Calculator
Solar System Sizing Calculator
Array size is the first decision in a photovoltaic project and the one every later choice inherits. Get it wrong upwards and you pay for capacity that exports at a poor price; get it wrong downwards and the fixed costs of scaffolding, design and connection are spread over too few kilowatts.
This calculator works backwards from the only number you can actually measure — your annual electricity consumption — to a module count and the roof area it demands.
Size the array
Take it from twelve months of bills, not one.
850–1,000 northern Europe, 1,100–1,350 central and southern.
Allowance for edges, vents, walkways and setbacks.
Target generation
4,050 kWh/yr
Capacity required
3.52 kWp
Modules needed
9
at 440 W each
Installed capacity
3.96 kWp
Rounded to whole modules
Expected production
4,554 kWh/yr
Share of consumption
101%
Annual total, not hour by hour
Module footprint
17.6 m²
Roof area needed
23.4 m²
at 75% packing
How the calculation works
Figure — The sizing chain
Decision tree
Fixing the array size
Size from the number you can measure, then test the answer against the three limits that actually cap it.
Do you have twelve consecutive months of consumption data?
- No
- Get it before sizing. A single quarter over- or under-states annual demand by a wide margin in either direction.
- Yes
- → go to step 2
Will a heat pump, electric vehicle or extension land within five years?
- Likely
- Add the expected load now. Retrofitting modules later means a second scaffolding visit and often a new grid application.
- Speculative
- Size for today but reserve roof space and a spare MPPT input. → go to step 3
- No
- → go to step 3
Does the unshaded roof area accommodate the calculated capacity?
Plan on 5.5 to 6.5 m² of gross pitched roof per kilowatt after setbacks, and 10 to 14 m² per kilowatt on a flat roof with row spacing.
- No
- Take the capacity the roof allows and accept lower coverage. Do not fill setbacks or access routes to hit a target.
- Yes
- → go to step 4
Does the network operator's export limit or connection band cap the system below that size?
- Yes
- Design to the limit, or price the upgrade before committing. This is the most common late-stage redesign.
- No
- Size is confirmed. Round up to whole modules and check the inverter DC/AC ratio next.
The calculation is a chain of three divisions. Target generation is annual consumption multiplied by the share you want to cover. Capacity is that target divided by the specific yield of your location — the kilowatt-hours a well-installed kilowatt produces in a year, which folds together irradiation, orientation, temperature and system losses. Module count is capacity divided by module power, rounded up to whole modules.
Roof area is not the same as module area. Fire setbacks, ridge and eave clearances, vents, dormers and maintenance access consume 20 to 35 percent of a typical pitched residential roof, which is why the packing efficiency input exists. Flat roofs on tilted frames are worse still: row spacing to avoid winter self-shading commonly halves usable density.
The formula
E_target = E_annual × offset% P_kWp = E_target / Y_specific N = ceil(P_kWp × 1000 / P_module) P_inst = N × P_module / 1000 E_prod = P_inst × Y_specific A_roof = N × A_module / packing% Y_specific = annual kWh per installed kWp, location and orientation dependent packing% = usable fraction of gross roof area after setbacks and obstructions
Assumptions behind the numbers
- Specific yield already includes inverter, cable, soiling and temperature losses; do not deduct them a second time.
- Annual coverage is not the same as self-consumption. A system generating 100% of your annual kWh typically supplies only 30 to 45 percent of your consumption directly without storage or load shifting.
- Module dimensions assume a standard 2.0 x 1.1 m format; check the datasheet for your actual product.
- Roof structural capacity, wind and snow loading are not considered and must be verified by a competent person.
- Export limits imposed by the network operator may cap system size below the figure calculated here.
How to read the result
Treat the capacity figure as the centre of a range, not a target. Because fixed costs dominate small installations, adding two or three modules to a nine-module design usually improves the levelised cost even if the extra energy is exported cheaply. The opposite is true once you exceed the point where a second inverter or a network upgrade becomes necessary.
If coverage above 100 percent looks attractive, check the export price first in the payback calculator. Under net billing the marginal exported kilowatt-hour is often worth a quarter of the imported one, and the economics of oversizing depend entirely on that ratio.
Worked example
A household in central Europe uses 4,500 kWh a year and wants to cover 90 percent of it. Local specific yield for a south-facing 30 degree roof is 1,150 kWh/kWp.
Target generation is 4,500 × 0.90 = 4,050 kWh. Capacity is 4,050 ÷ 1,150 = 3.52 kWp. At 440 W per module that is ceil(3,520 ÷ 440) = 8 modules, giving 3.52 kWp installed and about 4,050 kWh a year. Module footprint is 8 × 1.95 = 15.6 m², so at 75 percent packing the roof needs about 20.8 m² of clear, unshaded pitch.
Mistakes that skew the answer
- Sizing from a single winter or summer bill instead of a full year.
- Using the peak-sun-hours figure for a horizontal surface when the array is tilted, or vice versa.
- Assuming annual coverage equals bill reduction. Without storage the two diverge sharply.
- Forgetting that a heat pump or an electric car added next year changes consumption by thousands of kilowatt-hours.
- Filling the roof to the last square metre and leaving no access route for maintenance or firefighting.
- Ignoring the network operator's export limit until after the design is fixed.
Questions readers ask
- How many solar panels do I need for a typical home?
- For a European household using 4,000 to 5,000 kWh a year, eight to twelve modern modules of 400 to 450 W each — roughly 3.5 to 5 kWp — will cover most of the annual consumption on a reasonably oriented roof. Homes with electric heating or an electric vehicle commonly need double that. The honest answer always comes from your own twelve-month consumption figure rather than a household average.
- Should I size for today's consumption or future consumption?
- Size for the consumption you realistically expect within about five years, because retrofitting extra modules later usually means a second scaffolding visit, possibly a second inverter and sometimes a new grid application. If a heat pump or electric car is likely, include its expected load now. If it is speculative, at least leave physical roof space and a spare inverter MPPT input.
- Does a bigger system always pay better?
- Up to a point. Fixed costs — scaffolding, design, connection, labour mobilisation — are spread over more kilowatts as the system grows, so the cost per installed kilowatt falls. That benefit stops when the extra energy can only be exported at a low price, or when the next module triggers a larger inverter, a three-phase upgrade or a more involved grid connection process.
- How much roof area does one kilowatt need?
- With modern modules of around 440 W in a 1.95 m² format, one kilowatt occupies about 4.4 m² of module surface. Allowing for setbacks and obstructions, plan on 5.5 to 6.5 m² of gross roof area per kilowatt on a pitched roof, and 10 to 14 m² per kilowatt on a flat roof where rows must be spaced to avoid shading each other.
- What specific yield figure should I use for my location?
- Use a location-specific estimate rather than a national average. The European Commission's PVGIS tool returns modelled annual yield for a given coordinate, tilt and azimuth free of charge, and NREL's PVWatts does the same for North America. Both already account for typical system losses, so their output can be entered directly into the specific yield field here.