20kWh Home Battery Storage: Cost, Limits and Who It Suits
20kWh is as far as a single modular unit usually goes. What that ceiling means, what it costs, and why the next step up is about paperwork rather than batteries.
- Around 18 kWh usable
- Single-unit ceiling
- Independent advice
A 20kWh home battery is usually the most a single modular unit will hold, which makes it a meaningful landmark rather than just another rung. Expect around £3,800 to £4,700 for the hardware. It stays inside the simple grid connection process, it needs no second unit, and it is the last size on the ladder where everything remains straightforward.
18 kWh
Usable capacity
1 unit
No second system needed
~6 hrs
To charge from empty at 3 kW
Why 20kWh is a landmark, not just a bigger number
Modular home batteries are built as a base unit holding the electronics, plus expansion modules that add storage. Depending on the brand, a single unit tends to top out somewhere around 20kWh once it carries its full complement of modules. Go beyond that and you are not adding another module; you are adding a second complete system running alongside the first.
That matters more than it sounds, because two units running together roughly double the combined output. A single unit on a dedicated circuit sits at two or three kilowatts, comfortably under the 3.68 kW single-phase threshold your network operator cares about. Two units cross it. The battery is the same hardware; what changes is that the installation now needs approval before it goes in rather than a notification afterwards.
At 20kWh and below
One base unit with its modules. A qualified electrician runs a dedicated circuit, connects the system and notifies your network operator once it is done. No waiting, no application, no approval to chase. This is the simple route and it covers the great majority of British homes.
Above 20kWh
A second complete unit in parallel, which roughly doubles output and pushes the installation over the single-phase threshold. Your network operator now has to approve it before installation. Expect an application, a wait, and in some cases conditions attached to the connection.
None of that makes larger systems a bad idea. It does mean 20kWh is the point to stop and ask whether you genuinely need more, because the step after it costs more than the price list suggests.
Our take
If you are weighing 20kWh against something larger, price the whole job, not the battery. The second unit brings an application, a wait and possibly conditions on your connection. Plenty of households that thought they wanted 25kWh settle happily at 20 once they see the difference in process.
How much battery storage do you actually need?
Three figures decide almost everything: how much electricity your household uses in a year, how much surplus solar power you send back to the grid, and how much of that you actually use after sunset.
Annual consumption
Divide it by 365 to get your average day. At 9,000 kWh a year that is roughly 25 kWh per day, though a household at this level usually has that concentrated in heating and car charging rather than spread evenly.
Exported solar energy
This is the surplus a battery could have stored. At 20kWh you need a substantial array to fill the stack from the roof alone, so check this figure carefully rather than assuming.
Evening and overnight demand
Work out what you actually draw between the evening peak and the following morning. If that figure is well under 18 kWh, the extra capacity will not cycle.
Our guide to battery capacity works through the method in full, or run your figures through the configurator below.
What does 20kWh of battery storage cover in a real home?
Most lithium iron phosphate batteries release 90 to 95% of their nominal capacity, so a 20kWh battery gives you roughly 18 kWh in daily use. That is enough to change what the battery is for: below this level it covers an evening, here it covers a day.
| Scenario | Typical daily use | Share of 18 kWh usable |
|---|---|---|
| Household base load, 24 hours | 3-5 kWh | About 22% |
| Cooking, laundry and dishwashing | 3-4 kWh | About 19% |
| Heat pump, cold winter day | 8-10 kWh | About 50% |
| EV top-up, 40 miles of range | 10-12 kWh | About 61% |
| All of the above together | 24-31 kWh | Exceeds capacity |
Read the last row carefully, because it is the honest answer to the most common reason people reach this size. A household running a heat pump and charging a car at home uses more in a winter day than 18 kWh holds. 20kWh covers one of those two comfortably and both only partially. If you want to cover both, you are looking at a second unit, and the 25kWh guide explains what that involves.
Can you actually fill 20kWh?
This is where more 20kWh purchases go wrong than anywhere else. Charge rate does not rise with capacity: modular systems commonly take around 3 kW whether the stack holds 10kWh or 20kWh. Filling 18 kWh from empty therefore takes roughly six hours.
From a cheap tariff
Six hours is longer than most off-peak windows. On a shorter tariff you will routinely start the evening with a part-filled battery, and the unfilled portion earns nothing.
From the roof
You need an array of roughly 12,000 W, around twenty-four to thirty panels, to fill this regularly. That is a large roof, and in winter it will not manage it on any day.
Both together
The combination is what makes 20kWh work: solar through the day, a cheap window overnight for whatever the sun did not supply.
The practical test is simple. If you cannot describe how the battery gets filled on an ordinary Tuesday in November, you are buying capacity that will spend the winter half empty.
How much does a 20kWh battery cost?
Around £3,800 to £4,700 for the hardware, plus £250 to £500 for an electrician to run a dedicated circuit. This is the best cost per kilowatt-hour on the simple side of the ladder: you are spreading one base unit’s electronics and one circuit across the maximum storage a single system holds.
| Capacity | Hardware | Dedicated circuit | Grid connection |
|---|---|---|---|
| 5 kWh | £1,100 – £1,400 | £250 – £500 | Connect and notify |
| 10 kWh | £2,000 – £2,500 | £250 – £500 | Connect and notify |
| 15 kWh | £2,900 – £3,600 | £250 – £500 | Connect and notify |
| 20 kWh (this page) | £3,800 – £4,700 | £250 – £500 | Connect and notify |
| 25 kWh | £4,700 – £5,800 | £250 – £500 | Prior approval |
| 30 kWh | £5,500 – £6,900 | £250 – £500 | Prior approval |
Payback still depends far more on your electricity contract than on the purchase price, and at this size the contract is doing most of the work. A household on a time-of-use tariff shifting 18 kWh a day from cheap hours to peak hours is moving real money; the same battery on a flat tariff is an expensive way to store sunshine.
Is a 20kWh home battery right for you?
Pros
- Your annual electricity use is between about 7,000 and 9,000 kWh
- You run a heat pump and want it covered on the coldest days
- You have roughly 24 to 30 solar panels, or a long off-peak window, or both
- You want the largest system that still avoids prior grid approval
- You want the best cost per kilowatt-hour on the simple side of the ladder
- You are planning for consumption that is about to rise
Cons
- You cannot describe how the battery fills on a dull November day
- Your off-peak window is under four hours and you have little solar
- You have no electric heating and no EV charging at home
- Your evening and overnight use is well under 18 kWh
- You want whole-house backup, which is a different specification
- A 15kWh system is already reaching morning with charge left
Is 20kWh enough for backup power?
Comfortably, in energy terms. 18 kWh runs essential loads for the best part of four days. As always the constraint is output rather than capacity: a backup socket rated at around 3 kW cannot run an oven and an induction hob at once however full the battery is.
| Connected load | Power | Runtime on 18 kWh |
|---|---|---|
| Fridge, lights, router, phone charging | 150-250 W | Around 3-4 days |
| The above plus TV and a laptop | 350-450 W | 40-51 hours |
| Add a small heater or kettle in bursts | 1,000-1,500 W | 12-18 hours |
| Electric oven or induction hob | 2,000-3,000 W | 6-9 hours, at the output limit |
If uninterrupted whole-house power is the main reason you are buying, capacity is the wrong thing to shop for. That capability comes from a conventional installation with a hybrid inverter, and it is worth getting quotes for both approaches before you decide.
Discover smarter home energy with HomeBattery.com
A short introduction to how home batteries store solar energy, respond to electricity prices and fit into an ordinary household.
What to check before you buy a 20kWh battery
| Specification | Why it matters | Typical at 20 kWh |
|---|---|---|
| Usable capacity | What you actually get out of the battery each cycle | 17-19 kWh |
| Single-unit maximum | Whether this fits in one system or needs a second | Often around 20 kWh, varies by brand |
| Continuous discharge power | Whether it can carry heating and cooking together | Typically 2-3 kW on a dedicated circuit |
| Charge power | Whether you can refill inside your cheap tariff window | Up to about 3 kW, so roughly six hours from empty |
| Backup output | What you can run during a power cut, separate from the above | Up to about 3 kW with modules fitted |
| Cycle life and warranty | The real measure of lifespan, more than the number of years | 10 years or 6,000-8,000 cycles |
| Footprint and weight | A full stack is heavy and needs a sound floor and clearance | Several modules, tens of kilograms each |
| Protection rating | Required for outdoor placement | IP65 for outdoor use |
Footprint is worth real attention at this size. A full stack is a substantial piece of equipment, and the place you had in mind may not take it. Our capacity guide covers the connection rules and what your network operator needs.
20kWh or 25kWh?
On paper this is a 25% increase in storage for a similar increase in price. In practice it is the biggest step on the whole ladder, because it is where one unit becomes two.
| 20 kWh | 25 kWh | |
|---|---|---|
| Usable capacity | About 18 kWh | About 22.5 kWh |
| Hardware | £3,800 – £4,700 | £4,700 – £5,800 |
| Number of units | One | Two |
| Combined output | Typically 2-3 kW | Roughly double |
| Grid connection | Connect and notify | Prior approval needed |
| Floor space | One stack | Two stacks |
The higher output is a genuine benefit, not just a regulatory inconvenience: two units can carry heating, cooking and car charging at the same time in a way one cannot. But if your reason for going larger is purely storage rather than power, stop at 20kWh. You get the simple connection, one footprint, and the best value per kilowatt-hour available without an application.
Frequently asked questions
Is 20kWh the largest single battery you can get?
For most modular platforms, yes or close to it. A single base unit with its full complement of expansion modules tends to reach somewhere around 20kWh, varying by brand. Larger systems are built by running two or three complete units in parallel.
How long does a 20kWh home battery last?
Discharging at its full rate on a dedicated circuit it empties in six to nine hours. Running only essential loads it lasts several days. As a rule, 18 kWh usable divided by the connected load in kilowatts gives the runtime in hours.
How much does a 20kWh battery cost?
Around £3,800 to £4,700 for the hardware, plus £250 to £500 for an electrician to run a dedicated circuit. That works out as the best cost per kilowatt-hour available without stepping up to a second unit.
Can I fill a 20kWh battery overnight?
Not usually in one go. At a charge rate of around 3 kW the stack takes roughly six hours to fill from empty, which is longer than most off-peak windows. In practice a 20kWh system relies on solar through the day as well as cheap hours overnight.
Do I need approval from my network operator for 20kWh?
Not in advance. A single unit on a dedicated circuit stays under the 3.68 kW single-phase threshold, so your installer connects it and notifies afterwards. That changes at the next size up, where a second unit pushes the combined output over the threshold.
How many solar panels do you need for a 20kWh battery?
Roughly twenty-four to thirty panels, or about 12,000 W, to fill it from the roof regularly. Most British homes do not have that, which is why 20kWh systems usually depend on a time-of-use tariff as well as solar.
Is 20kWh enough for a heat pump and an electric car?
For one of them comfortably, for both only partially. A heat pump on a cold day plus a meaningful car charge exceeds 18 kWh of usable storage. If you need both covered, you are looking at two units and the grid approval that comes with them.
How much space does a 20kWh battery take up?
More than people expect. A full stack is several modules weighing tens of kilograms each, and it needs a sound floor, clearance for airflow and, if it goes outside, a suitable protection rating. Check the dimensions against the spot you have in mind before ordering.