Hybrid and off-grid solar systems

Hybrid or off-grid?

These are different problems that get discussed as if they were the same one.

Hybrid keeps a utility connection. Solar supplies the load, surplus charges the battery, and the grid covers what neither can. During an outage the system carries a defined set of backup circuits. The battery is sized around how long those circuits must run, not around total consumption.

Off-grid has no utility connection. Everything the site consumes must come from generation and storage, including through several consecutive poor-weather days. Off-grid systems are consequently larger and more expensive per unit delivered, and are chosen when a grid connection is genuinely impractical.

The backup load schedule

This is the part that determines the cost, and the part most often skipped. Before any battery is sized, we write down which circuits must remain live during an outage and for how long. That schedule is an engineering input and a commercial decision at the same time: adding air conditioning to a backup schedule can multiply the battery requirement.

Battery capacity is quoted as an indicative bracket until that schedule exists. Anyone offering a specific battery size before discussing which loads it must carry is guessing.

What actually happens during an outage

A hybrid system does not simply “keep the power on”. It carries a defined set of circuits, and understanding that distinction before installation prevents the most common disappointment in this category.

At the moment supply is lost, the inverter disconnects from the grid — a safety requirement, not a limitation, since exporting into a dead network endangers anyone working on it. It then supplies the backup circuits from the battery, topped up by whatever the array is generating at that moment. When supply returns, the system synchronises and reconnects.

Two consequences follow. First, only the circuits wired to the backup side stay live, which is why the backup load schedule is an installation decision rather than a setting. Second, an outage at 2 a.m. is carried entirely by the battery, while an outage at noon may be carried largely by the array — the same system, very different runtimes.

Sizing a battery honestly

Battery capacity is where most of the money and most of the exaggeration live. Four numbers separate a real estimate from a sales one:

• Nominal versus usable capacity. A bank is rated at a nominal figure; the portion you may actually use depends on chemistry and on the depth of discharge the manufacturer permits. Sizing against nominal capacity overstates runtime.
• Round-trip losses. Energy stored is not energy returned. Charging and discharging both cost something, and so does the inverter.
• The backup load, in watts and in hours. Lights, fans, a router and a refrigerator are a modest load that a small bank carries for a long evening. One air conditioner changes the arithmetic entirely.
• Autonomy, for off-grid. Not “hours of backup” but consecutive poor-weather days the site must survive without generation.

We quote an indicative bracket until the backup schedule exists, and a specific capacity afterwards. A specific battery size offered before that conversation has been had is a guess with a decimal point.

Battery chemistries, in general terms

Two families are in common use, and the choice is an engineering decision rather than a brand preference.

Lead-acid banks cost less to buy, tolerate less depth of discharge, need more usable capacity for the same delivered energy, and have a shorter cycle life. They still make sense where the budget is fixed and the duty is light.

Lithium banks cost more to buy, allow deeper discharge, deliver more cycles, and take less space and weight for the same usable energy. Over a system’s life the cost per usable cycle is usually the more relevant comparison.

Whichever is chosen, the battery needs ventilation, a temperature range it will actually live in, protection sized for its fault current, and a location that is accessible for inspection and eventual replacement. Batteries are the one component of a solar system that is certain to be replaced during its life; designing as though they are permanent is how a replacement becomes an excavation.

Off-grid: a different discipline

Off-grid sizing is governed by the worst month, not the average one. A system that comfortably covers a site in June may leave it dark in December, when generation is lowest and, in many buildings, demand is not.

That leads to three design realities:

• Deliberate oversizing of both array and storage relative to average consumption, so the worst weeks are survivable.
• A generator, or planned load shedding, as an honest part of the design for the days when neither is enough. Pretending otherwise produces systems that fail exactly when they matter.
• Load discipline as an engineering input. On an off-grid site, what you switch on is part of the system design. High-consumption loads — electric heating, large air conditioning, resistive water heating — may need to be redesigned rather than powered.

Common mistakes we are asked to correct

• A backup board that was never separated. Every circuit is on the backup side, so the battery is drained by loads nobody wanted to keep running.
• Air conditioning added to the backup schedule after installation, on a battery bank sized for lights and fans.
• A hybrid inverter specified for a site with no real backup requirement — paying for capability that the site never uses, when a straightforward grid-tied system with net metering would have delivered more for the money.
• Batteries installed somewhere hot, sealed, or unreachable, shortening their life and making replacement a building exercise.
• Generator and inverter fighting each other, because the changeover arrangement was assembled rather than designed.

Interaction with net metering

A hybrid system can still export surplus under net metering, but the system design and the approval process both change. See the net metering service and the LESCO guide.