
The point is the extraordinary difference behaviour makes.
A battery that seems inadequate while trying to reproduce normal life may be an extremely substantial emergency supply when concentrated on essentials.
What Might 3 kWh a Day Look Like?
Three kWh a day would require discipline, but it doesn't necessarily mean living in darkness.
A fridge and freezer cycle on and off rather than consuming their rated power continuously.
A broadband router doesn't require much.
LED lights can illuminate a house remarkably efficiently.
Phones require very little energy.
A laptop can provide entertainment, communication and useful work without consuming anything remotely like an electric oven or heating appliance.
One could have light, refrigeration, communication and computing while consuming surprisingly little electricity.
The objective isn't luxury.
It is maintaining the services that make the house safe, usable and connected.
A 10 kWh Battery Is Actually Quite Large—If You Use It Properly
The Energy Saving Trust describes around 10 kWh as a typical home battery capacity. Energy Saving Trust
Suppose an ordinary household had roughly 8 kWh genuinely available after reserves and losses.
At 8 kWh/day, that's approximately one day.
At 4 kWh/day, that's two days.
At 2 kWh/day, that's four days.
So asking:
"How long will a 10 kWh battery last?"
is rather like asking:
"How long will a tank of petrol last?"
It depends entirely upon what you do with it.
Solar Changes the Calculation Again
This is where a solar-and-battery system can become much more resilient than a battery on its own.
Imagine going to bed with 20 kWh remaining.
Without solar generation, that's all there is.
But if the following morning produces enough solar electricity to power the house and recharge some of the battery, the calculation begins again.
Instead of asking how many days the battery lasts, we begin asking whether:
daily solar generation >= daily emergency consumption
If it is, then in favourable conditions the house could potentially remain self-sufficient for considerably longer.
But there are two enormous qualifications.
First, the solar system must actually have been designed so that solar generation remains usable while the property is islanded from the grid. Battery ownership alone does not guarantee this. The Energy Saving Trust explicitly cautions that additional equipment and configuration are normally necessary for power-cut operation. Energy Saving Trust
Second, sunshine is not guaranteed.
A summer outage on a clear June morning is a very different proposition from a power failure at 5 pm on a dull December afternoon.
The Worst Time for My Power Cut
For my system, the interesting question isn't:
"Could 50 kWh keep me going?"
It almost certainly gives me substantial resilience if it is charged.
My real question is:
"What happens if the grid fails shortly after I have deliberately discharged the batteries?"
Imagine I have used stored electricity through an expensive tariff period and the batteries have fallen to 15% or 20%.
Then the power goes off.
My theoretical 50 kWh capacity is almost irrelevant.
What matters is what remains at that instant.
That has made me think differently about the concept of a battery reserve.
Perhaps, when severe weather is forecast or there is another credible reason to be concerned about supply, it would make sense to change the battery strategy temporarily and retain a larger reserve.
Instead of allowing the battery to drop to 10% or 20%, I might decide that resilience is temporarily more valuable than maximising tariff savings.
That is one of the benefits of a battery system that can be intelligently controlled.
Should We Keep 20% in Reserve All Winter?
Not necessarily.
There is a trade-off.
Keeping a large emergency reserve every day means that battery capacity is unavailable for normal tariff optimisation and self-consumption.
If I permanently reserve 20% of a 50 kWh bank, that's 10 kWh that I'm deliberately avoiding using.
For somebody with a smaller 10 kWh battery, reserving 20% removes 2 kWh from everyday operation.
So resilience has a cost.
The interesting possibility is dynamic resilience.
Normally use the battery efficiently.
But when bad weather is approaching or an outage has been announced, increase the reserve and perhaps deliberately charge the battery.
That could produce far more resilience than simply installing more and more batteries.
Don't Forget the Fridge and Freezer Already Contain Stored Energy
There is another form of energy storage sitting in many kitchens.
Cold.
A well-stocked freezer contains a considerable amount of thermal mass.
During a short power cut, constantly opening it to check whether everything is still frozen achieves precisely the opposite of what we want.
Keep the door shut.
UK Power Networks similarly advises keeping freezers closed during an outage because they can remain cold for several hours. UK Power Networks
This illustrates a broader lesson.
Resilience isn't always about generating more electricity.
Sometimes it is about not wasting the energy we already have.
What I Would Switch Off First
If my grid supply disappeared and I realised that it might be an extended outage, I would mentally divide the house into three categories.
Keep running
Fridges and freezers.
Necessary lighting.
Internet and networking.
Phones and communications.
Necessary controls and pumps.
Any genuinely essential medical or safety equipment.
Use occasionally
Kettle.
Microwave.
Computer.
Television.
Limited cooking equipment.
Possibly carefully managed space heating.
Switch off
EV charging.
Tumble drying.
Dishwashing.
Washing machine unless genuinely necessary.
Non-essential workshop equipment.
Unnecessary studio equipment.
High-power appliances being used merely for convenience.
That immediately turns a large all-electric household into a relatively modest electrical load.
Perhaps We Need an "Emergency Mode" for Houses
Modern smart homes have modes for almost everything.
Away mode.
Night mode.
Holiday mode.
Eco mode.
Why not a genuine power-cut mode?
One button could potentially reduce heating demand, disable EV charging, prevent certain appliances operating, shut unnecessary equipment down and prioritise refrigeration, communications and essential sockets.
A large battery would then become part of a resilience system, rather than simply being a large box full of electricity.
That is a much more interesting use of home automation.
So How Many Batteries Do You Actually Need?
There isn't a single answer.
A household wanting to maintain a fridge, freezer, router, lights and phone charging might be surprisingly resilient with a fairly modest battery.
A household wanting normal cooking, washing, entertainment and computing needs considerably more.
An all-electric household wanting to maintain normal winter heating throughout a prolonged outage may require a much larger battery capacity—and substantial inverter output.
And someone expecting to charge an EV normally throughout an outage is entering another category altogether.
The sensible calculation therefore starts from the load, not the battery catalogue.
Use this simple relationship:
Battery runtime = usable battery energy / average electrical demand
Or for a day-by-day calculation:
Days of backup = usable battery capacity / emergency daily electricity use
But remember that "usable battery capacity" means what is genuinely available at the moment the power disappears, not the number printed on the specification sheet.
The Real Test: Turn the Grid Off
There is one final point that is easy to overlook.
If you believe your house has backup capability, don't wait for a storm at midnight to discover whether it actually works.
A properly qualified installer can help verify how the backup arrangement should operate, what circuits are protected, the maximum permitted backup load, whether solar continues operating and what happens as the battery approaches its off-grid minimum state of charge.
Some LuxPower equipment, for example, specifically provides an Offgrid output setting and a Seamless switch option for transferring loads to inverter backup operation. That is a useful reminder that backup is an engineered feature, not simply something that happens because a battery exists. LuxpowerTek
Know what your system does before you need it.
My 50 kWh Battery: Would It Keep Me Going?
Yes—with an important qualification.
If my batteries were reasonably full and my backup system were configured correctly, 50 kWh gives me a substantial amount of resilience.
It could keep essential electrical services operating for far longer than a conventional small domestic battery.
But if I attempted to run my all-electric home precisely as though nothing had happened, that apparently enormous battery could disappear remarkably quickly.
And if the power cut arrived when the battery bank was already nearly discharged, "50 kWh" might become a rather misleading description of what I actually had available.
That is perhaps the most important lesson.
Resilience isn't simply about owning more batteries.
It is about knowing what matters.
Knowing what can wait.
Knowing how much power your inverter can actually deliver.
Knowing how much energy remains.
And designing your home so that, when the grid disappears, the electricity you have left is used intelligently.
Because when the lights go out, the most valuable kilowatt-hour may be the one you decide not to use.
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