Annual electricity use
What annual consumption figure has each installer used? If one company is designing around 3,000 kWh and another around 8,000 kWh, their recommendations may naturally be very different.
Solar questions answered
Three solar quotes. Three completely different systems. Which one is right?
One installer recommends 10 panels. Another wants to fit 18. One says you need a small battery. Another recommends two large ones. The inverter sizes are different and the prices may be thousands of pounds apart.
Before comparing the prices, establish whether the companies are actually proposing the same solution to the same problem.
Panels
Batteries
Inverters
Installation scope
Why this happens
Solar quotations can be different because the installers may be solving different problems.
Homeowners often assume that several solar companies visiting the same house should naturally produce roughly the same recommendation.
In practice, they can make very different assumptions.
✓ Different estimates of usable roof space
✓ Different approaches to shading
✓ Different expectations of household electricity demand
✓ Different panel wattages and equipment brands
✓ Different battery-sizing philosophies
✓ Different inverter and grid-connection strategies
✓ Different assumptions about EVs, heat pumps and future demand
✓ Different installation and warranty packages
Some of those differences may be perfectly reasonable. Others deserve further questioning.
Compare these first
Seven things to put side-by-side before comparing the total price.
Solar array size
Compare both the number of panels and total installed capacity in kWp. Different panel wattages can mean the same panel count produces a different-sized array.
Generation estimate
Check how much annual generation each proposal predicts and what assumptions have been made about orientation, roof pitch and shading.
Battery capacity
Compare nominal and usable storage, but also ask why that capacity was selected. A larger battery is only useful if the household has a realistic opportunity to charge and discharge it.
Inverter specification
Look at inverter capacity, hybrid capability, MPPT requirements, roof orientations, battery compatibility and whether grid approval affects the design.
Installation scope
Check whether quotations include electrical work, monitoring, bird protection, scaffolding, certification, grid applications, equipment relocation and other installation requirements.
Warranty and aftercare
Panel, inverter, battery, workmanship and insurance-backed warranties are not automatically equivalent simply because both quotations contain the word "warranty".
Question 1
Why has one company quoted 10 panels and another 18?
Start by asking each company to explain why its panel number is right.
Possible reasons include:
✓ Different roof measurements
✓ Different panel dimensions or wattages
✓ Different treatment of shaded roof areas
✓ Different electricity-demand assumptions
✓ Different views on north-, east- or west-facing roof space
✓ Different inverter or string limitations
More panels are not automatically better.
Fewer panels are not automatically more sensible either.
The correct question is: what panel layout makes sense for this particular roof and household?
Question 2
Why has one installer quoted one battery and another quoted two?
Battery recommendations can vary enormously because battery sizing is one of the easiest parts of a solar quotation to oversimplify.
A sensible recommendation should consider:
✓ Annual electricity consumption
✓ Daytime versus evening demand
✓ Expected solar generation
✓ Overnight electricity tariffs
✓ Maximum battery charge and discharge rates
✓ Electric vehicle charging
✓ Heat-pump consumption
✓ Future changes in electricity demand
If somebody recommends 5 kWh, 10 kWh or 20 kWh of storage, ask them to explain the demand profile that justifies that number.
Question 3
Why are the inverter sizes different?
The inverter is one of the most important components in the system and yet it is often reduced to a single number on a quotation.
Different inverter choices can reflect:
✓ Total solar-array capacity
✓ Number of roof orientations
✓ MPPT and string requirements
✓ Battery charge and discharge capability
✓ Household peak demand
✓ Export limitations
✓ G98 or G99 grid-connection strategy
A smaller inverter may sometimes be entirely deliberate. A larger inverter may be justified on a higher-demand property.
What matters is whether the installer can explain the engineering reason for the choice.
Question 4
Why are the prices thousands of pounds apart?
Sometimes one company is simply more expensive.
But first make sure you are comparing the same scope.
✓ Same number and specification of panels?
✓ Same battery capacity?
✓ Same inverter capability?
✓ Same scaffolding requirements?
✓ Bird protection included?
✓ Electrical upgrades included?
✓ G99 application included where required?
✓ MCS certification included?
✓ Comparable workmanship warranty?
✓ Comparable insurance-backed protection?
Only after those differences are understood does the total quotation become genuinely comparable.
Generation figures
Check the assumptions behind the savings projection.
Two quotations can contain similar equipment and still produce very different financial projections.
Look carefully at the assumptions being used for:
✓ Annual solar generation
✓ Electricity import price
✓ Export payment
✓ Percentage of solar used in the home
✓ Battery behaviour
✓ Future electricity-price inflation
✓ Equipment degradation
A spectacular-looking payback period is only as reliable as the assumptions used to calculate it.
A real example
Sometimes the larger system genuinely is the sensible one.
In one anonymised Central Scotland assessment, the household used approximately 8,725 kWh of electricity each year and had an air-source heat pump, electric hot water and an EV.
A twenty-panel array, approximately 18.8 kWh of battery storage and a 7 kW hybrid inverter therefore had a completely different justification from the same equipment being proposed for a small, low-consumption household.
The equipment numbers only make sense when the property and electricity demand behind them are understood.
✓ 8,725 kWh annual electricity use
✓ Heat pump
✓ Electric vehicle
✓ Electric hot water
✓ 20-panel design
✓ 18.8 kWh proposed storage
✓ Larger inverter
✓ G99 consideration
Warning signs
Be cautious when the recommendation appears before the questions.
A good solar recommendation should normally require some understanding of the property and household.
Ask more questions if you see things such as:
✓ A battery size recommended before electricity use is known
✓ A panel count proposed without clear roof measurements
✓ No explanation of shading or orientation
✓ No discussion of EV or heat-pump demand
✓ No explanation of the inverter choice
✓ A large system with no mention of grid-connection requirements
✓ Savings figures with assumptions that cannot be explained
✓ Pressure to sign before differences between options are understood
None of these automatically proves a quotation is bad. They are simply good reasons to ask for a clearer explanation.
Simple comparison checklist
Put every quotation onto one sheet of paper.
Solar
Panel make, model, wattage, quantity, total kWp, orientations and predicted annual generation.
Battery
Make, model, nominal capacity, usable capacity, charge/discharge capability and number of battery units.
Inverter
Make, model, AC rating, hybrid capability, MPPT arrangement and grid-connection implications.
Installation
Scaffolding, mounting, bird protection, electrical work, equipment locations, cable routes and monitoring.
Protection
Product warranties, workmanship warranty, certification and any insurance-backed protection.
Money
Total price, finance cost if applicable, generation assumptions, export assumptions and projected payback.
The key question
Which quote best fits your house — not which salesperson sounds most convincing?
The goal is not necessarily to find the smallest system, the largest system or the cheapest system.
It is to understand what the property actually needs and then judge each proposal against that requirement.
Once you know the likely panel capacity, battery requirement, inverter needs and installation constraints, comparing quotations becomes much easier.
You are no longer comparing sales presentations.
You are comparing proposed solutions.
Still completely confused?
Get the house assessed before choosing between the quotes.
If several installers have given you substantially different recommendations, an independent assessment can help establish a sensible baseline for comparison.
That means looking at the roof, electricity demand, panel layout, battery requirements, inverter choice and practical installation issues before deciding which proposal makes the most sense.