Three ways to set up solar
There are three common ways to connect a home solar system:
- Off-grid, or stand-alone: no connection to the utility. Panels, batteries and often a generator supply everything.
- Grid-tied: connected to the utility, usually with no batteries. Solar power runs the house first, and the grid fills any gap.
- Hybrid: connected to the grid and with batteries. It can store solar power and keep some loads running in an outage.
Text description
Three block diagrams side by side, showing power flows only, not wiring. Off-grid, with no utility connection: solar panels feed a charge controller, which charges a battery bank; an inverter-charger turns battery power into household power for the house loads, and a backup generator can charge the battery through the inverter-charger. In an outage there is no grid to lose; it keeps running from the battery. Grid-tied, usually with no battery: solar panels feed an inverter, which supplies the house loads; power flows both ways between the house and the utility grid. In an outage it shuts off, even in full sun, to protect line workers. Hybrid, a grid connection plus a battery: solar panels feed a hybrid inverter, which charges and draws from a battery bank, supplies the critical loads, and exchanges power with the utility grid. In an outage the critical loads keep running from the battery. A note says every system needs an electrical permit and inspection and a licensed installer, and grid-connected systems need utility approval.
The right choice depends on whether the grid reaches your land, what a connection costs, and how much you value backup.
The parts of an off-grid system
An off-grid system has a few main parts:
- Solar panels make DC power.
- A charge controller manages the flow from panels to batteries so they are not overcharged.
- The battery bank stores energy.
- The inverter turns DC into AC for the house. Many off-grid inverters are inverter-chargers that can also charge batteries from a generator.
- The load center, or breaker panel, splits power into circuits, each protected by a breaker.
- A backup generator tops up the batteries in long cloudy spells.
Name each numbered part of the off-grid system.
Text description (gives the answers)
A block diagram of an off-grid solar system. Solar panels (1) send direct current to a charge controller (2), which charges the battery bank (3) safely. An inverter (4) turns the battery’s direct current into household alternating current for the load center, or breaker panel (5), which feeds the house circuits. A backup generator (6) connects to the inverter-charger to recharge the batteries during long cloudy spells. This shows how power flows, not how to wire it; wiring is work for a licensed electrician.
Off-grid
An off-grid system must cover every kilowatt-hour you use, in every season. It is sized for the worst month, usually midwinter, with batteries for several days.
It makes sense when the grid is far away. Bringing power lines to remote land can cost a lot. DOE puts it at about $15,000 to $50,000 per mile, depending on terrain. Ask the utility for a quote.
The trade-off is that you are the utility. You manage the battery, the generator and the loads. When something breaks, the power stays off until you fix it.
Grid-tied
A grid-tied system sends extra solar power to the grid by day and draws from it at night. It is usually the lowest-cost way to use solar where the grid already exists. No battery is needed.
Many utilities credit you for power you send out. This is often called net metering. The rules and credit rates vary by state and utility, and they change over time.
Here is the surprise for many people: a standard grid-tied system shuts off in a power outage. This is a safety rule. It stops the system from sending power into lines that utility workers think are dead. So solar alone does not give you backup.
Text description
Three small block diagrams of a grid-tied solar system with no battery. On a sunny day, solar power runs the house first, and extra power flows out to the grid; many utilities credit this, often called net metering, under rules that vary. At night there is no solar, and the grid supplies the house. In a grid outage, the inverter shuts off, even in full sun, so no power flows anywhere; this protects utility line workers. A hybrid system with a battery is the option for backup.
Hybrid
A hybrid system adds batteries to a grid-tied system. When the grid goes down, it disconnects from the grid and runs selected loads from the battery and panels.
Most hybrids back up only some circuits, called critical loads, such as the fridge, lights, well pump and internet. Running a whole house from a battery takes a much larger system.
Hybrids cost more than grid-tied systems. They make sense where outages are common or long, or where utility rules reward storing your own power.
Which system fits
Which system type does each statement describe best?
- Lowest-cost way to use solar where the grid already reaches
- Often includes a generator for long cloudy spells
- Must be sized for the cloudiest winter week
- Has batteries and an agreement with the utility
- Goes dark when the utility power fails
- Keeps the fridge and well pump running in an outage, with the grid as backup the rest of the time
Rules and approvals
Every type needs an electrical permit and inspection. Grid-connected systems also need the utility’s approval, often called an interconnection agreement, before you switch on.
The National Electrical Code sets rules for solar and battery systems, and your state and town adopt their own version of it. Some places also have rules about how far panels sit from roof edges, for firefighter access.
Plan for this time and cost. Permits and utility approval can take weeks to months.
Pro and permit
Check
A family on the grid installs solar panels with no batteries so they will have power in storms. What happens in the next outage?
Summary
- Off-grid stands alone and is sized for the worst month. Grid-tied uses the grid as a battery. Hybrid adds batteries for backup.
- Standard grid-tied solar shuts off in an outage.
- Hybrids usually back up only critical loads.
- Every system needs permits and inspection. Grid systems also need an interconnection agreement.
Go deeper: why grid-tied inverters switch off
When the grid fails, line workers need the wires to be truly dead. A solar system that kept pushing power into the lines could shock them. This is called islanding. Grid-tied inverters are tested to detect a lost grid and stop within a fraction of a second. Hybrid systems use a transfer device to separate the house from the grid before they power it from the battery. That separation is the critical part, and it is why backup systems must be installed and inspected by professionals.