Customers who live with power cuts want solar that keeps the fans running. That means a battery and an inverter that can run without the grid. This lesson explains how hybrid and off-grid inverters behave, so you can set the right expectations and wire them correctly. Battery chemistry and sizing are covered in module m07.
The hybrid inverter's four ports
A hybrid inverter connects to:
- PV array through its MPPT inputs, like an on-grid inverter
- Battery through a bidirectional charger
- Grid through relays (it can import and export)
- Backup (EPS) port feeding a separate essential-load distribution board
When the grid is healthy, it powers the house, charges the battery and exports surplus. When the grid fails, anti-islanding still opens the grid relay, but the inverter now forms its own small grid on the backup port, fed from PV and battery. Loads on the normal board, which hang off the grid side, go dark.
Operating modes
Most hybrids offer some version of these settings:
| Mode | Behaviour | When to use |
|---|---|---|
| Self-use (self-consumption) | Solar runs loads, surplus charges battery, battery runs evening loads, grid fills gaps | Low export rates, ToD tariffs |
| Backup priority | Battery held near full; discharges only in outages | Frequent long cuts |
| Time-of-use | Charge and discharge on a schedule | Time-of-day tariffs (module m08) |
| Feed-in priority | Export first, battery second | Rarely sensible under Indian export rates |
| Off-grid | Ignores the grid | Unreliable or no grid |
Set a minimum state of charge reserve (for example 20–30%) in self-use mode, so there is always something left for an evening outage.
Transfer time
When the grid fails, the backup port is unpowered for a short moment while the inverter takes over.
- Many hybrids switch in about 10–20 milliseconds: lights and fans will not notice; a desktop computer may restart.
- Some models claim "UPS-grade" changeover under 10 ms, or seamless (0 ms) transfer.
- Check the datasheet figure and the test condition. For desktop PCs, medical devices and servers, keep a small dedicated UPS.
Battery voltage classes
- Low-voltage (LV) systems, usually 48 V nominal: common in homes; batteries are easy to add in parallel; high DC currents (100 A or more) need heavy cables and proper fusing.
- High-voltage (HV) systems, typically 150–600 V: stacked battery modules, lower current, higher efficiency; battery and inverter must be a compatible, approved pair.
Always use a battery listed as compatible by the inverter maker, with a working communication link (CAN or RS485) between the battery's BMS and the inverter.
Off-grid inverters and PCUs
Where there is no grid, or the customer does not want export:
- An off-grid inverter or PCU (power conditioning unit) runs everything from the battery, with solar charging it. Most can also charge from the grid or a generator when available.
- The charge controller may be MPPT (efficient, allows higher-voltage strings) or PWM (cheaper, requires array voltage close to battery voltage, and wastes more energy). For anything beyond small rural kits, choose MPPT.
- Common battery voltages are 12, 24 and 48 V. Higher system voltage means lower current and thinner cables.
- Older "solar UPS" products are home inverters with a solar charger added; they are fine for small loads but are not grid-interactive and cannot net meter.
Worked example: sizing backup output in Chennai
A family in Chennai gets one or two cuts a day, each up to 3 hours. They want these loads on backup:
| Load | Quantity × watts | Total |
|---|---|---|
| Ceiling fans (BLDC) | 6 × 35 W | 210 W |
| LED lights | 10 × 10 W | 100 W |
| Refrigerator | 1 × 150 W running | 150 W |
| Wi-Fi router and TV | 20 W + 100 W | 120 W |
| Running total | 580 W |
Inverter backup rating: the backup port must carry 580 W continuously and the refrigerator compressor's starting surge, often three to five times its running power for a fraction of a second (about 450–750 W at start). A hybrid with a 3 kW backup output and a short-time surge rating above that is comfortable.
Battery energy: 580 W × 3 h = 1.74 kWh. Allowing 90% usable depth of discharge for LFP and about 95% inverter efficiency: 1.74 ÷ (0.9 × 0.95) = 2.0 kWh. A 48 V, 100 Ah LFP battery (about 5 kWh) covers that with a healthy reserve for longer cuts and self-use in the evening. Module m07 goes deeper.
Switching from old 75 W fans to 35 W BLDC fans halved the fan load here. Suggest efficient loads before adding battery capacity; it is the cheapest backup.
Comparison at a glance
| On-grid | Hybrid | Off-grid / PCU | |
|---|---|---|---|
| Battery | No | Yes | Yes |
| Exports to grid | Yes | Yes | No |
| Works in outage | No | Backup port only | Yes |
| Transfer time | n/a | About 10–20 ms typical | n/a (always on battery) or a few ms if grid-charging |
| Net metering | Yes | Yes | No |
Key points
- A hybrid inverter has PV, battery, grid and backup ports; in an outage only the backup port stays live.
- Choose the operating mode with the customer and set a minimum battery reserve.
- Typical transfer times are about 10–20 ms; sensitive equipment still needs a small UPS.
- LV (48 V) batteries suit most homes; HV batteries need a compatible, approved inverter pairing.
- Size backup output for running load plus motor surge, and reduce loads with efficient appliances first.
Check yourself
- Why do the ACs go off during an outage in a hybrid home while the fans keep running?
- What does a "minimum state of charge" setting protect against?
- Backup loads total 800 W for 2 hours. Roughly how much LFP battery energy is needed at 90% DoD and 95% inverter efficiency?
Answers
- The ACs are on the normal board, which is fed from the grid side; the fans are on the backup board fed from the inverter's backup port.
- It keeps energy in reserve so the battery is not empty when an outage arrives, especially in self-use mode.
- 800 × 2 = 1.6 kWh ÷ (0.9 × 0.95) ≈ 1.9 kWh.