Vicaya GlobalCalculator · Solar · Simple calculator

How much solar do you actually need?

Start with your bill, your units or your roof — whichever you know. You get a system size, what it would generate, what it would save, what it would cost and how long it takes to pay for itself.

Step 1 · How would you like to work it out?

My monthly bill What you pay for electricity each month My monthly units The kWh figure on your bill My roof area How much space you have to fill

Step 1 · Tell us one thing about your home

Set your currency, then answer whichever of these you know. Any one of them is enough to size a system.

Currency

Changes how money is shown and labelled. Nothing is converted — enter costs and tariffs in your own currency and they stay in it.

Your monthly electricity bill £

Your monthly units

Your roof area

Location

Step 3 · Where is the roof?

Searches use OpenStreetMap. Nothing is stored.

Or pick an example location

Or enter coordinates directly

Latitude

Longitude

Searching or using your location fills these in, so you can always see exactly which point the figures are for.

Climate comes from PVGIS, the European Commission's reference dataset — the same source this model is validated against, for examples and searches alike.

Step 4 · The system you would buy

The price sets the whole budget; the panel rating decides how many modules that budget buys.

Watts per panel

The nameplate rating of one module, on the quote or the panel label. It changes the PANEL COUNT, not the system size — the kilowatts you need are the kilowatts you need. A 5.3 kWp system is 12 panels at 450 W, or 8 at 740 W.

Installed price per kWp £

Everything in, per kilowatt of panels — hardware, labour, paperwork. This is the single figure quotes differ on most, so put yours in: at £1,400/kWp a 4 kWp system is £5,600, and at £1,000/kWp the same system is £4,000 and pays back years sooner.

The fine print

Sensible starting points. Everything here is editable, and nothing is hidden from you.

What you are paid for what you export p/kWh

Electricity you generate but do not use goes to the grid, usually for far less than you pay to buy it back. Set it to zero if you are paid nothing. At 15p against a 27p tariff, an exported unit is worth a little over half a used one.

How much you use as you make it %

Solar arrives at midday; households mostly use power at breakfast and in the evening. Without a battery, 40% to 60% is typical. At 55% and a 27p tariff, an average generated unit is worth about 22p rather than the full 27p.

Panel efficiency %

The percentage on the datasheet, next to the wattage — how much of the light landing on the glass becomes electricity. It decides how big each panel has to be to make the watts you entered: ten 400 W panels at 21% cover 19 m² of roof, the same ten at 17% cover 24 m². The energy figures on this page do not move, because the wattage already decides those.

How much of a roof panels can use %

Setbacks, vents, chimneys and access paths take their share before any panel is placed. Only used when you size by roof area: at 70%, a 30 m² roof holds 21 m² of panels, which is about 4.4 kWp. At 90% the same roof holds 5.7 kWp.

Carbon in your grid electricity

Grams of CO2 per kWh, which varies hugely by country — a hydro or nuclear grid is under 100, a coal-heavy one is over 700. Every unit you generate displaces one of these. At 200 g, 4,000 kWh a year avoids 800 kg; at 700 g the same generation avoids 2.8 tonnes.

Result 1 Recommended solar plant size

The system size your answer points to, and what it would do on an average day where you are.

5.27kWp

Sized to cover the 5,333 kWh a year your answer implies, at 1,012 kWh per kWp a year in London, United Kingdom.

14.6 kWhgenerated on an average day

3.22hours of full-strength sun a day

12panels at 450 W

36 m²of roof it needs

Over a year this generates about 100% of the electricity you use. Not all of it arrives when you want it, which is what the next sections account for.

Hours of full-strength sun are not daylight hours. A panel only makes its rated output under 1,000 watts per square metre — roughly midday, clear sky, sun overhead. The rest of the day it makes less: at sunrise and sunset the light crosses far more atmosphere and strikes the panel at a glancing angle. So a twelve-hour day delivers the energy of about five hours at full strength, and that figure is what every generation number here is built on.

Result 2 Electricity generation

Total kWh this plant would produce. The 25-year figure allows for panels slowly losing output, so it is not simply twenty-five times the first year.

444 kWha month, on average

5,333 kWhin the first year

125,333 kWhover 25 years

Generation by month, kWh
Month	kWh
Jan	139
Feb	216
Mar	432
Apr	631
May	685
Jun	702
Jul	730
Aug	632
Sep	522
Oct	326
Nov	198
Dec	119
Total	5333

Monthly figures are for an average day in each month, scaled by its length. A single cloudy week will not match them; a year will come close.

Result 3 Financial savings

What that generation is worth to you. Electricity you use as you make it saves the full unit rate; the rest earns your export rate, which is usually a good deal less.

£96a month, on average

£1,152in the first year

£27,072over 25 years

At 55% used in the home, an average generated unit is worth £22 per 100 kWh — blending what you avoid buying with what you are paid for the surplus. The 25-year total is in today's money and assumes no price rises, which makes it a cautious figure rather than an optimistic one.

Result 4 Estimated project cost

Your installed price per kWp, multiplied by the size above and then divided into the parts a quote usually itemises.

Panels £2,952
Inverter £1,107
Mounting and racking £738
Wiring, protection and meter £590
Installation labour £1,402
Design, permits and inspection £590
Total installed cost £7,381

The total is your own figure — £1,400 per kWp, which you can change in step 4. Only the split between the lines is assumed, and it is indicative of a small rooftop job rather than a price this page stands behind. Real quotes vary widely, and this section does not know what solar costs where you live.

Result 5 Payback and return

How long the savings take to cover the cost, and what the system returns each year as a percentage of what it cost.

6.5 yrssimple payback period

15.6%annual return on investment

£19,691net gain over 25 years

Payback walks year by year with panel output declining, rather than dividing the cost by the first year's saving — that shortcut flatters the answer, because later years earn less than the first. The return figure is first-year saving over installed cost: a simple yardstick, not an internal rate of return, and it ignores both inflation and the cost of borrowing.

Result 6 Environmental impact

Every unit you generate is a unit the grid does not have to. What that is worth in carbon depends entirely on how your grid makes electricity, which is why it is a figure you set.

1,067 kgCO₂ avoided in the first year

25.1 tCO₂ avoided over 25 years

200 g/kWhthe grid intensity you set

Displaced grid emissions only. It does not net off the carbon of making and shipping the panels, which a rooftop system typically repays in one to three years of operation — a real figure, and one this page does not model rather than estimate loosely.

What this does not know

Shading. It assumes a clear sky in every direction. A chimney or a tree can cost far more than its share of the roof — the shading calculator models that properly.
Your money, not ours. Every figure in currency on this page — the unit rate, the installed price, the export rate — is one you typed or one we left as a starting point, and the starting points are UK examples. Nothing here knows what solar costs in your country, and nothing here should pretend to. Change the currency and the numbers are relabelled, never converted; where a country quotes tariffs in whole units rather than in cents or pence, the same figure also means something different, so replace it rather than trusting it.
When you use power. The self-use share is a single percentage, not a real load profile. If most of your usage is in the evening, the true figure is lower — the battery calculator is where that argument lives.
Price rises. Savings are in today's money throughout. If electricity gets dearer, the real return is better than shown.

For a system you have actually been quoted, the output calculator takes the panel count and wattage from the quote instead of estimating them.

Generation is modelled with the same physics engine as every other tool in this section, validated against PVGIS and reported openly on the methodology page. Money is not modelled at all — it is arithmetic on figures you supplied. That difference is deliberate, and it is why the two are kept apart.