Reference · Chapter 10
Glossary & red flags in offers
The key technical terms explained briefly — and the typical red flags where you should push back.
Glossary & red flags in offers
Two things on one page: the key terms that appear in PV and heat-pump offers, and the typical red flags where it's worth asking questions.
Key terms
kWp vs kWh — kWp (kilowatt-peak) = the PV array's power rating at standard conditions (a sizing metric). kWh = energy (battery capacity, annual yield). Don't conflate a 10 kWp array with a 10 kWh battery.
STC / NOCT — Standard Test Conditions (lab, 25 °C cell) vs. a more realistic operating temperature. Real roofs run hot, so NOCT/temperature behaviour predicts field yield better than STC.
COP / SCOP / JAZ — COP = instantaneous efficiency at one condition; SCOP = standardised seasonal; JAZ (Jahresarbeitszahl) = your actual installed seasonal average. JAZ determines running cost — ask for it at your flow temperature.
Flow temperature (Vorlauftemperatur) — the water temperature the heat pump delivers to radiators/underfloor heating. Lower = more efficient. The biggest single lever on running cost in a retrofit.
Heizlast (DIN EN 12831) — the calculated building heat load that should determine the heat pump's kW sizing. An offer without one is guessing.
Modulation — variable, inverter-driven output. The unit matches demand and runs efficiently at part load; the opposite (on/off) cycles harshly.
LFP vs NMC — battery chemistries. LFP (LiFePO4): safer, longer cycle life, cobalt-free — the home-storage default. NMC: denser but hotter-running and shorter-lived.
DoD / SoH — Depth of Discharge (how much you cycle) and State of Health (remaining capacity vs new). Warranties guarantee a minimum remaining capacity at end of term.
Hybrid inverter — an inverter with integrated battery management and often backup. DC-coupled is the efficient choice for a new PV+battery build.
Notstrom vs Ersatzstrom — Notstrom/UPS = automatic, near-seamless backup (ideally 3-phase). Ersatzstrom = manual, basic, often single-phase. "Backup-ready" often means neither is actually included.
R290 / R32 / GWP — refrigerants. R290 (propane, GWP ~3) = future-proof, efficient, favoured by the F-Gas phase-down; R32 (GWP ~675) current mainstream; R410A (GWP ~2088) legacy.
Monobloc vs split — Monobloc: all refrigerant outdoors, only water enters the house (no on-site refrigerant work). Split: indoor + outdoor units with refrigerant lines between them.
SG-Ready / EEBus / Modbus — open interfaces that let PV, battery, heat pump and energy manager talk for surplus-driven control. Avoid closed cloud-only ecosystems if you value integration.
BEG / Förderung — Germany's efficiency-building subsidy. Heat-pump grants can be large; eligibility depends on efficiency (e.g. JAZ), hydraulic balancing, and eligible equipment. Always compare after-subsidy net cost on a like-for-like basis.
Verfahren B — the detailed (room-by-room) hydraulic-balancing method required for BEG heat-pump funding and good efficiency.
Feed-in tariff / dynamic tariff — payment for exported PV vs time-variable electricity prices. The value of a battery + EMS increasingly comes from optimising against these.
Tier-1 — a Bloomberg bankability ranking of module makers (financing likelihood), NOT a direct quality grade — but it correlates with the maker surviving to honour a 25-year warranty.
Red flags — push back on these
Vague component names — "premium modules", "high-quality inverter", "powerful battery" with NO brand and exact model number. You can't verify a single spec without the model — insist on it.
Price before vs after Förderung mixed — quoting the after-subsidy number as if it were the price, or assuming a subsidy rate you may not qualify for. Always get NET, GROSS, and after-subsidy separately.
Performance vs product warranty conflated — a 25-year "warranty" that's only the output curve, with a short product/workmanship warranty hidden underneath. Ask for both, in years.
For heat pumps, only COP@A7/W35 — the flattering mild-condition figure. Without SCOP/JAZ and the A−7 cold-day COP you can't judge winter running cost.
No Heizlast / no emitter check — sizing by rule of thumb, with no confirmation your radiators work at low flow temp — the classic recipe for a disappointing, expensive-to-run retrofit.
"Backup ready" with no backup hardware — described as backup-capable but the backup box and wiring aren't in scope — you get nothing in a blackout.
Missing balance-of-system — no surge/arc protection, no proper metering, undersized cable, generic racking — corners cut where you can't see them.
High-pressure / "today only" pricing — artificial urgency, large upfront deposits, or lease/PPA terms that obscure the true cost of ownership. Slow down, compare like-for-like.
Subcontracted install, unclear warranty chain — the seller, the installer and the warranty-holder are different entities. Clarify who you call in year 8.
Cloud-only ecosystem — monitoring/control locked to a vendor cloud with no local or open-protocol access — an outage and lock-in risk, and a barrier to home-automation integration.
In short: demand exact model numbers, separated prices (net/gross/after-subsidy), both warranty types in years, a real Heizlast, and documented local access. If any of those is missing, that's your cue to push back.