Self-consumption vs export in UK solar: how the Smart Export Guarantee changes the maths

If UK solar quotes feel confusing, it is usually because two different “values” are being mixed together.

A kWh you use at home avoids buying a kWh at your import rate.

A kWh you export is paid at your Smart Export Guarantee (SEG) rate.

Those are not the same number, so the best system is rarely “the biggest system you can fit”. It is the system that matches your household’s timing and demand.

If you want a quick baseline for solar output where you live, start here: Find your postcode

We will use kW, kWh, and kWp (see Glossary) and we will refer to Smart Export Guarantee (SEG) (also in Glossary).

Assumptions and variability

• We assume a typical UK home where electricity use often peaks morning and evening, while solar generation peaks around midday.
• We assume you are comparing a self-used kWh (avoids import) with an exported kWh paid under Smart Export Guarantee (SEG) (see Glossary).
• We assume prices change over time, so we focus on a stable structure (the “two prices” model), not a fixed penny-per-kWh claim.
• What varies most in real homes: occupancy (work-from-home vs away), cooking habits, hot water setup, EV charging flexibility, and big flexible loads (heat pump, immersion heater, tumble dryer).
• Export caps and grid constraints can matter locally (DNO / G98 / G99, see Glossary), but the core self-use vs export logic remains the same.
• For how SolarByPostcode estimates output and savings by outcode, see: Data sources and methodology

The core idea: one kWh of solar can be worth two different amounts of money

A solar system produces electricity.

Your home can do three things with each kWh:

1) Use it immediately (self-consumption)
2) Export it (paid under Smart Export Guarantee)
3) Not use or export it (rare, but can happen with curtailment or limits)

Here is the mental model in one glance:

The practical takeaway is simple:

Your savings depend on the share of your solar that becomes self-consumption versus export.

Why timing drives everything

Solar is strongest around the middle of the day.

Many households are “evening-heavy”, which means the home wants electricity most when solar is fading.

That timing mismatch is the foundation for almost every “should I buy more panels / should I get a battery?” decision.

If you have not read it yet, the flagship guide that explains this clearly is:

• Daytime vs evening electricity use: why self-use matters more than kWp

How the same solar system behaves in different homes

Even with identical panels, households can end up with very different self-consumption percentages.

This is why one person says “export is fine” and another says “a battery changed everything”.

These are illustrative, but the direction is real: if your home is away in the day, you are more likely to export. If you can run daytime loads, you are more likely to self-consume.

The “two prices” model you should use in your head

You do not need perfect numbers to make good decisions. You need the right structure.

• Self-used kWh: worth roughly your import unit rate avoided
• Exported kWh under Smart Export Guarantee: worth your SEG export rate

So the question becomes:

Will the next kWh your panels produce be used at home, or exported?

That is why “more kWp” is not automatically better.

If you want the bigger system sizing framework (roof constraints, oversizing, diminishing returns), start here:

• Solar system sizing in the UK: choosing the right kWp without wasting money

A practical way to estimate your likely self-consumption

Most households do not need a spreadsheet. Use a simple three-step check:

1) Decide whether you are naturally “daytime-heavy” or “evening-heavy”

A quick test: on a normal weekday, is the house busy between 10:00 and 16:00?

If you are evening-heavy, export will be higher unless you shift some loads.

2) Identify flexible loads you can move without making life worse

The point is not lifestyle changes. It is defaults:

• dishwasher and washing machine at midday (when safe)
• dehumidifier in sunny hours
• immersion hot water (if you have it) as a “solar sponge”
• EV charging in a solar window, if possible

3) Sanity-check with your postcode output

Your roof and location change the amount of solar you have to work with.

Run your outcode and come back to this guide. Try a few different places to see range across the UK, for example:

• Yorkshire city centre: LS1 (Leeds)
• North West England: M1 (Manchester)
• Northern Scotland: IV1 (Inverness)

You can also zoom out with region pages:

• Yorkshire
• North West England
• North Scotland

When export is perfectly fine (and when it is a warning sign)

Export is not a failure. It is an expected outcome of midday generation.

Export is fine when:

• you already sized sensibly for your usage
• your household cannot (or should not) shift much daytime load
• your SEG export rate is decent relative to import
• your roof is large and the incremental cost per extra panel is low

Export is a warning sign when:

• your quote assumes very high “savings” without explaining the split
• the system size is being pushed up without a matching usage reason
• you are buying a battery mainly to “fix” a too-large system decision

If you want a clean explanation of why panels do not behave like their nameplate rating and why midday peaks matter, read:

• Why solar panels never hit their rated output (and why that is normal)

The most common mistakes installers and quotes make

Here are the classics:

• treating every kWh as if it is self-used (it is not)
• assuming export is worthless (it is not)
• ignoring timing and household pattern
• using “bill to zero” language (standing charges exist, and winter exists)
• bundling a battery without proving there is surplus to store

If you want a practical quote checklist, use:

• How to compare solar quotes in the UK: what matters and what is noise

What to do before buying more panels or a battery

This is the practical decision ladder.

Where batteries fit in this story

A battery is a timing tool: it can convert midday surplus (which would be export) into evening self-consumption.

That can be rational, but only if you can regularly fill the battery from solar and regularly use it later.

If you want the full battery decision framework:

• Solar batteries in the UK: who they make sense for (and who should skip them)

A simple “quote sanity check” you can use in one sentence

Ask your installer:

In your model, how many kWh per year are self-used, how many are exported under Smart Export Guarantee, and what rates are you using for each?

A good quote can answer that cleanly.

If it cannot, the “savings” number is not trustworthy.

Bottom line

• Solar has two money outcomes: self-consumption (avoids import) and export (paid under Smart Export Guarantee).
• The split between them is driven by timing and household pattern.
• More kWp can increase export more than savings if your daytime demand is low.
• Shift flexible loads first, then decide whether more kWp or a battery is actually the right lever.
• Use your postcode page to anchor expectations to real local output.

Next reads

• Daytime vs evening electricity use: why timing matters more than totals
• Solar payback periods in the UK: what actually drives the number
• Standing charge drag
• Solar batteries in the UK: who they make sense for (and who should skip them)
• How big should a home battery be? Avoiding the most common sizing mistake
• Do solar panels increase house value in the UK? The boring truth (and when it backfires)

Run the calculator for your postcode