Start with bills, not brochure photos
Annual kWh import is a start; better is hourly-ish load shape if you can approximate. A house importing 3,200 kWh/year mostly at night behaves differently from the same total skewed to remote-work daylight plug loads.
Roof capacity in plain English
Measure usable planes minus margins, vents, velux shadows and fire access strips designers must respect. “Could we squeeze another row?” sometimes fights building regs wisdom.
Inverter clipping is not a scandal
Oversizing arrays slightly versus inverter AC ratings can harvest low-light energy while sacrificing rare peak moments — designers trade summer spikes for broader daily energy, like choosing playlist depth over one loud chorus.
Typical sizes for UK families
| Household | Common kWp target* | Notes |
|---|---|---|
| 1–2 people, modest use | 3–4 kWp | Often export-heavy |
| 3–4 people, WFH mix | 5–7 kWp | Balances self-use |
| Heavy EV + heat pump | 7–10+ kWp | Check DNO early |
*Illustrative — not a specification.
Future-proofing for EVs and heat pumps
If adoption is 18–36 months out, discuss blank conduit, spare breaker capacity and inverter prep — cheaper now than stucco surgery later. Still, do not massively oversize purely for hypothetical cars you have not ordered.
Five questions worth sounding nerdy about
Ask how they model horizon shading, whether they include loss factors for soiling in your postcode band, and how they treat inverter AC clipping in annual kWh — polish signals competence.
Request a labelled roof diagram you can keep for insurers; future you will appreciate not relying on memory when the attic hatch confuses orientation.
String design limits you cannot bully with enthusiasm
Inverters accept maximum string voltages and currents; adding “just one more panel” sometimes violates cold-morning Voc limits or lands you in MPPT mismatch country. Designers are not refusing cheerfulness — they are avoiding warranty voids and arc fault risks.
Consumption spikes you forget to mention
Hot tubs, second freezers, grow lights (legal ones!), hobby workshops — each can quietly add hundreds of daytime kWh. Walk the house with an installer noting every hungry appliance; embarrassment now beats under-sizing regret later.
Export headroom and DNO paperwork
Some regions gate export upgrades behind applications with timelines measured in weeks. If you chase 8–10 kWp dreams, confirm the connection offer before fantasising about payback — paperwork denial rearranges budgets faster than shade does.
Oversizing arrays vs oversizing hope
Slight DC/AC oversizing can harvest diffuse light efficiently, but dumping ten extra panels into permanent shade buys ornaments, not electrons. Marginal panels need marginal irradiance — insist designers colour roof maps honestly, not cosmetically.
Future EV plans matter, but so does garage feasibility — blank conduit today beats tearing rendering off later.
Load shifting without buying a spaceship
Timers on washers, delay-start dishwashers and lunchtime laptop chargers increase useful self-consumption without new gadgets — free kWh harvested behaviourally beat expensive optimism about “AI will learn my soul.” Try two habit tweaks before upsizing inverter dreams.
Sizing mistakes we see in real kitchens
Oversizing purely because a chatbot said “max everything” ignores shade, export limits and inverter clipping laws — physics refuses SEO. Undersizing because horror headlines scared you ignores that panels now cost less per watt than a decade ago and labour often dominates — wasting viable roof can be false thrift.
Bring heat pump and EV plans, but label them “likely / unlikely” honestly — designers can stage inverter capacity without installing ghost panels for cars you will never buy.
Ask how many kWh per year they expect versus your historical annual import — if the ratio looks nonsense relative to orientation, slow down until the model explains itself in words you could repeat to a sceptical cousin.
Commercial install envy — resist politely
Warehouses host acres; your semi hosts grief from hips — do not let LinkedIn solar brags dictate domestic sizing. Engineers should defend modest, effective designs against machismo.
Historic homes with delicate slates may cap sensible capacity — honour fabric; silicon worship should not trash vernacular roofs.
Post-install optimisation without new panels
Timers and habit shifts sometimes recover more kWh than speculative fourth strings — try operational tweaks for one billing year before assuming hardware fault.
Carrying capacity of your patience for scaffold
Larger arrays sometimes imply longer roof days — confirm how many weather reserve blocks crews keep before you book annual leave around magic dates.
Ask whether scaffolding includes edge protection expectations for insurers — paperwork loves explicit photos.
If you plan Velux additions later, mention now — rail layouts hate surprise skylights aesthetically and electrically.