The Solar Guide

You don’t need to become a solar expert.

You need four things to stack up:

cost → generation → value → installation

The rest of this guide works through them one at a time.

The panels are only the most visible part of the system.

A complete installation normally includes things such as:

• solar panels
• inverter
• mounting equipment
• cabling and electrical protection
• design and survey work
• roof access or scaffolding
• installation labour
• testing and commissioning
• monitoring
• certification
• warranties and aftercare

So finding the retail price of a solar panel online and multiplying it by ten won’t tell you what your completed installation should cost.

There are two main sources.

Those two units of electricity do not necessarily have the same value.

That matters.

If imported electricity costs more per kWh than your export tariff pays, using solar electricity at home may be more valuable than exporting it.

A simple payback period asks how long the modelled annual benefit would take to equal the upfront cost.

For example:

If exactly the same system costs £10,000 but still delivers £800 per year, simple payback becomes 12.5 years.

The roof didn’t change.

The generation didn’t change.

The purchase price changed the investment.

That is why the price you actually pay matters so much.

Cheaper usually helps the economics.

But cheaper is not automatically better value.

Two quotes may differ in:

• equipment
• warranties
• electrical work
• system design
• aftercare
• generation assumptions

We’ll deal with that properly in Chapter 5.

For now, the important point is:

compare complete systems, not just headline prices.

This is one of the easiest solar terms to mix up.

For example:

That does not mean the system generates 4.6 kWh every hour.

Actual output rises and falls throughout the day and throughout the year.

For the financial calculation, the important figure is usually:

How many kWh will the system generate over a full year?

A south-facing roof will generally maximise annual generation in the UK.

But that does not mean east- or west-facing solar cannot work well.

Direction affects both how much electricity is generated and when during the day it is generated.

Roof angle also matters.

So does location.

And shading from trees, chimneys or surrounding buildings can reduce output further.

The useful question is therefore not:

“Is my roof perfect?”

It is:

“What can this roof realistically generate?”

More panels generally mean more annual generation.

But that does not automatically mean the largest possible array is the financially sensible choice.

System size has to balance:

So the aim is not necessarily to fit the maximum number of panels possible.

It is to find a system size that makes sense for the property and the economics.

Home by Numbers uses PVGIS, developed by the European Commission’s Joint Research Centre, as the technical basis for location-specific solar modelling.

PVGIS can estimate monthly and annual generation using information such as:

• location
• system capacity
• roof direction
• panel angle
• solar radiation
• temperature
• system losses

That lets us move from:

“A system this size normally generates X.”

to:

“A system this size, at this location and configured like this, is modelled to generate approximately X.”

That is a much stronger planning number.

Use that too.

A competent installer should provide an annual generation estimate based on the proposed system and property.

If their number differs materially from yours, don’t automatically assume one is wrong.

Ask:

“What assumption is different?”

Perhaps they have better shading information.

Perhaps the entered orientation differs.

Perhaps different system losses are being used.

Understanding the difference is more useful than simply choosing whichever estimate is higher.

Imagine your panels are generating strongly at midday.

Your home is only using a small amount of electricity.

The solar electricity first supplies the home.

Any surplus can then be exported.

Later that evening, when solar generation has dropped away, you may start buying electricity from the grid again.

A battery lets you store some of that midday surplus and use it later.

Because the electricity you avoid buying later may be worth more than the electricity you would have exported earlier.

For a simple example:

Using a solar-generated kWh at home could avoid roughly 30p of grid electricity.

Exporting it earns roughly 12p.

That creates potential value from storing some of the surplus.

But the battery itself costs money and energy is lost during charging and discharging.

So:

greater self-consumption does not automatically mean better overall economics.

Bigger is not automatically better.

A battery should broadly suit:

• how much surplus solar you produce
• how much electricity you use outside solar hours
• your tariffs
• the battery’s usable capacity
• its charge and discharge limits

If a large battery rarely fills or rarely empties, you may have paid for capacity you seldom use.

A battery might have:

That tells you roughly how much energy it can store.

It does not tell you how quickly it can supply your house.

That is determined by its power output.

So ask two separate questions:

1. How much energy can it store?
2. How much power can it deliver at once?

Both can matter.

A battery can sometimes have value even when solar generation is low.

On some time-of-use tariffs, electricity is much cheaper during certain periods.

A battery may be able to charge during those cheaper periods and discharge later when grid electricity costs more.

But again, the calculation needs to include:

• the cheap rate
• the expensive rate
• battery losses
• usable capacity
• how much expensive electricity can actually be displaced

The answer is still in the numbers.

No.

Exporting is sometimes talked about as though the electricity has been wasted.

It hasn’t.

If you are paid for exporting it, it has value.

So the choice is better thought of as:

use itstore itexport it

Each route can have a different financial value.

Getting more than one quote can help you understand:

• whether the price is broadly sensible
• whether installers agree on system size
• whether generation estimates are similar
• whether important work is included
• whether one proposal appears unusually optimistic

You do not need every installer to recommend exactly the same system.

But large differences are worth understanding.

When comparing quotes, look for:

• panel quantity and wattage
• total array capacity in kWp
• inverter make, model and power rating
• battery capacity, if included
• usable battery capacity
• battery charge and discharge power
• estimated annual generation
• electrical work included
• scaffolding or access costs
• monitoring
• product warranties
• workmanship cover
• total installed price

Only then do you know whether two prices are really comparable.

MCS provides a recognised standards and certification framework for small-scale renewable installations such as domestic solar PV.

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

But certification should not be the only check.

Also consider:

• experience
• reputation
• clarity of the quotation
• warranty arrangements
• how well questions are answered
• whether the proposed system actually makes sense

Many normal domestic rooftop solar installations can be carried out without a full planning application, subject to the relevant rules.

But circumstances vary.

Listed buildings, conservation areas and unusual installations can require additional consideration, and planning rules differ across the UK.

So the useful question is not:

“Does solar need planning permission?”

It is:

“Has the position for this property been checked?”

Your solar installation also connects to the local electricity network.

Depending on the system size and configuration, the installer may need to notify the network operator or obtain approval.

You do not need to become an expert in network-connection standards.

But it is reasonable to ask:

1. Who is handling the network application?
2. Does approval need to happen before installation?
3. Will the system have an export limit?
4. Can the system operate in the way I have been quoted?

This catches people out.

You might have:

Those are three different numbers describing three different parts of the system.

They do not have to match.