# 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.

[View the web page](https://heatuneed.com/solar-panel-calculator/)

## 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.

## 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.


## What the calculator asks for

### Locations

Named cities, grouped by region and ordered north to south within each, followed by five generic climate bands for anywhere not listed. Each entry carries a latitude, a yearly global horizontal irradiance in kWh per square metre and a mean air temperature. The latitude sets the sun's path, the irradiance pins the magnitude and the temperature drives the panel derate, so the model is only responsible for the shape.


**France**

- Lille — 50.63°, 1080 kWh/m2, 11.0 °C
- Paris — 48.86°, 1180 kWh/m2, 11.5 °C
- Strasbourg — 48.58°, 1180 kWh/m2, 11.0 °C
- Nantes — 47.22°, 1280 kWh/m2, 12.5 °C
- Lyon — 45.76°, 1360 kWh/m2, 12.5 °C
- Bordeaux — 44.84°, 1400 kWh/m2, 13.5 °C
- Nice — 43.7°, 1690 kWh/m2, 15.5 °C
- Montpellier — 43.61°, 1670 kWh/m2, 15.5 °C
- Toulouse — 43.6°, 1440 kWh/m2, 14.0 °C
- Marseille — 43.3°, 1720 kWh/m2, 15.5 °C


**Europe**

- Stockholm — 59.33°, 980 kWh/m2, 7.5 °C
- Dublin — 53.35°, 950 kWh/m2, 10.0 °C
- Berlin — 52.52°, 1060 kWh/m2, 10.0 °C
- Amsterdam — 52.37°, 1080 kWh/m2, 10.5 °C
- London — 51.51°, 1000 kWh/m2, 11.5 °C
- Brussels — 50.85°, 1060 kWh/m2, 11.0 °C
- Munich — 48.14°, 1180 kWh/m2, 9.5 °C
- Geneva — 46.2°, 1310 kWh/m2, 11.0 °C
- Milan — 45.46°, 1400 kWh/m2, 13.5 °C
- Rome — 41.9°, 1600 kWh/m2, 16.0 °C
- Barcelona — 41.39°, 1620 kWh/m2, 16.5 °C
- Madrid — 40.42°, 1750 kWh/m2, 15.0 °C
- Lisbon — 38.72°, 1780 kWh/m2, 17.0 °C


**North America**

- Vancouver — 49.28°, 1150 kWh/m2, 11.0 °C
- Seattle — 47.61°, 1180 kWh/m2, 11.5 °C
- Montreal — 45.5°, 1330 kWh/m2, 7.0 °C
- Toronto — 43.65°, 1330 kWh/m2, 9.0 °C
- Boston — 42.36°, 1400 kWh/m2, 11.0 °C
- Chicago — 41.88°, 1400 kWh/m2, 10.5 °C
- New York — 40.71°, 1450 kWh/m2, 13.0 °C
- Denver — 39.74°, 1720 kWh/m2, 10.5 °C
- San Francisco — 37.77°, 1750 kWh/m2, 14.5 °C
- Los Angeles — 34.05°, 1900 kWh/m2, 18.0 °C
- Phoenix — 33.45°, 2100 kWh/m2, 23.5 °C
- Houston — 29.76°, 1620 kWh/m2, 21.0 °C
- Miami — 25.76°, 1750 kWh/m2, 25.0 °C


**Somewhere else**

- Very sunny — around 1950 kWh/m² a year — 34°, 1950 kWh/m2, 20 °C
- Warm — around 1550 kWh/m² a year — 43°, 1550 kWh/m2, 15 °C
- Temperate — around 1250 kWh/m² a year — 47°, 1250 kWh/m2, 12 °C
- Cloudy — around 1050 kWh/m² a year — 51°, 1050 kWh/m2, 10 °C
- Far north — around 900 kWh/m² a year — 57°, 900 kWh/m2, 7 °C



### Roof orientations

Measured from due south, negative towards the east. East and west come out within about one percent of each other, west slightly behind because the afternoon is the hotter half of the day. No difference in cloud between morning and afternoon is modelled.

**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.

- South — 0°
- South-east — -45°
- South-west — 45°
- East — -90°
- West — 90°
- North-east — -135°
- North-west — 135°
- North — 180°


Roof pitch: 0 is flat, 30 to 40 degrees is a typical pitched roof, 90 is a wall.

Installed power: DC panel power on this group. The result uses the total across all groups; the size table scales panel and inverter powers proportionally.

Inverter power: AC output available to this orientation. Output above this rating is clipped; a lower rating can be economical but gives up the strongest sunny hours.

Panel type: Choose the type installed on this orientation. Bifacial panels also collect light on the back; this group's mounting determines how much reaches it.

Shaded by trees, chimneys or neighbours: 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.

System losses: Inverter conversion efficiency, cabling, mismatch and dirt. 14 percent is the usual default. Inverter clipping, panel temperature and reflection are worked out separately.

### Mounting, and what it does

Every panel group has its own mounting. It decides three things at once: what lies under that orientation, how much of that reaches the back of a bifacial panel, and how freely air moves behind it. The last applies to ordinary panels too - a flush pitched-roof array runs hotter, and hotter panels are less efficient. Bifacial gain is reported rather than assumed, and flush against a pitched roof it comes out near zero. Each entry below is listed as rear-light factor / ground albedo / NOCT.

- Flush against a pitched roof — 0.02 / 0.18 / 45 °C
- Raised frame on a dark flat roof — 0.3 / 0.15 / 44 °C
- Raised frame on a pale or white flat roof — 0.3 / 0.6 / 44 °C
- Ground mount or carport over grass — 0.7 / 0.2 / 43 °C
- Ground mount over gravel or concrete — 0.7 / 0.3 / 43 °C
- Ground mount where snow lies much of the winter — 0.7 / 0.45 / 43 °C


Bifacial panels flush against a pitched roof gain almost nothing. The roof surface is directly behind them and in their own shadow, so there is no light back there to collect - you would be paying the premium for a second face that never sees anything. They earn it on a raised frame over a pale flat roof, on a ground mount, or anywhere snow lies for months. Change the mounting and watch the figure move.

### Electricity use, prices and investment

- Price you pay for electricity — 0.17 USD/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.
- Annual electricity use — 5200 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.
- CO2 — 0.37 kg/kWh
- Installed cost — 0 USD. Starts at zero. Enter the total quote for panels, inverter, mounting and labour. The size table scales your figure in proportion to the array.
- Grants and tax credits — 0 USD

Currency: USD, EUR. 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.

### How the production is worked out

A representative day of each month is walked in hourly steps. Sun position comes from the declination and the hour angle at your latitude; the clearness index is scaled so the year adds up to the location's measured irradiance, with a seasonal shape because midwinter is cloudier as well as shorter. Page's correlation splits the light into beam and diffuse, a Hay-Davies sky puts both onto each entered roof plane, glass reflection is taken off with an incidence angle modifier, and the panel's own temperature is worked out from the irradiance and the air temperature. Each plane's profile is multiplied by its panel power, capped hour by hour at its inverter's AC power, then combined with the other planes.

## Your result

### 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** — kWh (Annual electricity use ÷ 12)

### How the verdict is decided

The verdict follows the simple payback on the cost after grants: under 8 years is a strong case, 8 to 14 years is worth doing, 14 to 22 years is borderline, and beyond that it does not repay on money alone.

- **Strong case - worth doing** — The array pays for itself well inside the life of the panels, and everything after that is free electricity.
- **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.
- **Borderline on money alone** — The electricity value is real but slow to repay the installation. A lower quote, available incentives or a less shaded layout are what move this.
- **Hard to justify on money alone** — At this electricity price and installed cost, the array does not repay itself in a reasonable time. Check the quote, the shading and whether the roof assumptions match the proposed design.

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.

## Common questions


### What does the yearly saving assume?

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.

### Does a north-facing roof rule solar out?

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.

### What if my panels are split across different sides of the roof?

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.

### Are bifacial panels worth the extra?

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.

### Should I fit as many panels as the roof takes?

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.

### Why is my winter production so low?

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.

- [Check app availability](https://heatuneed.com/app-store/)