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

If you are getting solar quotes, you will quickly run into a disagreement:

• One installer says: “3.6 kWp is plenty.” (kWp is your system’s rated panel capacity, see Glossary.)
• Another says: “Max out the roof. Bigger is always better.”
• A third tries to sell you a battery to “solve” the sizing problem.

All three can sound reasonable.

But solar sizing is not a personality test. It is an economics problem.

Solar is most valuable when you use it in your own home.
Solar is less valuable when you export it.

So the “right size” is the size that fits how you use electricity, not the size that looks best on a quote.

If you want numbers you can actually use, start here: Find your postcode.

This guide gives you a simple method that works for most UK homes, using realistic assumptions and postcode-level generation estimates.

Last updated: January 2026.

Quick answer: what size system should you buy?

A good starting workflow:

1) Find your annual electricity usage in kWh (energy, see Glossary) (from a bill or smart meter).
2) Run your postcode through the SolarByPostcode calculator for a few system sizes.
3) Choose the size where the extra generation still has a clear plan (self-use, battery, EV, or acceptable export).

Assumptions and variability

• We assume a “normal” UK household pattern unless you tell us otherwise (daytime usage is the big swing factor for self-use).
• We treat export as lower value than self-use (because import rates are usually higher than export rates), but your exact tariff mix can change the gap.
• We assume your roof is not heavily shaded, and that your installer designs the system competently (layout, inverter sizing, and wiring matter).
• Your postcode, roof direction, and roof pitch can move annual generation materially).
• Any £ examples are illustrative, not promises. For the underlying data assumptions and how we model variability, see Data sources and methodology.

Related guides that reinforce sizing decisions

• Why your solar panels will never hit their rated output (and why that’s completely normal)
• Solar panel cost in the UK: what you actually pay and why quotes vary
• Solar panels and heat pumps: why they are the perfect energy pair

The one idea that makes sizing make sense

Self-use kWh and export kWh are not worth the same

Every kWh you self-use avoids buying 1 kWh at your import unit rate.

Every kWh you export earns your export rate (SEG or similar, see Glossary).

In most households, the import rate is higher than the export rate. That means:

• The first chunk of generation that you self-use is usually the highest-value chunk.
• As you keep adding panels, you often increase export faster than you increase self-use, unless you also add flexibility (battery, EV charging, daytime loads).

“Paying the grid” vs “losing export”: the £ trade-off (in plain language)

The cleanest way to think about solar value is this:

• If you self-use 1 kWh, you avoid paying the grid for that 1 kWh.
• If you export 1 kWh, you get paid your export rate for it.
• If you could magically shift 1 kWh from export into self-use, your extra value is roughly:

(import rate) minus (export rate)

That difference is the value of load shifting, and it is why batteries and smart usage patterns can be worth real money.

A simple example (illustrative)

Imagine:

• Import rate: 28p/kWh
• Export rate: 15p/kWh

Then:

• A self-used kWh is worth about 28p
• An exported kWh is worth about 15p
• Shifting 1 kWh from export to self-use is worth about 13p extra

This framing also explains why “bigger panels” are not the same as “better economics”:

• If your extra panels mostly create export, the extra kWh are only worth export-rate money.
• If your extra panels create self-use, the extra kWh are worth import-rate money.

Sizing is about where the marginal kWh goes.

The 70/30 rule (and when it is wrong)

People often want a simple rule of thumb. The most useful one for sizing conversations is a ratio:

For many UK households with PV and no battery, a “good-fit” system often ends up somewhere around 70% self-used and 30% exported over a year.

But treat that as a starting point, not a law of physics.

It varies a lot with load profile:

• If you are out most weekdays, self-use can be much lower (and the split can move towards 40/60 or even 30/70).
• If you are home in the day, or you have controllable loads (EV charging, hot water diversion), self-use can be much higher.
• With a battery and a smart tariff, you can often push self-use materially higher, but batteries are their own economics problem.

Load profile matching: when you use power vs when panels generate

Solar generation is concentrated in daylight hours, with a strong midday peak in spring and summer.

So the question that decides your self-use share is:

Do you have meaningful electricity use during solar hours?

Typical day-time loads that increase self-use:

• People at home (work from home, retirees, childcare)
• Dishwasher and washing machine runs during the day
• Cooking earlier (or batch cooking)
• EV charging during daylight (even a few hours helps)
• Hot water heating or immersion diversion
• Heat pump operation (seasonal, but still relevant)

Typical patterns that reduce self-use:

• Home empty 9 to 5 most weekdays
• Low daytime base load (fridge + router only)
• Heavy usage mainly evenings (cooking, tumble dryer, entertainment)

This is why two neighbours can rationally choose different system sizes.

The stacked-bars picture: what happens as you go bigger (without a battery)

Most households see a pattern like this: as system size increases, export rises faster than self-use unless you also add flexibility.

The figure below is illustrative only. It is not “your house”. It shows the typical direction of travel for a household with no battery and a fairly normal day-time base load.

The practical takeaway is simple:

• If you are already exporting a lot, “more panels” is mostly an export decision.
• If you can use more of what you generate, “more panels” is usually a self-use decision.

Neither is automatically right or wrong. The mistake is pretending the two are the same.

A practical sizing method that works

You do not need perfect modelling to make a good decision. You need a decision that is directionally correct and avoids the common traps.

Step 1: start with your annual usage (kWh)

Find your annual electricity use from your latest bills (ideally 12 months). If you only have monthly bills, add them up.

Also note any changes coming soon:

• EV arriving within 12 months?
• Heat pump planned?
• Work-from-home pattern changing?

Those changes can shift you from “export-heavy” to “self-use-friendly”, which changes the sizing logic.

Step 2: compare a few realistic system sizes in your postcode

Run your postcode on SolarByPostcode and test a few sizes that installers commonly quote for UK homes (for example, a “small”, “typical”, and “larger” option).

Write down, for each size:

• Estimated annual generation (kWh)
• Any assumptions you used (roof direction, east-west split, etc.)

If roof direction is a factor, see:
- Roof direction penalty (east and west vs south)

Step 3: decide what your self-use story is (without forcing it)

You do not need an exact percentage, but you do need a realistic story.

Most homes fall into one of these:

Step 4: make the trade explicit in £ terms

Once you have:

• a realistic annual generation estimate for your postcode, and
• a plausible self-use vs export split for your household,

you can think in simple value buckets:

• self-used kWh are worth roughly your import rate
• exported kWh are worth roughly your export rate

That is enough to prevent most sizing mistakes.

If you want the “sanity-check” version:

• If your next 1 kWp is mostly export, you are buying export-rate energy.
• If your next 1 kWp is mostly self-use, you are buying import-rate energy.

Common sizing traps (and how to avoid them)

Trap 1: sizing to cover winter consumption

UK winter solar is limited. If you size to cover winter electricity use, you will almost certainly overproduce in summer.

That does not automatically make it wrong, but it changes the economics. You become more dependent on export value.

If your goal is bill stability rather than fastest payback, that can be a valid choice. Just do it with eyes open.

Trap 2: comparing quotes using only kWp

Two systems with the same kWp can perform very differently depending on:

• roof direction and tilt
• shading
• inverter sizing and clipping behaviour
• layout and stringing

If you want a refresher on why peak output is not the sizing benchmark, read:
- Why your solar panels will never hit their rated output (and why that’s completely normal)

Trap 3: assuming a bigger inverter is always better

Some systems are deliberately designed with slightly more panel capacity than inverter rating. That can cause occasional “clipping” on the very best hours, but it can still increase annual kWh and improve value.

Clipping is often a design choice, not a fault.

Trap 4: ignoring standing charges when thinking about “zero bills”

Even if solar covers a large share of your usage, standing charges can keep bills from going to zero.

This varies by region. If you want to understand how much standing charges drag the baseline in your area, see:
- Standing charge drag

What installers should give you (so you can compare fairly)

Ask each installer for the same three things:

1) System size (kWp)
2) Estimated annual generation (kWh)
3) A short list of assumptions (roof direction, shading allowance, export limitation, inverter type, optimisers if any)

If two quotes differ a lot, the assumptions almost always explain why.

If you want context on why quotes vary so widely even for similar sizes, see:
- Solar panel cost in the UK: what you actually pay and why quotes vary

Postcode examples (known-good links) to explore

If you want to click through a few real pages to build intuition, these examples already exist in our cost guide:

• GL19 (Forest of Dean)
• NW6 (Brent)
• CF10 (Cardiff)
• PH15 (Perth and Kinross)
• BT92 (Fermanagh and Omagh)
• BA1 (Bath and North East Somerset)

The point is not that these pages are your house. The point is that postcode and roof realities shape what “sensible” looks like.

Bottom line: a good sizing decision is a clear trade, not a guess

If you take nothing else from this guide, take this:

• Use postcode-based kWh estimates, not UK averages.
• Make a realistic call on self-use vs export based on your household pattern.
• Use the 70/30 rule as a starting point, then adjust based on your reality.
• Do not size for winter independence unless you explicitly accept export-heavy summers.
• Choose a size where the extra generation still has a plan (self-use, battery, EV, or “export is fine for me”).

Once you have that, the “right size” usually becomes obvious.

Next reads

• How much electricity does the average UK home actually use?
• Daytime vs evening electricity use: why timing matters more than totals
• Self-consumption vs export in UK solar: how the Smart Export Guarantee changes the maths
• Solar panel cost in the UK: what you actually pay and why quotes vary
• How to compare solar quotes in the UK: a numbers-first checklist

Run it for your house

Next step: run your postcode on the calculator and compare 2 or 3 system sizes.

Run the calculator for your postcode