The Battery Guide

There are two main routes.

Suppose adding the battery costs £5,000.

And the battery adds £500 per year of modelled savings.

Its simple payback would be around:

If it only adds £250 per year:

Same battery.

Very different investment.

And, as with solar, simple payback is a planning measure rather than a guarantee.

Tariffs change.

Usage changes.

Battery performance changes over time.

Energy is lost while charging and discharging.

This matters when judging a long payback period.

Home batteries have a finite working life, and the warranty, usage pattern and battery chemistry all matter.

So a payback calculation should not be looked at in isolation.

A battery with a relatively short modelled payback presents a very different proposition from one whose payback stretches well beyond the period you would reasonably expect to own or rely on the system.

Imagine a battery with:

It may be able to store around 10 kWh of electricity.

But it cannot necessarily supply a 7 kW household load entirely from the battery at that moment.

The remaining demand may still need to come from solar, the grid or another source.

The number in the product name is not always the exact amount of electricity available for everyday use.

A system may:

• reserve part of the battery
• limit how deeply it normally discharges
• keep some capacity available for backup
• have a quoted nominal capacity different from usable capacity

So when comparing batteries, ask:

“How many kWh can I actually use?”

not just:

“How big is the battery?”

Imagine your home regularly produces around 5 kWh of surplus solar during the day and then uses around 5 kWh after the sun has gone down.

A 5–6 kWh usable battery might have plenty of work to do.

A 15 kWh battery might spend much of its life partly empty because there simply isn’t enough surplus electricity available to fill it.

Alternatively, a household with high evening demand, an EV, electric heating or a large cheap-rate tariff window may have a very different requirement.

There is no universal perfect battery size.

Some battery systems allow part of the capacity to be kept in reserve for power cuts.

That can be valuable.

But capacity held back for emergencies is capacity that is not normally being used for everyday savings.

So:

more backup reserve → potentially more resilience

but

less everyday usable capacity → potentially lower financial benefit

Neither is automatically right or wrong.

It depends what you want the battery to do.

1. surplus solar
2. →battery
3. →used later at home

Solar often generates most strongly around the middle of the day.

Many homes use more electricity in the morning and evening.

Without a battery, surplus daytime electricity can be exported.

With a battery, some of it can instead be stored and used later.

That can increase the amount of your own solar generation that supplies your home.

But — as we saw in Chapter 2 — that does not automatically mean the battery pays for itself.

You must compare the additional saving with the additional battery cost.

1. cheap rate
2. →battery
3. →expensive period

Solar is not essential.

Some tariffs offer cheaper electricity during particular hours.

A battery can potentially charge during that cheaper window and then supply your home during more expensive periods.

For example:

There is clearly a potential price difference.

But the battery will lose some energy during charging and discharging, and the amount you can shift is limited by its capacity and charge/discharge power.

So you still need to model it.

For some households, the useful answer may be:

both.

In winter, when solar generation is lower, the battery may rely heavily on cheap-rate grid charging.

In summer, solar may provide much of the charging.

The battery does not have to behave identically every day of the year.

The controls and tariff can materially change how much value you get from the same battery.

Export is not failure.

If somebody is paying you a good rate for electricity, exporting it can be completely sensible.

Spending thousands of pounds on more battery capacity purely to avoid export may not make financial sense.

So don’t assume:

store = good

and

export = bad.

They are alternatives with different values.

Every time electricity goes into and later comes back out of a battery, some energy is lost.

That is why charging a battery at 20p/kWh to avoid buying electricity later at 21p/kWh is unlikely to be particularly useful.

The price difference needs to be large enough to overcome those losses and still provide useful value.

When comparing quotes, I would want to understand:

• battery make and model
• total battery capacity
• usable capacity
• maximum charging power
• maximum discharge power
• inverter make and rating
• whether the inverter is included
• solar compatibility
• grid-charging capability
• backup capability
• backup reserve settings
• monitoring/app functionality
• warranty
• installation work
• total installed price

Then you can compare systems properly.

You may see these terms in quotations.

You don’t need to become an electrical engineer.

At a simple level:

Neither label automatically means better.

The practical question is:

“Which arrangement makes most sense with the equipment already in my home and what I want the battery to do?”

Your installer should be able to explain that in plain English.

The battery capacity tells you how much energy can be stored.

The inverter and battery power limits help determine how much power the system can actually deliver.

So don’t just ask:

“How big is the battery?”

Also ask:

“How much power can it supply to my house at once?”

That can matter when several high-power appliances are running together.

No.

A battery in the house does not automatically mean your whole home continues operating normally during a power cut.

Backup capability depends on the battery, inverter, wiring and system design.

Some systems can provide backup.

Some may support selected circuits.

Others may not provide backup in the way you expect.

If backup matters to you, make it an explicit part of the quote and specification rather than assuming it comes with the battery.

A home battery is substantial electrical equipment.

Where it is installed matters.

The installer should consider the manufacturer’s requirements, electrical standards, accessibility, environmental conditions and safety requirements when choosing a location.

This is one area where the correct answer should come from the product specification and competent installer rather than an internet rule of thumb.

Battery storage connects to the electricity network and may need to be notified to, or approved by, the relevant network operator.

In Great Britain, government guidance says domestic battery storage is one of the energy devices that must be registered with the local Distribution Network Operator.

Northern Ireland has its own NIE Networks connection requirements.

You should not normally need to become an expert in the technical application yourself.

But it is reasonable to ask:

1. Who is dealing with the network application?
2. Does approval need to happen before installation?
3. Is there an export limitation?
4. Will that restriction affect how the battery or solar system operates?

MCS has a dedicated installation standard for electrical energy storage systems.

Using an appropriately certified installer can therefore be an important part of your checks.

But certification should not be the only check.

Also look at:

• installer experience
• clarity of the quotation
• warranty support
• aftercare
• the equipment specification
• whether the proposal actually fits your household