Backup runtime and load-shedding order
The question is not how long the battery lasts but what you give up and in what order. The calculator takes your appliances in three priority tiers and reports the runtime for each — plus the shedding order. It works from average load over the duty cycle, not from nameplate power.
- Even the essential loads only last 21.3 hours against a target of 24. Shedding load will not close that gap — the battery is too small.
TARGET_NOT_REACHED_ESSENTIAL
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-07-15
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Available energy | C * U_nenn * (SoC_start - SoC_min) * f_alter | 2,048 Wh | exact |
| Energy at the load | E_verfuegbar * eta_Entladepfad | 1,806.3 Wh | exact |
| Runtime of the essential loads | E_load / P_critical | 21.251 h | exact |
- Formula
t = E_available · η_discharge / (Σ P_appliance · duty cycle + P_idle)- Valid for
- Constant average load per tier, battery at room temperature.
- Not covered
- Start-up surges, temperature dependence of capacity, recharging during the outage. For lead chemistries the Peukert effect is not applied here — the Peukert calculator shows how much it matters.
- Data sources
- Aggregate of manufacturer datasheets, the EU energy label database EPREL and measurement series from off-grid practice · retrieved 2026-07-15
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
How long will my battery keep the house running during a blackout?
That depends on what you switch off: the calculator sorts your appliances into three priority tiers and reports a runtime for each — along with a shedding order, most dispensable loads first. It works from average load over the duty cycle rather than nameplate power: a 100 W fridge runs about a third of the time, and calculating with nameplate power underestimates the runtime threefold.
Why does the inverter's idle draw matter so much in a blackout?
The inverter's idle consumption runs whenever any AC appliance is on. In an outage with a 60 W base load, 20 W of idle draw is already a quarter of the consumption — exactly the magnitude that decides whether the battery lasts through the night. The calculator warns when idle draw is a significant share of the essential load.
Does the calculated runtime hold for lead-acid batteries?
Only with a caveat: with lead chemistries the usable capacity falls as discharge current rises, and this Peukert effect is deliberately not applied here — the calculator warns and points to the Peukert calculator, which shows how much it matters. Start-up surges, temperature dependence of capacity and recharging during the outage are also outside the model.
Why do 200 Ah not last 24 hours of basic supply?
Because of the 2,560 Wh of nameplate energy only about 1,806 Wh are actually usable — between start state of charge and cutoff, after discharge-path efficiency. Against the 85 W of the essential tier (heating control, fridge, router, plus 20 W inverter idle) that yields 21.3 hours — just under the 24 h target. The two strongest levers are already in the list: cut the idle (router and cool box on 12 V directly) or raise capacity by a quarter.
How do I read the shutdown order in practice?
Bottom up: with everything on the battery lasts 10.6 hours, without the dispensable tier 13.9, with only the essentials 21.3. The order tells you what to unplug immediately during an outage (kettle, freezer — well insulated it keeps for many hours without power) and what runs until last. Exactly this decision cannot be automated away — the calculator lets you play it through beforehand.
Will my heating keep running in a blackout?
Most gas and oil heating systems only need electricity for control and circulation pump — typically 20 to 150 W together. That is a stroke of luck for backup power: a mid-size battery carries heat supply for days if nothing else runs. Important: the heating must be on the backed-up circuit (electrician), and heat pumps with their kilowatts play in a different league — that is what the heat-pump calculator is for.
How long can I ride out a blackout with solar and a battery?
It depends decisively on the month. A 200 Ah LiFePO4 with a 61 W emergency load lasts 26.7 hours on the battery alone. With 1,000 W of modules in the Berlin region the array covers the emergency load completely in 9 of 12 months — in the worst month the stored energy still lasts 2.2 days. With 2,000 W it is 11 months and 96.9 days in the worst. With only 400 W, just 3 months and 1.4 days.
How much array power do I need for indefinite emergency supply?
More than the daily consumption suggests, because December sets the size. The emergency load in the example is 1,464 Wh a day. In June a small array covers that; in December central Europe needs a multiple, because irradiance drops to a fraction. The calculator therefore reports in how many months the array carries the load — not an annual average, which is useless in an actual outage.
Is a plug-in balcony system enough for emergencies?
In summer for the base load yes, in winter no — and only with a backup-capable inverter. A typical 800 W balcony system sits in the range of the 400 W row above: three months of full coverage. More importantly, most plug-in systems shut down on grid failure (anti-islanding) and then deliver nothing at all. Backup capability is a separate device property, not a question of wattage.
What should I switch off first during a blackout?
The calculator gives the order: the most dispensable tier first, and within a tier the appliance with the largest average load. That is more effective than any saving on an individual device, because runtime rises inversely with total load. Dropping from the comfort tier to the essential tier multiplies runtime in the example — without spending anything.
How long should emergency supply be designed for?
Germany's civil protection agency recommends supplies for 72 hours, and the calculator uses that as the default target. For electricity that is only half the answer: an outage beyond 72 hours is very rare in central Europe, while an outage of a few hours is realistic. Bridging 72 hours without recharging needs a battery sized well beyond the likely case — with PV the calculation changes fundamentally.