In short: A standard grid-tied solar system shuts down during a blackout, even in full sun, as a safety measure. To keep power on you need a battery plus the right equipment (a hybrid or battery inverter that can “island” your home, and a backup gateway or critical-loads panel). A battery doesn’t power everything for ever: how long it lasts depends on its usable kWh, how many kW your loads draw and whether your panels can recharge it during the outage.
Many people buy solar expecting it to keep the lights on when the grid fails, and are surprised when it doesn’t. This guide explains why, what a battery changes, how to estimate how long it will run, and what to check before you buy. For the basics, start with How Does a Home Solar System Work?
Why solar panels shut down in a blackout
Grid-tied inverters are required by safety standards (such as UL 1741 and IEEE 1547 in the US) to stop exporting power the moment the grid fails. This is called anti-islanding protection. It prevents your system from feeding electricity into lines that utility workers believe are dead and may be repairing.
The result: with a standard grid-tied system and no battery, your panels go quiet during an outage even if the sun is shining. Some specially designed inverters can offer limited daytime power without a battery, but that is the exception and needs specific equipment, so ask your installer.
What a battery changes
When the grid fails, a battery system with backup capability does three things:
- Detects the outage and safely disconnects your home from the grid (islanding).
- Creates a small local power supply for your home, or for the circuits you’ve chosen.
- Lets your solar panels keep working, if the system is designed for it: panels cover your loads during the day and recharge the battery with any surplus.
Exact behaviour depends on the inverter, the battery, the backup gateway and the loads connected, so never assume. Confirm in writing what your quote includes.
What equipment you need for backup
- A battery with backup capability. See Tesla Powerwall vs Enphase IQ vs Franklin for popular models.
- A hybrid or battery inverter that can run in islanded (off-grid) mode.
- A backup gateway or transfer switch that isolates your home from the grid and manages the switchover.
- A critical-loads panel (for partial backup), so only the circuits you choose are powered.
Having a battery doesn’t automatically mean you have backup. Some systems store energy for bill savings only, with no backup wiring. In the UK and elsewhere you may see terms such as “backup circuit” or “EPS output” in quotes, so ask exactly what stays on during an outage.
Whole-home vs partial backup
| Partial (essential loads) | Whole-home | |
|---|---|---|
| What stays on | Chosen circuits: fridge, lights, internet, outlets, maybe a well pump or furnace fan | Most or all circuits, including heavy loads |
| Equipment needed | Battery plus a critical-loads panel | Larger battery capacity and a more powerful inverter, often several batteries |
| Cost | Lower | Higher |
| Runtime | Longer, because fewer loads drain the battery | Shorter unless the battery is large |
| Risk of overload | Lower | Higher if you run big loads together |
| Best for | Most homes | Homes with frequent or long outages and heavy electric loads |
Many homeowners choose partial backup because it lasts longer and costs less. Heavy loads such as electric heating, central air conditioning, electric ovens and EV charging can drain a battery quickly.
How long will a battery last in a blackout?
Runtime (hours) ≈ usable battery capacity (kWh) ÷ average load (kW)
Check the basics in kW vs kWh. Usable capacity is a bit lower than the headline figure, and efficiency losses apply, so treat the result as an estimate.
Illustrative example: a battery with about 13.5 kWh of usable energy.
| Average load | What it might cover | Approx. runtime |
|---|---|---|
| 0.5 kW | Fridge, lights, router, phone and laptop charging | about 27 hours |
| 1 kW | The above plus TV, a few outlets and a furnace fan | about 13 hours |
| 3 kW | Essentials plus a well pump or some cooking | about 4.5 hours |
| 5 kW | Whole-home with heavy loads such as air conditioning | under 3 hours |
Solar recharging can extend these times a lot on sunny days, if your system supports it. On cloudy days or in winter, don’t count on it.
Typical running power of common loads
| Appliance (typical) | Running power |
|---|---|
| LED light | about 10 W each |
| Wi-Fi router | about 10–20 W |
| Laptop | about 50 W |
| Refrigerator | about 100–200 W while running (cycles on and off) |
| Furnace fan | about 400–800 W |
| Sump or well pump | about 500–1,500 W |
| Electric oven or hob | about 2–3 kW |
| Central air conditioning | about 3–5 kW |
| EV charger | about 7 kW |
These are rough figures: check the label on your own appliances. Motors such as pumps and air conditioners also draw a surge when they start, so the inverter’s peak rating matters, not just its continuous kW.
What limits your backup
- Usable capacity (kWh): how much energy is available.
- Inverter power (kW): how many loads can run at once, including start-up surges.
- Which loads are connected: partial vs whole-home.
- Solar recharge: sunny days help; cloudy days and winter reduce it.
- Backup reserve: most systems let you set a minimum charge to hold back for outages. If your battery is set to cycle down to a low level for bill savings, it may have little left when a storm hits.
- Length of the outage: a few hours is easy; several days is hard without a large battery or another source.
Other ways to get backup power
- Generator: lower upfront cost, but fuel-dependent and noisy. It can be combined with solar and a battery for long outages.
- Portable power station: a plug-in battery for a few critical devices. See Best Portable Solar Generators for Home Backup.
- Vehicle-to-home: some electric vehicles and chargers can power a house, but compatibility and cost vary.
- Off-grid system: full independence, at a much higher cost. See Grid-Tied vs Off-Grid vs Hybrid Solar Systems.
Questions to ask your installer
- Is the backup whole-home or partial, and which circuits stay on?
- Will my solar panels keep working and recharge the battery during an outage?
- What are the battery’s usable kWh and the inverter’s continuous and peak kW?
- Can it start my well pump, furnace fan or air conditioner?
- How do I set a backup reserve, and what is the default?
- What equipment is included: gateway, critical-loads panel, wiring and permits?
- How long will it run with my real essential loads? Can you show the calculation?
Before the next outage
- List your essential loads and what they draw.
- Set a backup reserve that matches the outages you worry about.
- Know how to check your system through the monitoring app.
- Test it if your installer says it’s safe to do so, rather than finding out during a storm.
- Keep a plan B for medical or other critical needs, such as a charged portable power station or a generator.
Common mistakes
- Assuming solar panels will work in a blackout without a battery and backup equipment.
- Assuming any battery gives whole-home backup.
- Ignoring the inverter’s kW rating and start-up surges.
- Leaving the battery set to low reserve before a storm.
- Expecting solar to fully recharge the battery on a cloudy day.
- Not asking what exactly stays on during an outage.
Next steps
- Decide how much backup matters to you: Do You Need a Solar Battery?
- List your essential loads and estimate your runtime with the formula above.
- Get quotes that state clearly what the backup covers: How to Compare Solar Quotes and Spot Red Flags.
- Compare battery models and their power ratings: Tesla Powerwall vs Enphase IQ vs Franklin.
Frequently asked questions
Do solar panels work during a power outage?
Not in a standard grid-tied system. The inverter shuts down when the grid fails, for safety. To use solar power during an outage you need a battery with backup capability and the right inverter and gateway, or an off-grid setup.
Can solar panels charge a battery during a blackout?
Yes, if the system is designed for it. The battery system creates a local power supply, and the panels can supply your loads and recharge the battery in daylight. Output depends on weather and season.
How long will a home battery power my house in a blackout?
Divide the usable kWh by your average load in kW. For example, 13.5 kWh at 1 kW lasts roughly 13 hours, but at 5 kW it lasts under 3 hours. Solar recharge can extend this on sunny days.
What is the difference between whole-home and partial backup?
Whole-home backup powers most or all circuits and needs more battery capacity and a more powerful inverter. Partial backup powers only chosen essential circuits, so it’s cheaper and lasts longer.
Do I need a backup gateway?
For most systems, yes: a gateway or transfer switch isolates your home from the grid and manages the switch to battery power. Some systems integrate this into the inverter, so ask your installer what yours includes.
Can a battery run my air conditioner or well pump?
Possibly, but it needs a battery and inverter with enough continuous and surge power. These loads can drain a battery quickly, so ask your installer to confirm with your actual appliances.