How to Size Your Solar Battery

If you’re looking at solar panels and battery storage in South Wales, one of the most common questions is “What size battery do I actually need?”. The answer is usually much simpler than most people think.

For most homes, the best battery size is not the biggest battery you can afford — it’s the battery that gets used properly every day.

At Ulex Energy, we normally recommend sizing a battery around your average daily usage, rather than worst-case winter scenarios.

How Does a Solar Battery Save Money?

A battery saves money by replacing expensive electricity from the grid with:

• cheaper overnight electricity, or

• your own solar energy stored during the day.

  
  

The key point is: A battery only saves money when it is actively being used all year around. That’s why correctly sizing the battery matters so much.

Typical Electricity Prices in the Wales (May 2026)

Typical electricity costs today based on Octopus Flux and Go (other similar tariffs are available from other providers):

Electricity Type	Typical Price
Standard electricity tariff (i.e. Ofgem Price Cap April-June 2026)	25p/kWh
Cheap overnight tariff	15p/kWh (Octopus Flux) 9p/kWh (Octopus Go with EV)
Export payments	29p/kWh 4pm to 7pm and 10p/kWh otherwise (Octopus Flux) 12p/kWh all day (Octopus Go with EV)

So if your battery charges overnight and avoids buying electricity later:

25p - 15p = 10p saving per kWh (Flux) 
25p - 9p = 16p saving per kWh (Go) 

After battery losses of 5% assuming a battery connected to a hybrid inverter, the real saving is usually closer to:

9p per kWh (Flux)
14p per kWh (Go)

Note that the savings are greater with Octopus Go but this requires owning an EV.

Note that with Octopus Flux you are better off charging your battery with excess solar during the day whilst with Octopus Go you are better off charging your battery overnight and exporting all excess solar during the day. With Octopus Flux, it may still be better to charge your battery fully overnight if the next day is going to be cloudy, this is where AI driven system that can control your battery charge according to the weather forecast can make a big difference.

Battery size ≈ your average daily electricity use

For example, if your home uses:

10 kWh per day

then a sensible battery size is usually:

~10 kWh

That allows most of your daily electricity to come from cheaper overnight charging.

Why Bigger Batteries Often Don’t Make Financial Sense

• Sizing your battery on your winter usage rarely makes sense:

  • The most cost-effective battery is usually one that fully cycles every day, all year around.

  • You will unlikely use the full capacity of your battery most of the year if it is oversized, hence diminishing your return on investment period.

    
    

• Solar panels top up:

  • If you are on an Octopus Flux style tariff, it makes sense to charge your battery with excess solar (as exporting only earns you 10p/kWh versus 15p/kWh if charged overnight, this is the opposite with Octopus Go style tariffs).

  • This means, you battery is being kept topped up by the solar and so doesn't need to cover a full day of consumption.

• Overnight usage is covered by cheap tariffs

  • Homes still use some electricity overnight.

  • For example:

    • 200W overnight baseload

    • over 5 hours

    • equals 200W × 5h = 1 kWh

  • This is means that this portion of your daily average use does not need to be covered by the battery.

    
    

• Battery lifetime:

  • Batteries are generally warrantied to retain 60% to 80% of their original capacity after 10 years. This means that you will likely need new batteries then if you wanted to keep the full cycle depths.

• You can always expand later

  • One of the biggest advantages of modern battery systems is flexibility.

  • Most systems are modular, which means:

    • you can start with a smaller battery,

    • see how your usage changes,

    • and add extra storage later.

  • This is often the most financially sensible approach.

• Arbitrage

  • Modern tariffs such as Octopus Flux can make battery arbitrage financially attractive because electricity can be bought overnight cheaply for 15p/kWh and exported at a higher price during peak window for around 29p/kWh, creating a meaningful margin even after battery losses.

  • However, if you were charging directly from excess solar, you would earn the full 29p/kWh hence providing a better return on investment.

Final Thoughts

The best solar battery size is usually based on how much electricity you can realistically shift every day —not worst-case winter scenarios.

For most homes in South Wales solar battery systems are sized around daily usage.

If you’re considering solar panels or battery storage in South Wales, Ulex Energy can help you choose a system that is properly sized for your home and energy usage.