kWp vs. kWh — the two numbers every solar buyer should know

Your utility bill shows kWh. Your panel datasheet shouts kWp. Installers quote array sizes in both. Let’s unpack why they’re different and how each one matters when you run the Solar PV Sizer wizard.

1. What is kWp?

kWp (kilowatt-peak) is the maximum DC power a solar array can deliver under Standard Test Conditions (STC): panel temperature 25 °C, irradiance 1 000 W/m², and cell spectrum AM1.5. Think of it as the engine size of your array—capacity, not energy.

10 × 600 W panels = 6 kWp array capacity

2. What is kWh?

kWh (kilowatt-hour) is energy: power × time. Your electricity meter counts kWh, your utility bills you per kWh, and our wizard sizes your daily load in kWh.

6 kWp array × 4.5 h of full-sun equivalent = 27 kWh produced

3. Factors that turn kWp into kWh

Sunlight hours: PVGIS gives you site-specific hours of 1 000 W/m² equivalent irradiance.
Orientation & tilt: South-facing at optimal tilt maximises winter yield in the Northern hemisphere. North-facing if you're in the Southern hemisphere.
System losses: Temperature, wiring, inverter efficiency—our backend assumes 14 % total.

4. Why installers oversize panels vs. inverter

A 5 kW inverter paired with 6 kWp of panels is common. Morning and late-afternoon power ramps faster, you clip only a few noon peaks, and the inverter saves cost.

5. How our wizard uses both metrics

You enter monthly kWh → we derive daily kWh.
PVGIS gives site yield (kWh per kWp per year).
Array kWp = Annual load ÷ Yield.
Inverter rounds kWp up to the next whole kW.

6. Quick sanity checks

Annual array kWh should be 4–5 × kWp in sunny climates, 3–4 × kWp in cloudy ones.
If your bill kWh < 365 × (Array kWp × Site yield ÷ kWp), you’re probably net-exporting.

7. Take-aways

kWp = hardware size; kWh = energy you actually pay for.
More kWp raises winter output, not just annual totals.
Always quote both numbers when you compare installer bids.

Ready to see how many kWp you need for your own roof? Head back to the Solar PV Sizer wizard and tweak your monthly kWh until the report meets your goals.