Costs
Solar Savings Calculator
This page does not produce a savings figure, and it explains why. What it does instead is set out what a calculator can honestly tell you, what only a survey can establish, and exactly which inputs a genuine estimate for your property would need.
- No figure without a survey
- Assumptions made visible
- Routes to a real design
Most solar calculators ask for a postcode, a rough monthly bill and a roof direction, and return a savings figure to the nearest pound. The arithmetic behind them is usually sound. The problem is what has been assumed in order to get there: an annual usage figure, a self-consumption rate, a tariff, an export rate, a shading allowance, a usable roof area and a view about future energy prices. Change any one and the answer changes, often substantially — and the person reading the result has no way of knowing which assumption is doing the work.
We would rather be useful than confident. Below is what a calculator genuinely can and cannot do, the list of inputs a real estimate needs, a check of how many of them you already have to hand, and what a survey has to establish before any figure means anything.
What a calculator can and cannot tell you
| A calculator can | A calculator cannot |
|---|---|
| Compare two system sizes on the same assumptions | Tell you what your roof will actually generate |
| Show how sensitive a result is to the self-consumption rate | Know what shades your roof at four o clock in February |
| Illustrate the difference between avoided import and export income | Know your tariff, your export rate or your occupancy pattern |
| Show the direction and rough scale of an effect | Establish whether your roof structure can carry an array |
| Frame a conversation before a survey | Tell you what the installation will cost on your property |
| Make its assumptions visible so they can be argued with | Replace a survey, a design, or a written quotation |
The inputs a genuine estimate needs
Nine inputs. Most estimates you will be offered online use assumed values for at least six of them without saying so.
| Input | What it has to be, to be worth using |
|---|---|
| Twelve months of electricity usage in kWh | From bills or a smart meter data export, not an estimate from memory. |
| Your current import unit rate and tariff structure | Flat rate, day and night, or time-of-use, with the periods and rates. |
| Your export rate, or the SEG offers available to you | Set by the supplier, not by Ofgem, and subject to its own contract terms. |
| When your electricity is actually used | Which loads run when, and which of them could move into daylight hours. |
| Roof orientation, pitch and usable area | Measured rather than estimated, and per roof face where the array is split. |
| What shades the roof, and when | Trees, chimneys, flues, dormers, aerials and neighbouring buildings, by time of day and season. |
| A proposed system specification | Array size in kWp, panel and inverter specification, and usable battery capacity if storage is included. |
| A fixed written quotation for that specification | Including scaffolding, electrical work and everything else in scope. |
| The assumptions to be used about the future | Whether energy prices are held flat or assumed to rise, and how array degradation is treated. |
Check how ready you are for a real estimate
Tick the inputs you already have. This produces no savings figure, no payback period and no generation figure — it simply tells you which of the nine inputs are still missing and where each one usually comes from. Nothing is sent anywhere and nothing is saved.
What only a survey can establish
These are the items that decide whether a system is suitable, how large it can be and what it will cost. None of them can be obtained from a form, and every one of them can change an estimate materially.
| What is established | Why an estimate depends on it |
|---|---|
| Roof structure and condition | Rafter centres and depth, the state of the covering, and whether any remedial work is needed before an array is fixed above it. |
| Usable array area | Measured properly, allowing for edge zones, obstructions, vents, flues and access requirements — usually less than the roof looks from the ground. |
| Real shading | Recorded on site across the day, rather than inferred from an aerial photograph taken at one moment in one season. |
| Electrical supply arrangements | Consumer unit condition and spare capacity, main earthing and bonding, and the suitability of the incoming supply. |
| Equipment positions and cable routes | Where the inverter and any battery can go, and how far the cabling has to travel to get there. |
| Access and scaffolding | Elevations, height, ground conditions, parking and any permissions the scaffold needs. |
Our page on what happens during a solar survey sets out the process in full, and how Solar Fx works shows where the survey sits between the first conversation and the fixed written quotation.
What we do instead
We produce a remote design first, from mapping and aerial imagery, and we label it as indicative because that is what it is: a desk exercise that cannot see your rafters, your consumer unit or the tree that has grown since the photograph was taken. If the property looks suitable and you want to proceed, someone attends and surveys it properly. The design is then rebuilt on measured information, and the written quotation follows.
Where we provide an estimate of generation or savings at that point, it states the assumptions on the face of it, separates avoided import from export income, and shows solar-only and solar-plus-battery as two cases rather than one. That is the level of detail at which a figure is worth having. Everything before it is conversation, and we are happy to have the conversation without dressing it up as a calculation.
VAT, and where it sits in a savings calculation
Any savings or payback estimate has an installed cost on one side of it, and the VAT position is part of that cost. Installation of qualifying energy-saving materials — including solar panels, and electrical storage batteries since 1 February 2024 — in residential accommodation in Great Britain is zero-rated for VAT from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027. An estimate produced for work completed either side of that date uses a different cost figure, so it is worth knowing which applies. The detail is in HMRC VAT Notice 708/6, cited below, and in our advice article on VAT on solar panels.
About any figure produced by any calculator
Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.
This page publishes no savings figure, payback period or generation figure. Illustrative figures published by independent bodies are industry figures produced on their own stated assumptions; they are not Solar Fx figures and they are not an estimate for your property.
Common questions
Why does this page not calculate a savings figure?
Because the inputs that decide the answer are not things a web form can obtain. A savings figure depends on your annual usage, your tariff structure, your export rate, when in the day you use electricity, the measured orientation, pitch and usable area of your roof, and what shades it at different times of year. A form that asks for a postcode and a rough bill and returns a confident number has substituted assumptions for all of that, and it does not tell you which ones.
Are online solar calculators useless, then?
No. A calculator that states its assumptions is a useful way to see how sensitive an answer is to a change in one of them, and to compare two system sizes on identical terms. The problem is not calculation, it is calculation presented as measurement. Use one to understand the shape of the answer, and a survey to find out what applies to your property.
What is the single input that changes the answer most?
The self-consumption rate — the share of generation used in the property rather than exported. Because avoided import is normally worth more per unit than export income, the same array can produce quite different annual benefit in two households with different occupancy patterns. It is also the input a calculator is least able to guess, because it depends on when people are home and when appliances run.
Can you give me a figure over the telephone?
We can talk through what is realistic on a roof of your type and what the main variables would be, and we will be straightforward about how wide the range is at that stage. What we will not do is give a confident savings or payback figure before the property has been surveyed and a design produced, because a figure given at that stage would be a guess wearing the clothes of a quotation.
What do I get instead of a calculator result?
An indicative proposal after a remote design, labelled as indicative, followed by a survey and then a written design and fixed quotation. Where we provide an estimate of generation or savings, it states the assumptions used, separates avoided import from export income, and shows the solar-only and solar-plus-battery cases separately.
Read next
Where the numbers actually come from, and what has to be established first.
- Solar panel paybackThe calculation itself, every assumption behind it, and savings against export income.
- Solar panel costEvery factor on the cost side, from array size and scaffolding to electrical work and access.
- Battery storage costWhat moves the price of storage, and why its value depends on cycling and on your tariff.
- How many solar panels do I need?Sizing an array against roof area and household usage rather than against a budget.
- Roof orientation and shadingHow direction, pitch and shadow change output and the shape of the generation day.
- Find the right system for your propertyTwelve questions about the property, and guidance on what to read and what a survey would check.
- What happens during a solar surveyWhat is measured and inspected before a design or a fixed price can be produced.
- Solar panel installationThe service itself: what is included, what is not, and how the quotation is built.
Important: Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.
Sources
- Solar panels — advice — Energy Saving Trust. Accessed 2026-08-23.
- Smart Export Guarantee (SEG) — Ofgem. Accessed 2026-08-23.
- Smart Export Guarantee — Consumers — MCS. Accessed 2026-08-23.
- Energy-saving materials and heating equipment (VAT Notice 708/6) — HM Revenue & Customs (GOV.UK). Accessed 2026-08-23.
Information reviewed on 2026-08-23.
Swap the calculator for a survey
Send us the postcode, the property and your usage. We will design it, survey it and put the assumptions in writing alongside any figure we give you.
Ask for a design and a written estimate
No obligation. We will review the details, come back to arrange the next step, and issue a written design and fixed quotation before anything is agreed.
Prefer to talk? Call 0118 227 7444.