Units and quantities
kWp (kilowatt peak)
Also known as: kilowatt peak, watt peak, Wp, installed capacity.
Definition
The kilowatt peak is the unit that measures the capacity of a solar system under standard test conditions. It is the main reference for sizing an installation and for comparing prices: cost is usually expressed in pesos per installed kWp.
One kWp is equal to 1,000 watts peak. It does not measure the energy the system produces, but the maximum power it would deliver under laboratory conditions: 1,000 W/m² of irradiance, a 25 °C cell temperature and a reference solar spectrum. In real operation those three conditions almost never coincide, so a system's instantaneous power usually sits below its nominal value.
The practical use of kWp is twofold. It serves for sizing — the kWp needed come from cross-referencing your monthly consumption in kWh with the peak sun hours at your location and with the system's estimated performance ratio — and it serves for comparing prices: dividing the total value of a quote by the peak watts gives the price per watt, the only metric that lets you compare proposals of different sizes on a level footing.
When reading a quote it is worth checking that the declared capacity matches the number of panels multiplied by the unit capacity of each one. A system of twelve 550 Wp panels is a 6.6 kWp system; if the proposal declares a different figure, ask for an explanation before comparing prices.
The essentials of kWp (kilowatt peak)
- 1 kWp = 1,000 watts peak (Wp).
- It measures power under laboratory conditions, not energy produced.
- System kWp = number of panels × unit capacity of each panel.
- Price per watt = total value ÷ (kWp × 1,000): the metric for comparing quotes.
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Frequently asked questions about kWp (kilowatt peak)
What is the difference between kWp and kWh?
kWp measures power (installed capacity) and kWh measures energy (what is actually produced or consumed over time). A 5 kWp system does not produce 5 kWh: it produces as many kWh a day as you get by multiplying its kWp by the peak sun hours at the location and by the system's performance ratio.
How many kWp do I need for my house?
It depends on your monthly consumption in kWh and on the radiation in your city, not on the size of the house or the number of rooms. The starting point is always the bill: you take the average daily consumption, divide it by the local peak sun hours and adjust for the system's performance ratio.
Why does the price per kWp fall on large systems?
Because part of a project's costs are fixed: the technical visit, the design, the connection process, mobilising the crew and the commissioning cost almost the same on a 3 kWp system as on a 10 kWp one. Spread across more capacity, the cost per watt falls.
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