Zero inflation
This is a useful stress test because it assumes the avoided electricity price does not increase merely to improve the solar return.
Solar savings and ROI explained
Why are solar payback figures so different?
One installer says the system pays for itself in seven years. Another says twelve. A third shows huge savings over 25 years.
The solar panels may be almost identical. The difference is often hidden in the assumptions behind the calculation.
The short answer
Two installers can both use correct arithmetic and still produce completely different answers.
Solar payback is calculated from a collection of estimates and assumptions.
These include:
how much electricity the panels are expected to generate;
how much of that generation the household will use itself;
the electricity import price being avoided;
the export tariff received for surplus electricity;
how a battery will operate;
future electricity-price assumptions;
panel and battery degradation;
future replacement costs;
and whether the system is paid for with cash or finance.
A payback figure without its assumptions is almost meaningless.
Ask to see the inputs as well as the result.
What does “payback” actually mean?
It is the point where estimated accumulated benefit catches the cost.
Imagine a system costs £10,000.
If it produced exactly £1,000 of financial benefit every year and absolutely nothing changed, a simple payback calculation would be:
£10,000 ÷ £1,000 per year = 10 years
Real life is more complicated.
Generation changes slightly over time. Electricity prices change. Export tariffs change. A battery may degrade. Equipment may need replacement.
A proper long-term projection therefore calculates the estimated position year by year rather than assuming every year is identical.
Before the money
First establish how much electricity the solar system should actually generate.
The current MCS standard solar performance calculation starts with physical facts about the installation.
kWp
Installed peak capacity of the solar array.
Kk
Location, orientation and pitch-specific irradiation figure.
SF
Shading factor applied to the array.
Estimated annual AC output = array kWp × Kk × shading factor
The calculation therefore takes account of:
postcode region;
roof orientation;
roof pitch;
installed panel capacity;
and shading.
If two quotes predict very different annual generation from the same roof, ask why.
A difference may be legitimate, but you should be able to see whether it came from panel capacity, orientation, shading or an alternative calculation methodology.
An independent benchmark
There is no single “Scottish solar payback period”.
Energy Saving Trust's July 2026 consumer figures illustrate the point.
For its modelled Stirling example with export payments, the stated solar payback is approximately:
Around 11–12 years depending on household occupancy.
That is not a prediction for your particular house.
Different system cost, electricity use, tariff, roof and battery arrangements can produce a very different answer.
It is useful mainly as a reminder that an extremely short payback deserves explanation rather than automatic belief.
Worked example
Watch what happens when we change only one assumption.
Illustrative £10,000 solar system
£10,000
System cost
4,000 kWh
Annual generation
60%
Solar electricity used in the home
40%
Exported electricity
Assume imported electricity costs 25p/kWh and exported electricity earns 15p/kWh.
2,400 kWh self-used × 25p = £600 saved
1,600 kWh exported × 15p = £240 earned
Total illustrative first-year benefit = £840
Simple payback = £10,000 ÷ £840 ≈ 11.9 years
This intentionally simplified example ignores degradation, tariff changes, inflation, maintenance and replacement costs so that the effect of changing individual assumptions is easy to see.
Same panels — different assumptions
Now watch the payback move.
| Illustrative scenario | First-year benefit | Simple payback |
|---|---|---|
|
Base case 60% self-use, 25p import, 15p export |
£840 | About 11.9 years |
|
Higher self-consumption 80% self-use, same tariffs |
£920 | About 10.9 years |
|
Lower export tariff 60% self-use, 25p import, 5p export |
£680 | About 14.7 years |
|
Higher avoided import price 60% self-use, 35p import, 15p export |
£1,080 | About 9.3 years |
Nothing happened to the panels.
We changed the financial assumptions and moved the simple payback from roughly nine years to almost fifteen.
Self-consumption
How much solar electricity will the household actually use?
This is one of the most important assumptions in the whole calculation.
A solar kWh consumed inside the home avoids buying a kWh from the electricity supplier.
A solar kWh exported instead earns the applicable export rate.
If import electricity is worth more than export electricity,
self-consumption generally has greater financial value.
But the household cannot simply claim that every generated unit will be self-consumed.
Panels generate electricity whether the homeowner is using it or not.
EPVS protection
There are limits on how aggressively self-consumption can be presented.
EPVS's published Solar PV Validation Guide limits the main self-consumption savings presentation against the customer's current annual grid consumption.
The published limits are:
75%
Maximum of current annual grid consumption used for the main
solar-only savings presentation.
90%
Maximum used for the main solar-plus-battery savings presentation.
Bill
Current annual consumption and electricity price should normally
be evidenced from recent customer information.
Future EVs, electric heating or other new loads can be shown as additional scenarios.
But future demand should not quietly be treated as though the customer already uses it.
The main projection should remain grounded in the household's current consumption.
This is why the electricity bill matters
If the installer does not know your consumption, what exactly are they claiming to save?
A savings calculation needs a credible starting point.
EPVS guidance expects evidence of:
recent annual electricity consumption;
the customer's current electricity unit rate;
and the relevant tariff structure.
If a bill genuinely cannot be obtained after reasonable attempts, the published guide allows documented reasonable assumptions.
A made-up annual consumption can create a made-up saving.
That is why good installers ask for the electricity bill even when it feels inconvenient.
Export tariffs
A generous export assumption can make an average proposal look spectacular.
Smart Export Guarantee rates vary significantly between suppliers.
Some tariffs also have:
supplier eligibility requirements;
technology requirements;
import-tariff conditions;
time windows;
or promotional terms.
EPVS therefore restricts the export rate used in its approved savings presentation.
The published EPVS methodology says the assumed export rate should be generally available from at least two SEG providers.
It should not rely on one unusually high, short-lived promotional offer simply to improve the payback illustration.
Batteries
A battery can improve the savings — while simultaneously making the system more expensive.
Batteries can increase self-consumption by moving solar electricity from daytime into evening and night.
They can also charge cheaply from the grid on suitable time-of-use tariffs.
But the battery has a purchase cost.
So there are two separate questions:
Does the battery reduce the electricity bill?
Does it reduce the bill enough to recover the additional cost of the battery?
Those are not the same question.
A battery can save money every year and still lengthen the total system payback if its additional purchase cost is greater than the additional savings it creates.
Battery assumptions
Be careful when overnight tariff savings appear in the projection.
A projection that includes cheap overnight battery charging needs to reflect what the actual equipment can do.
How much usable battery capacity is available?
How quickly can the inverter charge the battery?
How quickly can it discharge?
How long is the cheap tariff window?
What are the round-trip losses?
How much household demand exists during the expensive period?
An impressive savings figure built around a charging strategy that the actual battery cannot physically complete is not useful.
Electricity-price inflation
This is where long-term savings numbers can become enormous.
If electricity becomes more expensive in future, every solar kWh the household avoids buying becomes financially more valuable.
Compound that increase over 20 or 25 years and the projected lifetime savings can rise dramatically.
But nobody knows the future electricity price.
Inflation assumptions are scenarios — not promises.
The 7.04% question
Does EPVS say electricity prices will rise by 7.04% every year?
No.
This point is easy to misunderstand.
EPVS's published validation guide says the level of future fuel inflation is impossible to predict.
The guide contains an illustrative high inflation scenario of 7.04%, based on historical ONS RPI statistics used for that example.
It also shows lower scenarios.
7.04% is not a guaranteed future electricity-price forecast.
It demonstrates what the long-term result could look like under a high historical inflation assumption.
Why several scenarios are better
Do not ask which inflation prediction is “correct”. Ask what happens under several possibilities.
Moderate inflation
Shows the potential effect of electricity becoming progressively more expensive over time.
Higher historical scenario
Demonstrates the much larger financial value solar could create if electricity prices rise strongly over the long term.
Compare all of them
A customer can then see how much of the claimed long-term return depends on the inflation assumption.
A strong solar case should not need one optimistic inflation number hidden in the small print.
Show the customer what changes when the assumption changes.
Degradation
Panels do not normally produce exactly the same annual output forever.
Solar-module performance gradually changes over time.
Battery capacity can also decline through age and cycling.
EPVS guidance says extended projections should account for:
solar panel degradation;
battery degradation where applicable;
and appropriate replacement costs.
The exact values should reflect the products and assumptions used in the proposal.
Replacement costs
A 25-year projection should not pretend nothing ever needs replacing.
Solar panels can have very long operating lives.
Other components may not necessarily last as long as the modules.
Depending on the warranties, technology and projection period, allowance may be needed for future equipment replacement.
Lifetime benefit should mean net benefit after relevant costs.
A projection that counts every saving but ignores every future expense will naturally look better.
Cash versus finance
Two identical solar systems can have different financial payback because they were bought differently.
Imagine the cash price is £10,000.
If finance interest means the customer ultimately repays £13,000, calculating the return against only £10,000 would make the financed purchase look artificially attractive.
The customer's real cost is what the customer actually has to pay.
EPVS's published guidance therefore says consumer-finance projections should display the total contract value including interest.
Same first-year saving, different purchase method
Finance can move the apparent payback significantly.
| Illustrative purchase | Total cost used | £840 annual benefit |
|---|---|---|
| Cash | £10,000 | Simple payback about 11.9 years |
| Finance | £13,000 total repayment | Simple equivalent about 15.5 years |
Simple illustration only. Real finance repayments and system benefits occur at different times, so a proper projection should model the actual payment schedule.
EVs and heat pumps
Future electrification can make today's solar system much more valuable.
A household that currently uses 3,000 kWh per year might later add:
an electric vehicle;
a heat pump;
electric hot water;
or other substantial electrical loads.
That can significantly increase the amount of solar electricity the household can use.
It may therefore justify designing a larger system now.
Future usage should still be labelled as future usage.
A proposal should distinguish clearly between the customer's current consumption and an additional scenario based on planned electrification.
EPVS
This is exactly the problem independent validation is designed to reduce.
EPVS is an additional independent certification and validation scheme used by participating installers.
It is not a legal requirement that every solar installer in Britain must belong to EPVS.
But where an installer is an EPVS member, the scheme is intended to provide greater confidence that the performance and financial calculations have been produced using approved methods and appropriate evidence.
EPVS currently requires approved calculation tools or an approved methodology for its member installations.
Validation does not guarantee the future.
It helps establish that the figures shown at the point of sale were based on recognised methods rather than simply invented to make the proposal look attractive.
What EPVS actually checks
The calculation should connect back to evidence from the property and the contract.
The published EPVS solar validation process includes information such as:
system size and panel wattage;
orientation;
roof pitch;
shading;
solar generation calculations;
electricity usage and unit rate;
self-consumption;
export assumptions;
savings projections;
system price;
and finance information where applicable.
If the site survey later discovers that important design information differs from what was originally assumed, the performance estimate should be revised.
Ten numbers to ask for
Put competing quotes on the same footing before comparing their payback.
1. What is the total system price?
Use the cash price or, where financed, understand the total repayment including interest.
Use the cash price or, where financed, understand the total repayment including interest.
2. What annual electricity consumption have you used?
It should relate to the household's actual bill unless an assumption has been clearly identified.
It should relate to the household's actual bill unless an assumption has been clearly identified.
3. What import electricity rate have you used?
A high import price makes every self-consumed solar unit appear more valuable.
A high import price makes every self-consumed solar unit appear more valuable.
4. What export tariff have you used?
Check that it is realistically available rather than a temporary outlier.
Check that it is realistically available rather than a temporary outlier.
5. What annual solar generation are you predicting?
Ask how orientation, pitch, postcode and shading produced that figure.
Ask how orientation, pitch, postcode and shading produced that figure.
6. What percentage of generation is assumed to be self-consumed?
This can dramatically affect the result.
This can dramatically affect the result.
7. What exactly is the battery assumed to do?
Check solar storage, overnight charging, usable capacity and charge/discharge power.
Check solar storage, overnight charging, usable capacity and charge/discharge power.
8. What degradation and replacement costs are included?
A 20- or 25-year projection should explain them.
A 20- or 25-year projection should explain them.
9. What inflation assumption has been used?
Ask to see the zero-inflation result as well as higher scenarios.
Ask to see the zero-inflation result as well as higher scenarios.
10. Which figures describe the house today and which describe future EV or heat-pump use?
Current and future demand should not be mixed without explanation.
Current and future demand should not be mixed without explanation.
Red flags
I would ask more questions if a proposal does any of these.
No electricity bill
A very precise savings claim has been produced without establishing what the household currently consumes or pays.
One huge lifetime number
Twenty-five-year savings are displayed prominently without showing how sensitive they are to inflation assumptions.
Unusually high export rate
The calculation relies on an exceptional tariff without explaining its eligibility or whether it is likely to continue.
Almost 100% self-consumption
The proposal assumes nearly every solar unit saves the full import rate without demonstrating how the home will actually use it.
Finance interest disappears
Payback is calculated against the cash price even though the financed customer will repay considerably more.
No explanation of replacement costs
A decades-long projection assumes every component works forever without discussing warranties, degradation or replacement.
Payback versus lifetime saving
These figures answer different questions.
Payback period asks:
“When does the estimated accumulated benefit equal what I paid?”
Lifetime benefit asks:
“What might the total financial benefit be over the full projection period?”
Return on investment may express the gain relative to the capital invested.
They are related, but they are not interchangeable.
A £50,000 “25-year saving” does not mean the customer receives £50,000 today.
It is an accumulated future projection built from assumptions that may change considerably during those 25 years.
What should a customer believe?
Believe the calculation you can interrogate.
The most attractive payback figure is not necessarily the most honest.
The longest payback is not necessarily the most conservative either.
What matters is whether you can trace the result back to evidence.
Measured or properly assessed roof information
Recognised generation methodology
Real household consumption
Realistic tariffs
Defensible self-consumption
Transparent battery behaviour
Visible inflation scenarios
Real finance cost
Degradation and replacement assumptions
The strongest quote is not the one with the shortest payback.
It is the one that can show you how the answer was produced.
Frequently asked questions
Solar payback and ROI answered.
Why do solar installers give different payback periods?
Different assumptions about generation, self-consumption, electricity prices, export tariffs, batteries, inflation, degradation, replacement costs and finance can all materially change the result.
What does solar payback actually mean?
It usually means the point where the estimated accumulated savings and income equal the amount paid for the system.
How does MCS calculate expected solar generation?
The standard method uses installed kWp, postcode-specific solar irradiation, roof orientation, pitch and shading to estimate annual AC output.
Why does self-consumption matter?
Electricity used inside the home avoids the import price, while exported electricity earns the export tariff. Those two values can be very different.
Can an installer use any export rate?
EPVS's published validation methodology says the main export rate used should be generally available from at least two SEG providers rather than relying on one unusual promotional tariff.
Does EPVS predict 7.04% electricity-price inflation?
No. Its published guide says future inflation is impossible to predict and uses several illustrative scenarios. The 7.04% example is based on historical ONS RPI data rather than being a guaranteed future forecast.
Should I ask for the zero-inflation figure?
Yes. It provides a useful comparison showing the estimated economics without depending on future electricity-price inflation.
Does finance affect solar payback?
Yes. Interest increases the amount ultimately paid. EPVS guidance says financed projections should include the total contract value including interest.
Should degradation and replacement costs be included?
Long-term projections should take relevant degradation and appropriate replacement costs into account rather than assuming every component remains unchanged indefinitely.
Does adding a battery always improve payback?
No. It may improve annual savings while increasing the initial investment. Whether it improves overall payback depends on the additional savings relative to the additional cost.
Can future EV or heat-pump use be included?
It can be shown as an additional scenario. The proposal should clearly distinguish future planned demand from the household's current consumption.
How should I compare two payback illustrations?
Ask both companies to show the same underlying assumptions: purchase cost, electricity usage, import and export rates, generation, self-consumption, battery behaviour, degradation, replacement costs, finance and inflation.
Official guidance checked
A financial projection should be transparent enough to challenge.
This guide was reviewed against current material available in August 2026 including:
- MCS — MIS 3002 Solar PV Systems, Issue 6.0, 2026.
- MCS — domestic solar PV self-consumption guidance.
- EPVS — Solar PV Validation Guide.
- EPVS — current membership and approved calculation requirements.
- EPVS — homeowner validation and consumer-protection guidance.
- Energy Saving Trust — Solar Panels: Costs, Savings and Benefits.
- Energy Saving Trust — Smart Export Guarantee and battery guidance.
Solar performance and future energy prices cannot be predicted with certainty. Payback and lifetime savings are projections rather than guarantees. The customer's actual property, consumption, tariffs, system performance and future behaviour determine the eventual result.
Related Tom Solar guides
Understand the assumptions behind the quote.
Tom Solar view
A payback calculation should reduce uncertainty — not disguise it.
Nobody can tell a homeowner exactly what electricity will cost in 2040.
Nobody can guarantee exactly how the household will use electricity for the next 25 years either.
What a competent proposal can do is show:
what we know;
what we have calculated;
what we have assumed;
and what happens if those assumptions change.
Do not choose the installer who promises the fastest payback.
Choose the proposal whose figures make sense when you look behind them.