In short
- Each kW of panels produces between about 1,000 kWh (London) and almost 1,900 kWh (Guadalajara, Mexico) a year, according to the World Bank’s Global Solar Atlas.
- Grid-tied, you size with the yearly average. A home using 300 kWh a month in Madrid needs about 2.3 kW: 6 panels of 450 W.
- Off-grid, you have to size for the worst month and for battery losses: the same home needs 9 panels, and in London, 24.
- Shade from trees or buildings isn’t included in these figures and can cut production a lot.
What a kWp is
Panel power is given in kWp (“kilowatt peak”): what they deliver in full sun under laboratory conditions. One kWp is 1,000 W of panels, for example two 500 W panels or four 250 W ones. Each panel’s power is on its label or spec sheet, often as Pmax. What really matters is how many kWh they produce over a day or a year, and that depends mostly on how much sun there is where you live.
How much 1 kWp produces in each city
The Global Solar Atlas, from the World Bank and its energy programme ESMAP, works out for any point in the world how much 1 kWp of panels produces, using many years of satellite data. Its figure already deducts the normal losses of a system: the inverter, dirt, cables and panel heat, since panels perform worse the hotter they get. It assumes the panels are well tilted and facing the equator. These are its figures for 24 cities, from sunniest to least sunny:
| City | kWh per year per kWp | kWh per day, average | kWh per day, worst month |
|---|---|---|---|
| Guadalajara (Mexico) | 1,884 | 5.2 | 4.5 (July) |
| Mexico City (Mexico) | 1,814 | 5.0 | 4.3 (September) |
| Los Angeles (USA) | 1,792 | 4.9 | 3.9 (December) |
| Santiago (Chile) | 1,770 | 4.8 | 3.0 (June) |
| Guatemala City (Guatemala) | 1,715 | 4.7 | 4.0 (June) |
| Seville (Spain) | 1,709 | 4.7 | 3.4 (December) |
| Quito (Ecuador) | 1,682 | 4.6 | 4.3 (February) |
| Santo Domingo (Dominican Republic) | 1,666 | 4.6 | 4.3 (December) |
| San Juan (Puerto Rico) | 1,660 | 4.5 | 4.2 (November) |
| Madrid (Spain) | 1,655 | 4.5 | 3.0 (December) |
| Miami (USA) | 1,654 | 4.5 | 3.9 (December) |
| Havana (Cuba) | 1,639 | 4.5 | 3.9 (December) |
| Monterrey (Mexico) | 1,638 | 4.5 | 4.0 (December) |
| Buenos Aires (Argentina) | 1,606 | 4.4 | 3.2 (June) |
| Caracas (Venezuela) | 1,578 | 4.3 | 4.0 (June) |
| Barcelona (Spain) | 1,576 | 4.3 | 3.1 (December) |
| Medellín (Colombia) | 1,570 | 4.3 | 4.1 (April) |
| Sydney (Australia) | 1,531 | 4.2 | 3.3 (June) |
| Houston (USA) | 1,477 | 4.0 | 3.1 (December) |
| Bogotá (Colombia) | 1,433 | 3.9 | 3.6 (May) |
| New York (USA) | 1,414 | 3.9 | 2.7 (December) |
| Lima (Peru) | 1,397 | 3.8 | 2.4 (July) |
| Toronto (Canada) | 1,351 | 3.7 | 1.8 (December) |
| London (UK) | 1,027 | 2.8 | 1.1 (December) |
The daily average is the yearly figure divided by 365. The worst-month figure is what that month produces divided by its days. Note that the worst month isn’t always in winter: in Guadalajara it’s July and in Bogotá, May.
How many panels you need: a worked example
The maths is simple. It’s the same as our solar panel calculator does:
- What your home uses per day: the kWh per month on your bill ÷ 30.4. For 300 kWh a month (the average for a home in the European Union, according to the Odyssee-Mure project), that’s 300 ÷ 30.4 = 9.86 kWh a day.
- What 1 kWp gives per day: in Madrid, 1,655 ÷ 365 = 4.53 kWh. If the panels go on a house roof, a bit less: in the Atlas’s own examples, a rooftop system gives 3 to 4% less, because the panels run hotter and get dirtier. With 4% less, 4.53 × 0.96 = 4.35 kWh.
- The kWp you need: 9.86 ÷ 4.35 = 2.27 kWp.
- Number of panels: 2,266 W ÷ 450 W = 5.04, so 6 panels of 450 W.
With those 6 panels (2.7 kWp) the home would produce about 2.7 × 1,655 × 0.96 = 4,290 kWh a year, a bit more than the 3,600 it uses. In the darker months it will produce less than it uses and in the sunnier ones more: on the grid, the grid covers what’s missing.
Off-grid, you need far more
If the home has no grid connection and relies only on panels and batteries, two things change:
- You size for the worst month, because the panels have to be enough then too. In Madrid that’s December, with 3 kWh per kWp a day instead of the 4.5 average.
- Energy is lost in the battery. According to the U.S. national energy laboratory (formerly NREL, now NLR), a home battery gives back 85% of the energy that goes in. At night, everything you use has gone through the battery first.
For the same home in Madrid: 3 × 0.96 × 0.85 = 2.45 kWh a day per kWp, so you need 9.86 ÷ 2.45 = 4.03 kWp: 9 panels instead of 6. In London, where December gives only 1.1 kWh per kWp a day, you’d need about 10.8 kWp, 24 panels, compared with 9 on the grid. You also need the batteries themselves, for the night and cloudy days: work out which with the battery calculator and see which type suits you.
How to look up your home’s figure
- Open globalsolaratlas.info and click on the map where your home is, or type the address in the search box.
- In the panel that opens, find “Specific photovoltaic power output” (PVOUT): it’s what 1 kWp produces per year, in kWh/kWp.
- For the worst month, look at the monthly chart for that same figure and divide the lowest bar by the days in the month. In Madrid, 93 kWh in December ÷ 31 days = 3 kWh a day.
Enter those two figures in the calculator, under “Somewhere else”.
What can lower production
- Shade. The Atlas only accounts for shading from mountains and hills, not from trees, buildings, chimneys or antennas. Before installing panels, check where shadows fall through the day and the year.
- Direction and tilt. The Atlas assumes panels facing the equator (south in the northern hemisphere, north in the southern) at the tilt that collects the most sun. A roof facing another way produces less.
- Ageing. The Atlas figures are for new panels; they don’t include wear over time.
Before you install
These figures are for getting an idea and comparing quotes. Connecting panels to your home’s wiring, let alone to the grid, is a job for a qualified installer or electrician. If you’re on the grid, also ask your electricity company what it allows and what paperwork is needed in your country.
Sources and references
- Global Solar Atlas 2.0 (World Bank and ESMAP, data by Solargis; data version 1.7, updated April 1, 2026). globalsolaratlas.info, its methodology and its FAQs. City figures retrieved October 3, 2026.
- National Laboratory of the Rockies (NLR, formerly NREL; U.S. Department of Energy), Annual Technology Baseline 2024. Residential Battery Storage.
- Odyssee-Mure (European Commission project). Electricity consumption per dwelling.
Frequently asked questions
How much does a 450 W solar panel produce per day?
It depends on where you live. Using Global Solar Atlas figures, 1 kWp produces on average about 4.5 kWh a day in Madrid or Miami, 5.2 in Guadalajara and 2.8 in London. A 450 W panel is 0.45 kWp: in Madrid, about 0.45 × 4.5 = 2 kWh a day on average, a bit less on a roof.
How many solar panels do I need for a house?
Divide what your home uses per day by what 1 kWp gives per day where you live, then by each panel’s power. A home using 300 kWh a month in Madrid needs about 2.3 kWp on the grid: 6 panels of 450 W. Off-grid, 9.
Why does off-grid need more panels?
Because you have to size for the least sunny month, not the average, and because energy is lost passing through the battery: according to the U.S. national energy laboratory, a home battery gives back 85%. You also need the batteries.
