The answer
Why is the lowest quote hard to judge?
Compare quotes only after you know they are solving the same problem. Put the design assumptions, equipment, enabling work, exclusions, expected performance, commissioning and support side by side; a lower total can hide a smaller or incomplete scope.
01 / 02
Create one comparison brief
Give suppliers the same property information and desired outcome. Ask each to state the calculations or site facts behind its design and to identify work left to others. For heat pumps this may be radiators and electrical upgrades; for solar it may be scaffold, roof condition and grid work; for cooling it may be drainage and making good.
02 / 02
Compare the ownership experience
Check warranties, who commissions and hands over the system, what monitoring is provided and who handles faults. Separate a justified alternative design from an unexplained cheaper specification. If a scope item is unknown, ask how and when it will be resolved before treating the totals as comparable.
Where this guidance comes from
Check the original guidance.
These primary sources support the technical or scheme points above. Rules can change; follow the current source before making a commitment.
Apply this to your property
Get a property-specific scope to compare.
A free survey can establish the actual building needs so proposals can be judged against the same brief.