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Would solar panels actually pay for themselves?

Describe each panel group on your roof and get their combined yearly production, electricity value, CO2 avoided and payback.

Heatuneed

Would solar panels actually pay for themselves?

Prepared

Turn the shape and location of your roof into a yearly solar estimate

How much an array makes is geometry: the sun's path at your latitude against every plane your panels sit in. This tool models each orientation and pitch hour by hour through a representative day of every month, combines their production, then values every generated kilowatt-hour at the electricity price you enter.

Only the currency the figures are shown in changes. The prices stay exactly as you typed them, so set them to what you actually pay.

Start from a typical case

Your roof

Sets the latitude and the yearly sunshine. Pick the nearest city, or one of the climate bands at the end of the list. For the exact figure at your address, look the site up on PVGIS or NREL's PVWatts.

Panel groups

Add one group for each distinct orientation or roof pitch. Give every group its own panel type, mounting, panel power and inverter power; the results combine them into one system.

%
The share of the year's light lost to whatever stands in front of the panels. A dormer that clips one string at breakfast is a few percent; a mature tree to the south is far more.
%
Inverter conversion efficiency, cabling, mismatch and dirt. 14 percent is the usual default. Inverter clipping, panel temperature and reflection are worked out separately.

Electricity and investment

$ / kWh
Every generated kWh is valued at this rate. That matches full retail credit; if your exports earn less, treat the saving as an upper-bound estimate.
kWh/year
Shown on the monthly chart as a dotted average (annual use divided by 12). It is a visual benchmark and does not change the flat-credit saving.
$
Starts at zero. Enter the total quote for panels, inverter, mounting and labour. The size table scales your figure in proportion to the array.
$
Starts at zero. Enter any tax credits, national schemes or local top-ups you expect to receive. The size table scales your figure in proportion to the array.

Your inputs

Your result

Updated live as you change the inputs.

Worth doing

A payback in this range is normal for a rooftop array, and the panels should still be making most of their output long after they break even.

Saved per year —
Payback after grants —
Made per year —
Cost after grants —
Net over 25 years —
Yield per kW installed —
Lost to the roof angles —
Panel area needed —
CO2 avoided per year —

Estimated production by month

Energy and electricity value for a typical year. Each color is one panel group; the dotted line is annual use divided by 12. Actual weather will vary.

Monthly total kWh + electricity value Daily average kWh/day + electricity value Average monthly use —

    Estimates from an hour-by-hour model of a representative day in each month: sun position for your latitude, a clearness index that carries the season, Page's split into beam and diffuse light, a Hay-Davies sky, reflection off the glass, panel temperature and inverter clipping. Every generated kWh is valued at the electricity price entered; lower export rates or limited credit will reduce the real saving. Real production moves with the weather of the particular year, detailed shading and installation quality.

    Calculated with the free tool at heatuneed.com/solar-panel-calculator/

    How the calculation works

    The tool builds a year of production from every panel group on your roof, values their combined output at the electricity price entered, and compares that annual value with the net installed cost.

    1

    A year of sunshine on your roof

    A representative day of each month is walked hour by hour. The sun's position comes from your latitude, the clearness of the sky is scaled so the year matches the measured irradiance where you are, and the light is split into a beam and a diffuse part before both are projected onto every plane the panels sit in.

    2

    What the panels do with it

    Reflection off the glass is taken off at whatever angle the sun happens to strike, then the panel's own temperature is worked out from the irradiance and the air temperature - which is why a hot climate does not return its sunshine one for one. The installer's system losses and any shading come off next, then each orientation's hourly output is capped at its inverter's AC power.

    3

    Valuing the production

    Every generated kilowatt-hour is valued at the electricity price you enter. This is the right shortcut where full retail credit applies; where exports earn less, it is a transparent upper bound rather than a promise about the bill.

    4

    Money, carbon and payback

    Payback is the installed cost after grants divided by the yearly electricity value. The twenty-five year figure also takes off the output lost to ageing and one mid-life inverter replacement.

    Common questions

    Every generated kilowatt-hour is valued at the electricity price you enter. That matches full retail credit or an arrangement where all production offsets electricity at the same rate. Annual electricity use appears as a flat monthly benchmark in the chart, but it does not cap this value. If surplus exports earn less, the result is an upper-bound estimate; use your installer or supplier's tariff rules for a bill-specific forecast.

    It costs you a lot but not everything: at temperate latitudes a north-facing pitch makes roughly 55 to 60 percent of what the same array would make facing south. That is because much of the light in a cloudy climate arrives diffuse, from the whole sky rather than from the sun's direction. East or west is far less painful - about 15 percent down. West produces marginally less than east in this model because the afternoon is warmer and hot panels are slightly less efficient.

    Add one panel group for each side, then enter that group's orientation, panel type, mounting, pitch, panel power and inverter power. The calculator models and clips every group separately before adding its hourly and monthly production, so an east-west split — including different mountings, inverter ratios, or a mix of standard and bifacial panels — is not treated like one south-facing array. Location, shading and system losses still apply to the whole project.

    Only if something behind them is lit. A bifacial module has cells that work on the back too, but on a pitched roof the back faces the tiles, from a few centimetres away, inside the panel's own shadow - the calculator puts the gain there at well under one percent, which does not repay the premium. Raise the same panels on a frame over a pale flat roof and the gain is around twelve percent; over gravel or where snow lies for months it is more again. Switch the mounting and watch the figure move: the panel is not what decides this, the ground is.

    The table shows the physical output and flat-rate value of each size, while scaling your cost and grant in proportion. Under that assumption, production and value rise with the array. The real choice can still be smaller when roof space, budget, grid limits or a lower export tariff apply; those project-specific constraints are outside this simplified model.

    Because midwinter is cloudier as well as shorter. Geometry alone would put December at about half of July; the measured ratio in a temperate climate is nearer a fifth, and the model carries that. The monthly chart makes this visible so a strong annual total is not mistaken for steady production through winter.

    Want to know how much heat your house is losing first?

    Heatuneed scans each room with LiDAR and works out its heat loss through walls, windows, doors and roofs - the number that tells you how much electricity you would be generating for in the first place.

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