Take this to every appointment. The value is not in any single question, it is in asking the same ones of each installer so you can see where the answers diverge.
About the company
- Are you certified under the Microgeneration Certification Scheme (MCS), and which technologies does the certificate cover?
- Do you belong to a consumer code, and which one? What deposit protection does it give me, and what happens to my deposit if you cease trading?
- Who will actually carry out the work, and are they employed or subcontracted?
- How long have you been installing, and roughly how many systems a year?
- What insurance do you carry?
- Can I see a recent installation nearby, or speak to a recent customer?
About the system
- What is the system size in kW? Not the number of panels.
- Which panels, which inverter, and why those?
- What is the estimated annual generation in kWh, and how did you work it out for my roof?
- What self-consumption percentage have you assumed, and why?
- Is this a G98 or G99 connection? If G99, has the application gone in and how long will it take?
- Is export limiting being used, and at what level?
- What happens if the network operator restricts the system after I have signed?
About my roof
- Have you been up on the roof or assessed it in person?
- What shading have you identified, and how does the design handle it?
- Are optimisers or microinverters needed here, and what do they add?
- Is the roof in good enough condition, and did you check the timbers?
- How will the roof be made watertight, and what warranty covers that work?
- How many roof faces are being used, and does that change the equipment?
About a battery, if you are having one
- What is the usable capacity in kWh, as opposed to the headline figure?
- What is the power output in kW?
- Where exactly will it go, and how does that comply with BS 7671 Amendment 4 and PAS 63100?
- Can it charge from the grid on a schedule, for a time-of-use tariff?
- Who sets that up, and what happens if I change tariff later?
- Will it provide backup in a power cut? If not, what would that cost?
About the price
- What is the cash price, including everything?
- What exactly is included: scaffolding, certification, network notification, waste removal, making good?
- What is not included?
- What deposit do you require, and is it protected?
- What would the next size up cost, and what would it add in generation?
About warranties
- What is the panel warranty, and does it cover performance as well as product?
- What is the inverter warranty, and what does replacing one cost?
- What is the battery warranty, and is it stated in years, cycles or throughput?
- What is your workmanship warranty, and what does it cover?
- What happens to these warranties if your company ceases trading?
That last one is not rude. It is the single most useful question on this list, because a twenty-five year warranty from a firm with no insurance-backed guarantee is worth whatever the firm is worth in twenty-five years.
About the work itself
- How long will the installation take, and how many days will scaffolding be up?
- Will the power be off, and for how long?
- Who notifies the network operator, and when?
- When will I get the MCS certificate, and what do I need it for?
- What do I need to do to start being paid for exports?
- Is there a handover, and will somebody show me how the monitoring works?
Comparing the answers
Put your quotes side by side on these five things and the picture usually becomes obvious:
- System size in kW and estimated generation in kWh
- What is included, particularly scaffolding and certification
- The grid connection route, G98 or G99
- Warranty terms, all four of them
- The assumed self-consumption behind any savings figure
If one quote is much cheaper, work out which of those five is different. There is almost always a reason, and it is worth knowing what it is before you decide whether it matters.