Energy basics: kW, kWh, and what "kWp" actually means

Almost every misunderstanding in a PV or heat-pump offer starts with the same mix-up: power and energy get used interchangeably. This piece clears that up first, because everything that follows depends on it.

Power is a speed, energy is an amount

Picture a car. Kilowatt (kW) is like speed — how fast energy is flowing right now. Kilowatt-hour (kWh) is like distance travelled — how much has added up in total, over some period of time.

• A 10 kW PV system running flat-out for one hour delivers 10 kWh.
• The same system running at half power for two hours also delivers 10 kWh.

Power tells you how strong something is right now. Energy tells you what ends up on the bill — or in the battery.

What "kWp" means

kWp (kilowatt-peak) is the nameplate rating of your PV modules — the maximum power they'd produce under standardised lab conditions (Standard Test Conditions, STC: 25 °C cell temperature, a fixed light intensity). It's a sizing figure for the array, not a promise about any particular day.

On a real roof, actual output diverges from the peak figure: clouds, season, sun angle, and the fact that cells run hotter in the field than in a test lab (hotter cells produce slightly less power). That's why kWp is the number arrays are sized and compared by — not the number that shows up directly on your electricity bill.

The mix-up that happens most often: a "10 kWp array" is not the same thing as a "10 kWh battery." One is a power rating for the modules, the other is an amount of energy a battery can store. Both numbers appear side by side in every offer — and still get confused constantly.

Where you'll meet these terms again

• PV array: sized in kWp (the modules), measured in kWh (the annual yield — how much a given array actually produces at your location depends on orientation, tilt, and shading; more on that in the next chapter).
• Battery storage: rated in kWh (its capacity) — more on this in the battery-basics chapter.
• Heat pump: heating output is stated in kW (how much heat it can deliver at most), electricity use in kWh (what ends up on your power bill).
• Your electricity meter: counts kWh — that's the figure that gets billed (or credited, if you export).

A quick reality check for your own house

To make the numbers concrete, rough Germany-wide orders of magnitude:

• Annual consumption of a household without a heat pump/EV: often ~3,500–4,500 kWh.
• A heat pump adds roughly ~3,000–4,000 kWh/year.
• An EV adds roughly ~2,000–3,000 kWh/year.
• A ~15 kWp PV array produces roughly ~14,000 kWh/year in Germany — spread across the year, most of it in summer.

What this means for you

If someone quotes you a bare number — "the system is a 10" — ask whether they mean kW, kWh, or kWp. The three aren't interchangeable, and an offer that keeps them clearly separated is a good early sign.

With this distinction in place, the next chapters — how a PV system actually works, how batteries get sized — will make a lot more sense.