Quick answer
A photovoltaic DC maximum-power reference rating at standard test conditions, equal to 1,000 watt-peak, distinct from field output and energy production.
Answer: Kilowatt-peak (kWp) expresses a photovoltaic module or array’s rated DC maximum power at standard test conditions, with 1 kWp equal to 1,000 Wp. It is a reference capacity, not an annual energy quantity or an absolute field-output ceiling. To estimate kilowatt-hours, use a site-specific production model and state the energy boundary, weather assumptions and losses.
What the reference rating means
PVsyst’s STC documentation describes the conventional reference conditions: 1,000 W/m² irradiance, 25°C module temperature and an AM1.5 spectrum. The temperature reference is not outdoor air temperature. Confirm the exact module datasheet’s test and rating basis, including any separate bifacial front/rear rating information.
The “peak” label refers to maximum power on the device’s current–voltage curve at those conditions. It does not mean that STC represents the strongest possible sunlight or the highest power the device could ever produce. Cold cells or irradiance above the reference can change field power; the connected inverter and other equipment impose their own limits.
Sandia’s publication abstract identifies Wp as the DC module rating at STC. The reference rating provides a common capacity basis; a separate performance model estimates output for a particular design and location.
Calculating an array’s kWp
Sum the selected modules’ stated reference powers and divide watts by 1,000. For a hypothetical array:
25 modules × 400 Wp / 1,000 = 10 kWp
12 modules × 410 Wp / 1,000 = 4.92 kWp
Keep the exact model, count, rating basis and substitutions in the design/as-built record. A nameplate tolerance or flash-test result is separate evidence; do not silently redefine rated array capacity or assume a tolerance proves electrical compatibility. Roof area also needs module geometry, layout, access and structural review rather than a universal square-metres-per-kWp rule.
kWp, kW AC and kWh
| Term | Interpretation |
|---|---|
| kWp | PV DC reference rating under stated test conditions |
| kW DC at STC | Equivalent capacity notation when the same rating basis is used |
| kW AC | AC power rating or measured active power; specify which |
| kWh | Energy produced or consumed over an interval |
| kWh/kWp for a year | Annual energy divided by the stated PV reference capacity |
Use kilowatt for the general power concept and standard test conditions for the reference definition. A 10 kWp array does not imply a 10 kW AC inverter or a fixed DC-to-AC conversion ratio.
Converting capacity into a conditional energy estimate
Annual energy divided by kWp is specific yield. Specify whether the energy is inverter output, energy delivered at the grid boundary or another quantity, and which year/weather data and losses are included.
Suppose a hypothetical project model, not a regional benchmark, predicts 1,050 kWh per kWp for the modeled year at the declared AC boundary. Then:
4.92 kWp × 1,050 kWh/kWp = 5,166 kWh
For an annual energy target of 5,000 kWh, the same preliminary assumption gives 5,000 / 1,050 = 4.76 kWp, rounded. Module count, roof constraints, inverter compatibility, weather uncertainty and the final model still need review. Matching annual consumption on paper does not prove 100% instantaneous self-supply, backup or bill savings.
There is no universal annual kWh per kWp, even within a country. Orientation, shading, weather, cell temperature, losses and operation affect results. Sandia’s PVWatts model explanation explicitly relates modeled DC power to irradiance and cell temperature; it is not a promise that every module or software uses the same model.
Compare proposals without hiding the basis
For hypothetical like-scope quotes, $22,000 for 10 kWp is $2,200/kWp; $19,000 for 8 kWp is $2,375/kWp. The first has the lower capacity-normalized price but the higher total cost. This does not establish better lifetime value: compare included work, equipment, energy assumptions, financing and exclusions.
Commissioning comparisons need measured environmental conditions, the correct measurement boundary and agreed acceptance criteria. Output below kWp is not automatically normal or faulty. Warranty degradation terms, measured performance and the reference nameplate rating are separate concepts.
Frequently asked questions
Is kWp a different size unit from kW?
It adds a PV reference-rating meaning to kilowatts. A measured kW value may be DC or AC and is not automatically the STC rating.
Does 1 kWp produce a fixed number of kWh?
No. Energy depends on time and site/system conditions. Use a documented model and identify the output boundary.
Can field power exceed the kWp rating?
The rating is a reference value, not an absolute physical ceiling. Actual conditions and equipment limits determine operating output.
Does higher kWp guarantee greater savings?
No. Savings also depend on production timing, loads, tariffs, export rules, costs and financing. Larger reference capacity alone does not settle the financial comparison.
Related Glossary Terms
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Editorial contributor · SurgePV
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