Backup Power Calculator: how long can you bridge a power outage?
The power goes out – how long can you manage with what you have? This calculator tiers the answer into three levels: essentials only, basic needs, and full comfort. Because the biggest reserve isn't in the battery but in deciding what really has to keep running.
Particularly important in winter: a gas or oil heating system only needs its circulation pump and controls, together about 80 W. Even a small power station keeps that going for a whole day.
Result
Essentials last
1.1 days
4 devices, 1,605 Wh/day
- Basic needs
- 1 day
- 5 devices, 1,769 Wh/day
- Comfort
- 21 h 19 min
- 6 devices, 2,075 Wh/day
- Usable energy
- 1,843 Wh
- 90% of rated capacity
- Highest continuous load
- 100 W
- inrush up to 300 W
- Essentials mode adds
- 6 h 15 min
- compared with full comfort
- Fridges and pumps draw up to 300 W at start-up. Your unit's surge rating has to be above that, otherwise it shuts down.
- Almost everything runs through the inverter. Phones, lights and a radio directly on 12 V or USB save the conversion losses and the idle draw – quickly 10–15 % in an emergency.
- Leave the refrigerator and freezer closed at first: they hold 4–6 and 24–48 hours on their own. Only then cool them down for a few hours at a time.
- Connecting heating or a distribution board to a backup source belongs in qualified hands. Grid-tied systems without a backup function shut down during an outage.
Show calculation
- 1Usable energy: 2,048 Wh × 90% = 1,843 Wh
- 2Essentials (4 devices): 1,605 Wh/day → 1.1 days
- 3Basic needs (5 devices): 1,769 Wh/day → 1 day
- 4Comfort (6 devices): 2,075 Wh/day → 21 h 19 min
- 5Running essentials only buys roughly 6 extra hours over full comfort.
How it's calculated
In a power outage not everything keeps running – and that's exactly where the reserve lies. This calculator therefore doesn't give one number but three levels: how long your storage lasts running only the essentials, covering basic needs, or keeping everything going as usual.
| Level | What runs | Typical consumption |
|---|---|---|
| Essentials | Cooling at reduced use, heating pump, lights, phones | 1,000–1,800 Wh/day |
| Basic needs | plus router, laptop, well pump | 1,500–2,500 Wh/day |
| Comfort | plus television, hot plate, other devices | 3,000 Wh/day and more |
usable energy [Wh] = storage × usable fraction
consumption per level [Wh/day] = Σ (power × runtime), AC devices ÷ inverter efficiency
duration [days] = usable energy ÷ (consumption − recharge)Heating is the most important load – and the cheapest
Gas and oil heating systems produce their heat without electricity. What needs power is only the circulation pump and the controls, together about 80 W. A 2 kWh storage unit therefore keeps the heating running for roughly a day. In winter that's the difference between uncomfortable and dangerous. Heat pumps, by contrast, need the full heat output as electricity – they cannot sensibly be run from a power station.
Refrigerator and freezer: do nothing at first
Both hold their cold for a while on their own. A closed refrigerator stays cold enough for about 4 to 6 hours, a full freezer for 24 to 48 hours, a half-full one for roughly half that. What matters is keeping the door shut. Only afterwards is it worth running them for a few hours at a time – which is why the presets use 6 hours of runtime instead of the usual 8 to 12.
Worked example
Single-family home with gas heating and a 2 kWh power station: refrigerator at reduced use, heating pump, LED emergency lighting, phones and Wi-Fi router – without recharging.
Inputs
- Devices during an outage:
- Kühlschrank: 100 W × 6 h/day × 1 1–3 × 1 1/0
- Heizungspumpe: 60 W × 12 h/day × 1 1–3 × 1 1/0
- LED-Notbeleuchtung: 15 W × 5 h/day × 1 1–3 × 0 1/0
- Handys laden: 10 W × 3 h/day × 1 1–3 × 0 1/0
- WLAN-Router: 12 W × 12 h/day × 2 1–3 × 1 1/0
- Fernseher: 90 W × 3 h/day × 3 1–3 × 1 1/0
- Storage capacity: 2,048 Wh
- Recharge per day: 0 Wh
- Usable fraction of capacity: 90 %
- Inverter efficiency: 88 %
Result
1.1 days
Essentials last
- Basic needs
- 1 day
- Comfort
- 21 h 19 min
- Usable energy
- 1,843 Wh
- Highest continuous load
- 100 W
- Essentials mode adds
- 6 h 15 min
Calculation
- Usable energy: 2,048 Wh × 90% = 1,843 Wh
- Essentials (4 devices): 1,605 Wh/day → 1.1 days
- Basic needs (5 devices): 1,769 Wh/day → 1 day
- Comfort (6 devices): 2,075 Wh/day → 21 h 19 min
- Running essentials only buys roughly 6 extra hours over full comfort.
The variables explained
- Devices: power, runtime, level, output
- What should run in an emergency, for how long per day, from which level on, and whether via the inverter (AC) or directly on 12 V.
- Level (1–3)
- 1 = essentials (indispensable), 2 = basic needs, 3 = comfort. This is what creates the tiering in the result.
- Storage capacity (Wh)
- Power station, home battery or a vehicle battery with V2L. 2,048 Wh is a common power station; home batteries hold 5,000–10,000 Wh.
- Recharge per day (Wh)
- What comes in during the outage: a solar panel or generator. Considerably less in winter than in summer.
- Usable fraction (%)
- Power stations deliver 85–92 % of rated capacity. Home batteries often reserve an additional backup share.
Common mistakes
- Switching the fridge back on immediately: closed, it stays cold for 4–6 hours on its own, the freezer for up to two days.
- Counting on a heat pump: it needs the heating power as electricity – not feasible from a power station. Gas and oil systems only need the pump.
- Assuming summer solar yield: in December a panel delivers a fraction of the summer figure – exactly when outages hurt most.
- Overlooking inrush currents: fridges and pumps draw two to three times their rated power at start-up.
- Running everything through the inverter: phones and lights straight on 12 V or USB save the conversion losses and the idle draw.
- Leaving the storage unit empty: a power station only helps if it's charged – top it up every few months.
Assumptions and limits
- Each device's consumption is assumed to be spread evenly over the day.
- The thermal inertia of fridge and freezer is reflected in the preset runtimes (reduced use), not in a separate calculation.
- Recharging is assumed to be evenly distributed; in reality it depends on the weather.
- Temperature effects on storage capacity are not included: in a cold room the battery delivers less.
- Whether your home battery supports backup power at all depends on the unit – many systems disconnect during an outage and deliver nothing.
Frequently asked questions
How long does a power station last in a blackout?
Running essentials – fridge, heating pump, lights and phones – a household needs around 1,600 Wh a day. A 2 kWh power station therefore lasts a good day, a 5 kWh unit almost three days. Add comfort items like a television or hot plate and the time halves quickly.
Will my heating keep running in a power outage?
Gas and oil systems produce heat without electricity but need the circulation pump and controls – together about 80 W, i.e. roughly 1,000 Wh a day at twelve hours of operation. Even a small power station manages that. Heat pumps need the full heat output as electricity and cannot be run this way. Important: connecting heating or a distribution board to a backup source belongs in qualified hands.
How long does a freezer stay cold without power?
A full freezer stays cold enough for 24 to 48 hours if kept closed, a half-full one for roughly half that. A refrigerator lasts 4 to 6 hours. The single most important measure is not opening the door – every opening costs hours. After that it's usually enough to run the appliance for a few hours at a time.
What storage size do I need for a blackout?
About 2 kWh for a day of essentials, around 5 kWh for three days, 10 kWh and more for a week. Far cheaper than a large battery is reducing consumption: staying in essentials mode gets by on a third of the capacity.
Can I use my solar system during an outage?
Usually not: grid-tied inverters shut down during an outage for safety reasons (anti-islanding). Only units with an explicit backup or island function deliver power then – normally through a separate outlet on the battery. Check the data sheet before relying on it.
Is my home battery backup-capable?
Not automatically. Many batteries disconnect completely when the grid fails. Backup-capable units either have an emergency outlet or a full backup changeover for the house connection – the latter requires installation by a qualified electrician.
What does a solar panel contribute during an outage?
A lot in summer: 400 W delivers 1,500–2,000 Wh on a good day and can carry the essentials on its own. In December it's only a few hundred watt-hours – so enter a realistic winter figure when planning for emergencies.
How often does the power actually fail?
In countries with a well-developed grid, average interruption times are measured in minutes per year. Long, widespread outages are rare but not impossible; preparedness guidance typically assumes several days.
Sources and background
- Ready.gov: Power outage preparedness guidance
- Bundesamt für Bevölkerungsschutz und Katastrophenhilfe (BBK): preparing for a power outage
- Manufacturer data sheets of power stations and home batteries (usable capacity, backup function, continuous output)