01 / The basics
First, learn kW and kWh.
kW is power at a moment in time. kWh is energy used or generated over time. A 450 W panel is rated at 0.45 kW; ten of them make a 4.5 kWp array. If a system produces 4 kW for two hours, it has generated 8 kWh.
Maximum rated panel power under test conditions.
The peak DC rating of the complete array.
Panels do not sit at their rated output all day. Production changes with cloud, season, sun angle, temperature, roof direction and shading. The useful question is not simply “how many panels?” but “how much energy will this roof produce, and when?”
02 / Roof & sunlight
Your roof is the first piece of equipment.
South-facing roofs tend to maximise annual UK generation, but east and west roofs can be very useful because they spread production across the morning and afternoon. A complicated roof should be treated as several separate solar surfaces rather than one roof.
What should be measured?
- Usable roof area and panel dimensions
- Roof pitch and direction
- Chimneys, dormers, valleys, vents and roof edges
- Nearby buildings and trees
- Seasonal shadow paths
- Access, fixing points and roof condition
A chimney that barely matters in June can cast a much larger shadow in winter because the sun sits lower in the sky. Good solar modelling uses roof geometry, annual sun paths and shading rather than just looking at an aerial photograph.
Flat roofs can work very well.
Panels can be installed on low-angle frames or ballasted mounting systems. Flat roofs offer layout flexibility, but designers must account for wind loading, ballast weight, drainage, membrane protection and row-to-row shading.
03 / Panels & inverters
Panel wattage is only one specification.
Higher-wattage panels are not automatically better. Check physical dimensions, efficiency, degradation, temperature coefficient, product warranty and manufacturer reputation. On an awkward roof, smaller modules can sometimes fit around obstructions better and produce more total installed watts.
Cost-effective and efficient for uncomplicated, largely unshaded roofs.
Useful where panels experience different shading or orientations, often with panel-level monitoring.
Each panel operates more independently; useful for complex roofs and phased expansion.
Combines PV and battery functionality in a single architecture.
Understand inverter clipping.
A 6 kWp array paired with a 5 kW inverter is not necessarily badly designed. Panels rarely operate at nameplate power, so modest DC oversizing can improve generation in weaker conditions even if the inverter occasionally caps peak output. That capped energy is called clipping.
Ask how different roof faces are separated.
Modern inverters often have multiple MPPT inputs. East, south and west strings should be designed so different orientations do not unnecessarily drag one another down.
04 / Batteries
A battery changes when your solar is useful.
Without storage, unused daytime solar is exported and the home buys power back later. A battery can move some of that energy into the evening. Smart tariffs can also allow batteries to charge from the grid when electricity is cheaper and discharge later.
How much energy the battery can store.
How quickly it can charge or supply the house.
A large-capacity battery with a low discharge rate may still need grid support when several high-power appliances run together. Always check usable capacity, maximum charge/discharge power and round-trip efficiency.
AC-coupled vs DC-coupled
DC-coupled systems can send solar directly into a battery through a hybrid inverter. AC-coupled batteries use their own inverter and can be easier to retrofit. There is no universal winner: compatibility, efficiency, backup capability and upgrade path matter.
Backup is not automatic.
Normal grid-connected solar shuts down during a power cut for safety. If backup matters, ask whether the system can island from the grid, whether it supports whole-house or essential-load backup, and whether solar can continue charging the battery during an outage.
05 / Grid, export & MCS
Your house is connected to a wider electrical system.
Domestic solar installations have to comply with distribution-network requirements. You may hear installers discuss G98, G99 and export limits. A commonly encountered single-phase figure is 3.68 kW of inverter/export capacity, but that does not mean a house can only have 3.68 kWp of panels. Larger arrays can sometimes be installed with appropriate inverter design, approval or export limitation.
Your installer should explain the DNO pathway and handle the relevant notification or application.
MCS and export payments
For most UK householders, using an MCS-certified installer is the straightforward route. Certification is commonly relevant to Smart Export Guarantee eligibility and gives you a recognised installation standard and consumer-protection framework.
Export has value.
Surplus generation can be paid for under export tariffs. Rates vary by supplier and change over time, so a long-term financial model should not assume today’s rate will stay unchanged for decades.
06 / Economics
Ignore magical payback numbers.
A quote that says “7-year payback” is only as good as the assumptions behind it. Ask to see the model.
Battery-stored solar is not automatically worth the full import electricity price. If you could have exported that unit instead, the battery’s incremental value is closer to the difference between the import price and export price, after losses.
Use your consumption profile, not just your annual bill.
Two homes can both consume 5,000 kWh a year but have very different solar economics if one uses most energy during daylight and the other uses it in the evening. Half-hourly smart-meter data gives a much better basis for sizing.
Do not automatically undersize the array.
Panels are only part of installation cost. Scaffolding, labour, electrical work and administration are substantial fixed costs. If the roof has good additional space, the marginal cost of extra modules can be attractive.
07 / Future-proofing
Design for the house you are becoming.
Future electricity demand can change dramatically with an EV, air conditioning, heat pump, electric hot water or a larger household. A system sized only around today’s electricity bill can become undersized quickly.
Solar pairs particularly well with cooling because the hottest, sunniest periods tend to be the same periods when PV output is strongest. Heat pumps are different: their largest demand occurs in winter, when UK solar production is weakest, so summer surplus cannot simply be carried into January with an ordinary domestic battery.
Think about software too.
Modern systems increasingly coordinate panels, battery, EV charger, heat pump, smart meter and tariff. Check whether the system offers useful APIs, local control, charge scheduling, forced charge/discharge and sensible behaviour if a manufacturer cloud service is unavailable.
08 / Installer checklist
Questions worth asking before you sign.
- How many kWp of panels are proposed?
- What exact panel model and physical dimensions?
- What annual generation is predicted, and using what assumptions?
- What shading losses have been modelled?
- Can I see the roof layout?
- What inverter model and AC rating?
- Why is that inverter size appropriate?
- String inverter, optimisers or microinverters — and why?
- How many MPPTs are used and how are roof orientations separated?
- What is the battery’s usable capacity?
- What is its maximum charge and discharge power?
- What is its round-trip efficiency?
- Can it charge from the grid on a time-of-use tariff?
- Can stored electricity be exported if the tariff permits it?
- Does the system provide backup during a power cut?
- Is backup whole-house or limited to selected circuits?
- What happens to the solar array during an outage?
- What monitoring is included?
- Is panel-level monitoring available?
- Is a DNO application required and who handles it?
- Will there be an export limit?
- Is the installation MCS certified?
- What are the panel, inverter and battery warranties?
- What battery throughput/cycle limits apply to its warranty?
- What workmanship warranty is provided?
- How will the roof be fixed and weatherproofed?
- Are bird guards included or recommended?
- Can the battery or array be expanded later?
- What assumptions are used for electricity prices and export rates?
- Can I see the underlying savings calculation rather than just a payback headline?
Next step
Model the roof before choosing the hardware.
SurreySkies’ solar planner is an indicative feasibility tool for exploring roof layouts, representative system components and predicted annual generation before a qualified installer completes the final design.
Open solar planner