Almost every solar conversation starts in the wrong place. Someone measures their roof, works out that twelve panels will fit, and asks what that will save. The right question comes first: how much electricity does the house actually use, and when does it use it.
A 4 kW array on a house that consumes 1,800 kWh a year produces far more than it can use, and most of that surplus is exported at a rate well below what the same unit costs to buy. A 2.5 kW array on a house that consumes 6,000 kWh will save more money per unit installed. Roof area is a constraint on the size, not a target.
So the takeoff runs in this order: consumption, usable yield, array size, then components. Everything else follows from those four numbers.
Read the consumption properly
Take twelve months of bills if you have them, and note two things: total annual consumption in kWh, and the shape of daily use.
The shape matters because solar produces in the middle of the day. A household that is empty from nine to five and cooks in the evening will use perhaps 30 percent of its own generation without a battery. A household with someone at home, or with a heat pump running in daylight, will use 50 to 70 percent.
If you have no history, estimate from the appliances rather than guessing. A fridge and freezer is 500 to 800 kWh a year. Lighting is 300 to 800. A washing machine and dishwasher used daily is 500 to 900. An electric water heater is the largest single item in many homes at 1,200 to 2,500. A heat pump is 2,000 to 4,000. An electric vehicle at 10,000 km a year is roughly 1,700 kWh.
Add them, then apply judgement. Two similar houses can differ by a factor of three.
Yield per installed kilowatt
A well oriented array in a temperate climate produces somewhere between 850 and 1,050 kWh for every kW of installed capacity in a year. In a sunny climate it is 1,400 to 1,700. In a cloudy northern location it can be 750.
That number is called the specific yield, and it is the one figure to get from a local source rather than from a book, because it varies more between locations than any other number in the calculation.
Estimate the array like this: annual consumption divided by specific yield, multiplied by the fraction of generation the house can use without export. A house using 4,200 kWh with a specific yield of 950 and a self consumption of 45 percent needs about 4,200 / 950 = 4.4 kW to cover its total consumption, and it will still import in winter and export in summer.
Orientation and pitch, and the derating table
A roof facing the equator is the ideal. Anything else loses output, and the loss is steeper than most people expect:
Orientation away from ideal
Output retained
|---|---|
0° (ideal)
100%
30° east or west
95 to 97%
60°
88 to 92%
90°
78 to 84%
Facing away from the equator
55 to 65%
Pitch matters less than orientation. On a 30 to 40 degree roof you are near the optimum for annual yield in most temperate climates. A flat roof needs tilted frames, which add cost and spacing, because rows must be spaced to avoid shading each other.
Shading is the killer. A chimney that shades one panel in a string can cut the output of that string by far more than the shaded fraction, because panels in series behave like a chain. Do a shade survey at the times of year that matter and price the optimisers or micro inverters if shading is unavoidable.
Panel sizing and roof area
Modern domestic panels are 380 to 450 W each, and roughly 1.7 to 1.9 m² in area. That works out at around 5 m² of panel per kW, before spacing and access.
A 4 kW array is therefore about 20 m² of panel, plus walkways, plus the spacing needed to keep panels out of each other's shadow on a flat roof. On a pitched roof with a clear slope, 20 m² of panel area is a fair assumption. On a flat roof with tilted rows, allow 8 to 10 m² of roof per kW.
Count panels, not kilowatts, when you order. Panel count sets the number of mounting hooks, the rail length, the number of clamps and the DC cable length, and all four are quantities.
Inverter, battery and the balance of system
The inverter converts DC to AC, and it is usually rated slightly below the array because a 4.4 kW array rarely produces 4.4 kW at once. A DC to AC ratio of 1.1 to 1.3 is normal, so a 4 kW inverter for a 4.4 kW array is a sensible pairing.
A battery is sized on evening and night consumption, not on array size. A household using 6 kWh between sunset and sunrise needs 6 kWh of usable storage, which with a depth of discharge of 90 percent means a nominal 7 kWh battery. Sizing a battery larger than the evening load simply leaves capacity unused.
The rest of the system is where the line items hide:
Mounting rails and hooks, by panel count and rail length
Clamps, mid and end, by panel count
DC isolator, AC isolator, and a generation meter
DC cable, in metres, from the array to the inverter
AC cable from the inverter to the consumer unit
Earthing and bonding
Cable tray or conduit on the roof, if specified
Surge protection, increasingly required
A common oversight is the cable route. A 15 m run of DC cable from a roof to a garage inverter is 30 m of positive and negative, and it needs to be sized for voltage drop, which can push it up a size.
A worked example
A family house, 4,500 kWh a year, roof facing south at 35 degrees, no shading, 4.2 kW planned.
Specific yield locally: 950 kWh per kW, so gross generation is 4.2 x 950 = 3,990 kWh
Self consumption at 45 percent: 1,795 kWh used on site, 2,195 kWh exported
Inverter: 3.6 to 4.0 kW, one unit
Panels: 10 panels of 420 W, each about 1.8 m², so 18 m² of panel area
Mounting: 20 hooks, 4 rails of 4.3 m, 18 mid and end clamps
DC cable: 30 m of 6 mm², two runs
AC cable: 12 m of 6 mm² from inverter to consumer unit
Isolators: two, plus a generation meter
Battery: optional, 7 kWh nominal for an evening load of 6 kWh
That is a complete order at around 4 kW. Add 10 percent for cable, connectors and the small parts that never appear on a plan.
What the numbers do not tell you
Payback is the figure everyone asks for and the one least worth quoting precisely. It depends on the export tariff, the import price, how much the household shifts its use, and whether the roof will need attention in the next twenty years.
What is worth doing is showing the three cases: no battery, battery, and battery with load shifting. On most houses the third saves more than the second, because using your own generation at seven in the evening beats exporting it at midday.
Also worth saying plainly: a solar array is a twenty five year commitment on a roof. If the roof covering has less than ten years of life left, the sensible sequence is roof first, then panels. Lifting panels to replace a roof costs almost as much as installing them.
Permissions, connection and the paperwork
Three items that are not materials and still belong in the estimate:
Grid connection application and any fee, plus the waiting time
Building or electrical notification, where the jurisdiction requires it
Structural check if the roof is old or the array is heavy, which on a tiled roof with a battery is worth doing
In some markets the connection process alone determines whether the project happens this year or next, so check it before ordering equipment.
The roof is a structure, not just an area
Before panel count is fixed, somebody has to look at what holds the roof up. A tiled roof designed for tiles and a bit of snow has spare capacity for a typical domestic array. A lightweight industrial roof, a fifty year old truss or a flat roof with a timber deck may not.
The load is not huge but it is concentrated. A panel and its mounting is 12 to 15 kg per square metre, and on a pitched roof the hooks transfer that load into rafters at about 1.2 m intervals. That is usually fine. What causes problems is a roof with an existing defect, a rotten purlin, or an array mounted on battens rather than rafters.
The cost of this check is a survey. The cost of skipping it is a roof repair with panels on it, which is the most expensive kind of roof repair.
Export, tariffs and the arithmetic of self consumption
The economics of a domestic array are decided by two numbers: what you pay for a unit you import, and what you are paid for a unit you export. The gap between them is why self consumption matters so much.
If import costs three times the export rate, then every unit you use in the house is worth three units exported. A battery, a timer on the water heater, or simply running the washing machine at midday can shift a third of the consumption into daylight hours.
When you price a system for a client, show the three scenarios with their own self consumption assumptions rather than a single payback figure. It is more honest, and it stops the conversation about payback turning into an argument about a number nobody can guarantee.
Monitoring, faults and the second visit
A solar system has no moving parts and still needs attention. Inverter faults need reading, panels need cleaning in dusty climates, and a string that has dropped output is invisible without monitoring.
Include a monitoring device and the commissioning visit in the estimate, and tell the client what the normal output looks like in each season. A client who knows that a 4 kW array produces 120 kWh in January does not call an engineer when it does.
Frequently asked questions
How many panels do I need for 4,000 kWh a year? Roughly 9 to 11 panels of 420 W in a temperate climate, depending on orientation and how much of the generation you can use on site.
How much roof area does 1 kW of solar need? About 5 m² of panel on a pitched roof, and 8 to 10 m² of roof on a flat roof with tilted rows.
Is a battery worth it? If the evening load is significant and the export rate is low, usually yes. If the household is out all day and the export rate is high, sometimes not.
Does shade matter that much? Yes. One shaded panel in a series string can pull down the whole string, which is why optimisers exist.
How long do panels last? Most carry a performance warranty of 25 years at around 85 percent of rated output. The inverter usually needs replacing at 10 to 15 years.
Can I add panels later? Sometimes, but mixing panel models on one inverter is awkward. Oversizing the inverter and the roof space at the start is cheaper than a second system.
Size it in FRELUX
The FRELUX solar sizing tool takes consumption, orientation and pitch, returns array size, panel count, mounting and balance of system quantities, and prices them from your market's price book with a local specific yield.