How many solar panels do I need?

The starting point is your monthly consumption in kilowatt-hours (kWh), which appears on your bill. Since consumption varies from month to month, the ideal is to average several months — ideally a year — so you have a realistic base and are not sizing on an atypical month.

Dividing that monthly consumption by 30 gives you your approximate daily consumption, the value the sizing calculation works with.

If you are billed every two months, divide by the days in the period rather than by 30. And if you have just moved in or changed appliances, use consumption from the months after the change: sizing on a pattern that no longer exists is the most expensive mistake at this stage.

Averaging twelve months gives a solid figure, but looking at the month-by-month curve adds something more: it tells you whether your consumption is steady or has marked peaks. That shape influences how much it makes sense to cover.

Across much of Colombia residential consumption is fairly flat through the year, because there is no heating and temperature variation is moderate. In hot regions, by contrast, air conditioning introduces seasonality and also concentrates consumption in the hottest hours, which coincide with the sunniest ones. That overlap works in favour of self-consumption.

If your consumption has very marked peaks, mention it to the installer: a system sized on the average will leave surplus in the low months and fall short in the high ones, and net metering handles that mismatch better in some cases than in others.

The system capacity in kilowatts peak (kWp) is estimated by dividing daily consumption by the peak sun hours in your area multiplied by the system's performance ratio.

As a formula: capacity (kWp) = daily consumption (kWh) ÷ (peak sun hours × performance ratio). The three terms in the denominator are the ones to understand properly, because they introduce most of the error in home-made estimates.

The greater the radiation in your area, the less capacity you need to install to cover the same consumption, because each kWp generates more energy. That is why the same consumption leads to systems of different sizes in La Guajira and on the highland plateau.

Peak sun hours (PSH) are a compact way of expressing daily radiation: they are equivalent to the number of hours the sun would have to shine at a reference intensity of 1,000 watts per square metre to deliver the same energy the surface actually receives over the course of the day.

They are not hours of daylight. A day can have twelve hours of light and four and a half peak sun hours, because intensity varies with the height of the sun, cloud cover and humidity.

In Colombia PSH range roughly between 3.5 and 5.5 depending on the area, with the highest values in La Guajira and the Caribbean region and the lowest in areas of heavy cloud cover such as the Pacific. IDEAM's Solar Radiation Atlas is the official reference for the value in your region.

A structural advantage of Colombia over high latitudes: sitting on the equator, radiation varies little between seasons. That makes output more predictable through the year and simplifies sizing.

The overall performance ratio is the factor that turns the theoretical energy the panels receive into the real energy that reaches your panelboard. It usually sits between 0.75 and 0.85 on well-executed installations.

It is not an arbitrary number: it bundles specific losses, all of them real. Cell temperature, which in Colombia usually runs well above the 25 °C of laboratory conditions and takes off several percentage points. Inverter losses when converting direct current to alternating. Voltage drop in the wiring. Dirt accumulated on the glass. Mismatch between panels in the same array. And shading losses, if there are any.

Using 0.80 as a starting value is reasonable for an initial estimate. If your roof has partial shading or is in a very dusty area, a more conservative value reflects reality better.

Once you know the total system capacity in kWp, the number of panels is that capacity divided by the capacity of each panel.

Current residential panels usually sit between 400 and 600 peak watts per unit, so the exact figure depends on the model chosen. With higher-capacity panels you cover the same need with fewer units and less area, which matters if your roof is small.

The result is almost never a whole number, and it is rounded according to what the roof and the electrical configuration allow. Panels are grouped into strings connected to the inverter, and that grouping imposes its own constraints: the number of panels per string has to keep the voltage within the inverter's working range. That is why the installer may propose a slightly different number from the one the theoretical calculation gives.

The theoretical calculation can run into a physical limit: the usable area of your roof. As a rough reference, each kWp requires around 5 to 7 m² of well-oriented, unshaded surface.

'Usable area' is not the same as total area. You have to subtract maintenance walkways, edge setbacks, areas taken up by tanks, aerials, skylights or air-conditioning units, and the strips shaded by features of the roof itself.

If the space is not enough to cover 100% of your consumption, a system covering a significant part is still worthwhile. In fact, it is the most common situation in urban housing and does not invalidate the project at all.

Three geometric factors can change the result of the calculation considerably, and none of them appears in the basic formula.

Orientation. In the northern hemisphere the optimum is to face south, but Colombia is practically on the equator and there the penalty for orientation is far smaller than at high latitudes. East- or west-facing roofs lose some total output, though they shift generation towards the morning or the afternoon, which sometimes fits the consumption pattern better.

Tilt. Near the equator, optimal angles are low. But too flat a tilt accumulates dirt because rain does not wash the dust off well, so a minimum angle is usually kept for cleaning reasons even when the theoretical optimum is lower.

Shading. It is the most underestimated and most damaging factor. Partial shade over a few panels can affect the output of an entire string, not just the shaded panels. That is why growing trees, a neighbouring building adding a floor or a badly placed tank have a disproportionate effect. If your roof has unavoidable shading, there are solutions — microinverters or optimizers — that limit the damage to the affected panels.

Suppose a consumption of 300 kWh a month, that is, around 10 kWh a day, in an area with around 4 peak sun hours.

Applying the formula: 10 ÷ (4 × 0.80) = approximately 3.1 kWp. With 550 Wp panels, that is 3,100 ÷ 550 = 5.6 panels, which in practice rounds to 6 panels and around 3.3 kWp installed.

In surface terms, those 3.3 kWp would take up on the order of 17 to 23 m² of usable roof. And the estimated annual output, multiplying the capacity by the peak sun hours, by the performance ratio and by 365 days, would come to around 3,850 kWh: somewhat above the annual consumption of 3,600 kWh, which leaves a reasonable margin to cover losses and variation.

It is only an estimate. The final number is adjusted by the installer according to your real roof, the electrical configuration and the equipment available, but it helps you reach the quoting stage knowing what order of magnitude to expect and spotting proposals that fall well outside it.

Covering 100% of consumption sounds like the natural goal, but it is not always the most cost-effective decision.

The first reason is diminishing returns. The first kWp installed replace energy you consume directly and are the ones that give the best return. As you grow, a larger share of generation becomes surplus that goes to the grid, and its economic value depends on how it is offset, not on the tariff you pay.

The second is space and budget: the last kWp are usually the ones that complicate the installation most, because they occupy the worst-oriented or most shaded parts of the roof.

That is why many residential installations are sized to cover a high share of consumption without forcing 100%. If your goal is energy independence rather than pure return, the answer may be different, and it is a conversation worth having explicitly with the installer.

When making an estimate on your own, it is worth avoiding a few frequent slips that lead to oversizing or falling short.

This estimate is there to help you arrive informed, not to replace the design. The technical visit is what turns an approximate number into an executable project.

During it the genuinely available area is measured, shading is assessed through the day and the year, the condition and structural capacity of the roof are checked, the panelboard and the space for the inverter are inspected, and the cable route is defined. Any of those points can change the final size of the system.

Arrive at that visit with your last twelve months of bills and with your estimate done. It completely changes the conversation: instead of listening to a proposal, you can ask why the proposed number differs from yours, and that question usually reveals a good deal about the installer's rigour.