A hybrid system combines solar, a battery and the grid so the home keeps running during power cuts and, where tariffs allow, uses more of its own solar. It is more complex than an on-grid system: there are more operating modes, a second distribution board, heavy DC currents at low voltage, and a battery that stores a lot of energy in one box. This lesson covers how to put it together safely.
DC-coupled or AC-coupled?
DC-coupled (hybrid inverter): PV strings and the battery both connect to one hybrid inverter. Solar charges the battery directly on the DC side, with one conversion. This is the usual choice for new home systems.
AC-coupled: an ordinary on-grid inverter runs the PV, and a separate battery inverter connects on the AC side. Solar energy going into the battery is converted DC→AC→DC→AC, so a few percent more is lost. It is the natural choice for adding a battery to an existing on-grid system without touching the PV side, and it can scale well on larger sites.
| DC-coupled | AC-coupled | |
|---|---|---|
| Best for | New systems | Retrofits, larger sites |
| Efficiency of solar into battery | Higher | A little lower |
| Number of inverters | One | Two |
| Works during outage | Yes, if inverter has backup port | Yes, if the battery inverter forms the grid and the PV inverter follows it |
The backup circuit
A hybrid inverter normally has two AC ports: - Grid port: connects to the main DB and through it to the net meter. - Backup (EPS/load) port: feeds an essential-load DB that stays live during outages.
Move the agreed essential circuits (lights, fans, sockets for router and fridge) from the main DB to the essential-load DB. Heavy loads (geysers, ACs, pumps) stay on the main DB unless the system was sized for them.
Neutral and earth in backup mode
When the grid fails, the inverter opens its internal grid relay and supplies the backup port on its own. The backup supply then needs its neutral referenced to earth, or RCCBs on the essential-load DB will not work properly. Many hybrid inverters do this with an internal N-E link relay that closes in backup mode; others expect an external arrangement. Read the installation manual and wire exactly as it shows. Never improvise a permanent N-E link in the essential-load DB if the inverter already provides one.
Never connect the backup output to the grid side
The backup port must feed only the essential-load DB. A changeover switch that can connect the backup output to the main DB while the grid is live will destroy the inverter or back-feed the grid. If a manual bypass is needed for servicing, use a proper make-before-break bypass switch designed for the inverter, or a three-position switch with an OFF position.
Operating modes
Most hybrid inverters offer some version of these: - Self-use: solar runs the home first, then charges the battery, then exports. The battery discharges in the evening. - Backup priority: the battery is kept full for outages; solar surplus exports. - Time-of-use: charges and discharges in set time windows, for ToD tariffs. - Off-grid / zero export: no export at all, for sites without net metering approval.
Choose the mode with the customer and write it into the handover notes. A customer on plain net metering who wants backup should usually be on backup priority with a reserve of, say, 50–100%, because self-use cycling saves them nothing (lesson 4) and wears the battery.
Battery cables and protection
Low voltage means high current. A 5 kW hybrid inverter on a 51.2 V battery draws:
I = 5,000 ÷ (51.2 × 0.95) ≈ 103 A at full load
Worked example: cable and fuse
- Design current: 1.25 × 103 = 129 A
- Cable: 35 mm² copper single-core, flexible, (rated around 150 A or more in free air for short runs; check the maker's table for your installation method)
- Voltage drop for 2 m one way (4 m loop): R = 4 × 0.0172 ÷ 35 = 0.00197 Ω; Vd = 103 × 0.00197 = 0.20 V, about 0.4%. Good.
- Protection: a DC-rated fuse or breaker of about 125–160 A, as specified by the battery and inverter makers, fitted within a short distance of the battery terminals. Lithium batteries can deliver thousands of amps into a short circuit, so the fuse must have a high DC breaking capacity (class T or NH-type DC fuses are common).
Use crimped lugs with a hydraulic crimper, heat-shrink, and torque every terminal to the value in the manual. A loose battery lug at 100 A gets hot enough to melt plastic.
Battery safety in Indian homes
Where to put it
- Indoors, shaded, ventilated, between about 15 and 30 °C if possible. Never on a rooftop cabin, in a tin shed or on a west-facing wall in direct sun.
- Not in a bedroom, kitchen, or on the escape route (under the staircase, beside the main door).
- On a solid wall or floor that can carry the weight; above flood level in areas that waterlog.
- Away from water tanks, gas cylinders and fuel.
- With clearances for airflow and service, as the manual specifies.
Chemistry-specific points
- Lead-acid: gives off hydrogen while charging. The room needs ventilation near the ceiling, no sparks nearby, and an acid-resistant floor or tray. Wear goggles and gloves when topping up.
- LFP: no gassing in normal use, but a damaged or overheated cell can vent flammable gas. Never install a dented, swollen or water-damaged pack, and keep combustible stores away.
Fire and emergency
- Fit a suitable fire extinguisher near the battery and DB area, and follow the battery maker's emergency instructions.
- Put a laminated shutdown procedure next to the inverter: open the AC grid MCB, open the backup MCB, switch off the battery breaker, open the PV DC isolator.
- Label the battery with voltage, chemistry and "Do not open".
Key points
- DC coupling suits new hybrid systems; AC coupling suits adding batteries to existing on-grid systems.
- The backup port feeds only the essential-load DB; never connect it to the grid side.
- Follow the inverter manual for neutral-earth bonding in backup mode so RCCBs work.
- 48 V systems carry over 100 A at 5 kW: size cables for 1.25 × current and fit a DC fuse with high breaking capacity close to the battery.
- Install batteries cool, ventilated, away from escape routes, with a posted shutdown procedure.
Check yourself
- Why is AC coupling convenient for adding a battery to an existing system?
- What current does a 3 kW hybrid inverter draw from a 25.6 V battery at full load (95% efficiency)?
- Give three places you should not install a home battery.
Answers
- The existing on-grid inverter and PV wiring stay unchanged; the battery inverter is added on the AC side.
- 3,000 ÷ (25.6 × 0.95) ≈ 123 A.
- Any three of: bedroom, kitchen, escape route or under the stairs, rooftop cabin or tin shed, in direct sun, near gas cylinders or fuel, in a flood-prone basement.