Solar Power Outage Backup: Which Systems Keep Your Lights On?

A common misconception is that solar panels keep your home powered during a blackout. In reality, most grid-tied solar systems shut down completely when the grid goes down. Whether your solar system provides backup power depends entirely on the inverter type and system architecture you have installed.

This guide explains the three solar system types — grid-tied, hybrid, and off-grid — and how each behaves during a power outage. You will learn about critical load panels, battery sizing for backup, generator integration, and which configuration is right for your home.

Grid-Tied Solar Systems: Why They Shut Down During Blackouts

A standard grid-tied solar inverter is designed to synchronise with the utility grid's voltage and frequency. It exports excess solar power to the grid and imports power when the sun is not shining. This design includes a mandatory safety feature called anti-islanding.

Anti-islanding detects when the grid goes down and immediately shuts off the inverter within milliseconds. This protects utility line workers who may be repairing power lines — they need to know that the lines are de-energised. Without anti-islanding, your solar panels could backfeed into the grid and electrocute a lineman.

Even with the sun shining brightly, a standard grid-tied solar system will not produce any power during a blackout. You will be in the dark just like your non-solar neighbours — unless you have battery storage and a hybrid or off-grid inverter.

Hybrid Inverters with Battery Backup

A hybrid inverter combines solar and battery management in one unit and can operate in both grid-tied and off-grid modes. During normal operation, it behaves like a grid-tied inverter — exporting excess, importing when needed, and charging the battery. When the grid fails, the hybrid inverter disconnects from the grid via an internal transfer switch and creates its own island of power using the battery bank and available solar generation.

This is what enables backup power. The inverter senses the grid outage, opens its grid-side relay to prevent backfeed, and continues powering your loads from the battery. If solar is available during the outage, the inverter also charges the battery from the panels while running your loads — extending backup duration indefinitely as long as the sun shines.

For a detailed comparison of hybrid inverter models, battery chemistry compatibility, and sizing guidance, see our Hybrid Inverters Guide.

Off-Grid Solar Systems: Always in Backup Mode

Off-grid solar systems operate completely independently of the utility grid. By definition, they are always in backup mode. An off-grid inverter (often called a stand-alone inverter or battery inverter) creates its own AC waveform from the battery bank. Solar panels charge the battery through a charge controller, and the inverter draws DC from the battery to produce AC power for your loads.

Off-grid systems are the most reliable for continuous backup because they do not need to detect a grid outage or switch modes. They simply keep running. However, they require a properly sized battery bank and solar array to meet all loads at all times — there is no grid to fall back on during cloudy periods or at night. This means the system must be sized for the worst-case scenario: minimum sun, maximum load.

Off-grid systems are ideal for remote cabins, rural homes in areas with unreliable grids, or anyone who prioritises energy independence. The trade-off is cost: you need a larger battery bank and solar array compared to a hybrid system that can rely on the grid 95% of the time.

The Critical Load Panel: Separating Essential from Non-Essential

Whether you choose a hybrid or off-grid system, you cannot simply connect the inverter output to your main breaker panel during an outage. There are two reasons: safety and capacity. Your main panel likely has large loads (water heater, EV charger, central AC) that would instantly drain the battery or overload the inverter. And backfeeding the main panel through the house wiring when the utility expects a disconnect is dangerous.

The solution is a critical loads sub-panel (also called a backup loads panel or essential loads panel). This sub-panel is wired directly to the inverter's backup output port. Only essential circuits are moved to this panel:

• Always include: LED lighting circuits, refrigerator, freezer, well pump or water pump, WiFi router, security system, phone charging outlets, one general-purpose outlet circuit.
• Consider including: Gas furnace controls (low power), gas water heater circulator pump, garage door opener, TV and entertainment.
• Do not include: Electric water heater, electric oven/range, central air conditioning, EV charger, pool pump, sauna, large resistive heaters.

The sub-panel approach keeps the installation code-compliant, ensures the inverter never sees loads larger than its backup rating, and prevents accidental backfeed to the grid. Most modern hybrid inverters accept a separate AC input for the grid and a separate AC output for the backup panel, with automatic transfer switching built-in.

Battery Sizing for Backup Power

Battery sizing for backup starts with two questions: how many watts do you need, and for how many hours? The product of these gives you the required kilowatt-hours (kWh) of usable energy.

Example: A home running lights (0.2 kW), refrigerator (0.3 kW), internet (0.02 kW), fans (0.2 kW), and a TV (0.15 kW) totals 0.87 kW. For an 8-hour overnight outage: 0.87 × 8 = 6.96 kWh of usable energy needed.

But batteries are not 100% usable. You must account for depth of discharge (DoD) and round-trip efficiency (RTE). Lithium LiFePO4 batteries offer 90% DoD and 95% RTE, while tubular lead-acid batteries offer 50% DoD and 80% RTE. The actual battery bank capacity required is:

Lithium requires roughly half the physical capacity of tubular lead-acid for the same usable backup energy. At 48V, a 200 Ah lithium battery (9.6 kWh) covers this scenario, while tubular needs 400 Ah to achieve the same usable energy. See our Solar Battery Size Calculator for an interactive version of this calculation with variable voltage options.

Your battery bank voltage also affects the Ah rating. A 12V system requires 4 times the Ah of a 48V system for the same kWh. For backup systems over 2 kWh, always use 48V to keep cable sizes manageable and inverter efficiency high. Our Battery Bank Voltage Guide explains the trade-offs between 12V, 24V, and 48V architectures.

Generator Integration for Extended Outages

No battery bank is infinite. During extended blackouts lasting multiple days — such as after storms, natural disasters, or grid infrastructure failures — even a large battery bank can be depleted. Generator integration solves this by allowing the inverter to charge the battery bank from a backup generator when solar is insufficient.

Most hybrid and off-grid inverters include a generator input terminal (AC input). When the battery state of charge drops below a programmable threshold (typically 20-30%), the inverter sends a start signal to the generator, which turns on and charges the batteries through the inverter's built-in charger. Once the battery reaches a high threshold (typically 80-90%), the inverter signals the generator to stop.

Generator integration best practices:

• Size the generator to 1.5x the inverter's battery charging current. A 5 kW hybrid inverter with 100A battery charging at 48V needs about 5 kW of generator output.
• Use an inverter generator (not a conventional synchronous generator) for clean power delivery and lower fuel consumption. Inverter generators produce a stable sine wave that hybrid inverters can synchronise with reliably.
• Install a manual transfer switch or interlock if the generator also needs to power loads directly (bypassing the inverter). This is common when the inverter is being serviced or the battery bank is completely depleted.
• Program a generator cooldown period (typically 2-5 minutes) before shutdown to prevent thermal shock to the generator engine.

Which System Should You Choose?

The right backup solution depends on your outage frequency, budget, and energy independence goals.

For most homeowners in areas with moderate grid reliability, a hybrid inverter with 5-10 kWh of lithium battery storage is the sweet spot. It provides backup for essential loads during typical outages, enables solar self-consumption savings every day, and can be expanded later with additional batteries or a generator. If you already have solar panels, adding an AC-coupled battery system like the Tesla Powerwall or a third-party 48V battery inverter is the most practical retrofit path.