Backup explained: Notstrom vs Ersatzstrom

"Backup-capable" sounds good but means very different things — and in an offer often means the actual backup hardware isn't included at all. Here are the types and the right questions.

The two big terms

• Ersatzstrom: a simple, often manual switch-over to a limited emergency supply. Frequently only single-phase and only selected sockets/circuits.
• Notstrom / UPS: an automatic, near-seamless switch-over (ideally 3-phase, whole house), so devices simply keep running through an outage.

The three questions that make the difference

1. Seamless or with an interruption? Seamless (<~20 ms) means the computer/network keeps running without a dropout. A "switch-over time" of seconds means a brief outage; devices restart.
2. Whole house or selected circuits? "Selected circuits" only backs up what you define in advance — not automatically everything.
3. How many amps per phase? Determines which loads can run at once (e.g. is it enough for the heat pump?).

The hidden cost factor

Backup is almost always a separate line item — a backup box plus wiring. "Backup-ready" often just means the system could do it, but the hardware isn't in the price. Check whether the backup box is actually included, and exactly what it powers.

What to watch for

• For genuine whole-house backup you need a 3-phase backup-capable hybrid inverter + backup box, correctly wired.
• Get it in writing: seamless yes/no, whole house or which circuits, amps per phase.
• Clarify whether the heat pump runs on backup (large inrush current!).

In short: Ersatzstrom = simple/manual/often single-phase; Notstrom = automatic/seamless/3-phase. "Backup-capable" without a backup box is not backup. Ask about seamlessness, scope, and amps per phase.

Next, the grid rules that often cause worry but are usually harmless: §9 EEG, §14a and iMSys.