Off-grid vs hybrid solar systems: architecture and sizing compared

"Off-grid" and "hybrid" are often used loosely in marketing material, but they describe fundamentally different system architectures with different design goals, cost structures, and failure modes. An off-grid system is designed to meet 100% of a site's electricity demand from local generation and storage, with no utility connection at all. A hybrid system is grid-connected but includes battery storage and, in many designs, backup capability, so that it behaves like a grid-tied system most of the time and like a limited off-grid system during an outage.

Choosing between them is rarely a matter of preference; it is usually dictated by whether a grid connection exists or is economically reachable at all. This article compares the two architectures, explains how autonomy is sized for off-grid systems, covers generator backup integration, and details what actually happens to a hybrid system during a blackout. It builds on the sizing approach in home battery storage explained and the grid interconnection rules discussed in grid connection and export rules explained.

Architecture comparison

An off-grid system has no utility connection and must supply all demand from PV, battery storage, and typically a backup generator, sized for the worst realistic combination of low solar yield and high demand. A hybrid system remains connected to the utility grid, uses the grid as the primary backup for extended low-generation periods, and adds a battery mainly for self-consumption and, if specified, a limited backup circuit for outages.

The economics differ correspondingly: an off-grid system is only cost-competitive against the alternative of extending a grid connection over distance, since PV, battery, and generator capacity all have to be oversized to cover worst-case conditions rather than average conditions. A hybrid system's battery, by contrast, only needs to cover typical daily shifting needs, because the grid remains available as an unlimited backup source whenever generation and storage fall short.

Sizing for autonomy

Off-grid systems are sized around a target number of "autonomy days" — the number of consecutive days the system must supply full demand with no solar contribution at all, typically driven by a run of overcast winter days. This is a fundamentally different design target from a hybrid battery, which is sized around a single evening's typical load.

A practical off-grid sizing sequence:

1. Establish average daily energy demand (kWh/day) from a full annual load list, including seasonal variation.
2. Choose an autonomy target, commonly 2-5 days for residential off-grid, based on regional weather persistence and acceptable risk of load shedding.
3. Size battery usable capacity as daily demand x autonomy days, then add the DoD margin appropriate to the chemistry (see LFP vs NMC batteries and battery degradation and cycle life for how DoD affects long-term capacity).
4. Size the PV array against the worst-month solar resource, not the annual average, since an off-grid system that is only sized for annual average yield will regularly fail to recharge fully in winter.
5. Size a backup generator to cover the gap between worst-case PV yield and demand during extended low-irradiance periods, since battery autonomy alone cannot economically cover multi-week low-yield periods.

Worked example

A rural off-grid cabin has average daily demand of 8 kWh, with a target autonomy of 3 days and a decision to run the battery at 80% DoD on an LFP pack (95% usable-capacity fraction within that DoD window).

• Raw energy needed for autonomy: 8 kWh/day x 3 days = 24 kWh
• Battery capacity required at 80% DoD: 24 kWh / 0.80 = 30 kWh usable-designed capacity
• Nameplate capacity accounting for the 95% usable fraction within that window: 30 / 0.95 = ~31.6 kWh, rounding up to a 32 kWh product tier

For the PV array, if the worst-month average daily irradiation yields only 2.2 peak sun hours (a realistic winter figure for many mid-latitude European sites) and the system needs to fully replace the 8 kWh daily demand plus recharge some autonomy buffer, a conservative sizing target of 6 kWp of PV (6 kWp x 2.2h x ~0.80 system efficiency = ~10.6 kWh/day in the worst month) provides a margin over bare average demand, with the generator covering any residual shortfall during unusually long low-irradiance runs. This kind of worst-month-driven oversizing is the main reason off-grid PV arrays are typically 2-3x larger relative to daily demand than grid-tied arrays sized against annual yield.

Generator backup integration

Nearly all off-grid systems include a backup generator, sized to cover both battery recharge during extended low-solar periods and any load that exceeds inverter or battery discharge capacity. The generator is normally integrated through the inverter's AC input, allowing the inverter to use generator power to simultaneously supply load and recharge the battery, rather than running the generator directly against the load alone.

Generator sizing follows a different rule from solar: it should cover the larger of (a) peak simultaneous load plus battery charge current, or (b) the minimum runtime efficiency point of the generator itself, since running a generator lightly loaded for long periods causes incomplete combustion (wet-stacking) and accelerated wear in diesel units. A generator sized purely to match peak load, with no charging margin, will run for many more hours per year than one sized with charging headroom, since it will never be able to both supply load and meaningfully recharge the battery at the same time.

What happens to a hybrid system during a blackout

This is the most commonly misunderstood aspect of hybrid systems. A hybrid inverter connected to the grid, even one with a battery, will not automatically continue supplying the house during a grid outage unless it has backup/islanding hardware specifically installed: an automatic transfer switch and inverter firmware capable of grid-forming operation once disconnected. Anti-islanding protection required under IEC 62109-2 and equivalent grid codes mandates that inverter-based generation disconnects from a de-energised grid, both to protect utility workers and to prevent uncontrolled back-feeding.

Where backup hardware is fitted, it almost always covers only a pre-wired "critical loads" sub-panel rather than the full household distribution board, because the battery's continuous power rating (commonly 3-6 kW) is far below the instantaneous demand of a household running an oven, electric shower, and other high-draw appliances simultaneously. Correctly specifying which circuits sit on the critical loads panel — typically refrigeration, some lighting, networking, and perhaps a well pump or heating circulation pump — is an installation design decision, not something the battery handles automatically.

Reliability and maintenance implications

Off-grid systems place far greater maintenance responsibility on the owner, since there is no utility fallback if a component fails: generator servicing, battery state-of-health monitoring, and PV array maintenance all directly determine whether the site has power. Hybrid systems shift most of that reliability burden back onto the grid operator for the majority of operating hours, with the battery and any backup circuit only mattering during outages, which are comparatively rare in most developed grids (typically well under 0.1% of annual hours in most EU distribution networks, though rural and weather-exposed areas see materially more).

Standards and references

• IEC 62109-1/-2 — safety requirements for power converters in photovoltaic systems, including the anti-islanding and grid-forming behaviour relevant to backup operation.
• IEEE 1547 — interconnection and interoperability requirements for distributed energy resources with the grid, covering anti-islanding and ride-through behaviour (US context, broadly analogous to EU grid codes).
• IEC 62548 — design requirements for PV arrays, including array sizing and protection relevant to both off-grid and hybrid systems.
• IEC 60364-7-712 — low-voltage electrical installation requirements for PV power supply systems, applicable to wiring of off-grid and hybrid installations alike.
• ISO 8528 (relevant series) — reciprocating internal combustion engine driven generating sets, referenced for generator sizing and performance rating in off-grid backup applications.

Sources

• NREL — Distributed Generation and Microgrids research
• Fraunhofer ISE — Off-grid and Hybrid Systems
• IEA PVPS
• PVEducation — Standalone PV systems

Frequently asked questions

Will my hybrid solar and battery system keep my house running during a power cut?

Only if it includes dedicated backup/islanding hardware — an automatic transfer switch and grid-forming inverter capability — and even then, typically only for a pre-wired critical loads sub-panel rather than the whole house. A standard grid-tied hybrid system without this hardware shuts down during an outage for the same anti-islanding safety reasons that apply to any inverter-based generation connected to the grid.

How many days of autonomy should an off-grid system be sized for?

Most residential off-grid systems in temperate climates target 2-5 days of autonomy, chosen based on how persistent low-irradiance weather runs are in the specific region and how much risk of temporary load shedding the household is willing to accept. Sites in regions with long, reliably overcast winter periods often size toward the higher end, accepting the higher upfront battery and PV cost that autonomy requires.

Do off-grid systems need a generator?

Nearly always, yes, unless the site has an unusually generous PV and battery allowance relative to demand. Battery autonomy is economical for a few days of low generation, but covering weeks of unusually poor solar resource purely with battery and PV capacity is rarely cost-effective compared with a backup generator that only runs occasionally.

Is it cheaper to go off-grid or connect to the utility grid?

For any site within reasonable distance of existing grid infrastructure, grid connection is almost always cheaper than a fully autonomous off-grid system, because off-grid PV, battery, and generator capacity all have to be oversized for worst-case conditions rather than average conditions. Off-grid systems become cost-competitive mainly where the cost of extending a grid connection over distance is very high, such as remote rural sites.

What is the difference between "hybrid" and simply "grid-tied with a battery"?

They generally describe the same thing: a system connected to the utility grid, using a hybrid inverter capable of managing PV, battery, and grid connections together. The term "hybrid" specifically emphasises the inverter's ability to combine these functions, as opposed to older configurations that used entirely separate inverters for grid-tied PV export and battery charging, which is closer to what is now called AC-coupled retrofitting.