Hybrid (Grid + Battery) System Design
The best of both worlds: export and import like an on-grid system, plus a battery that powers your critical loads through outages and stores midday solar for the evening.
How it works
A hybrid inverter manages three energy sources — PV, battery and grid — and decides every second where power should flow. Loads are split into a backup (critical) panel and normal loads.
Operating modes
The mode you choose drives how you size the battery.
Backup priority
Battery stays full and is used only when the grid fails. Size battery for critical load × outage hours.
Self-consumption
Store daytime surplus, discharge in the evening, import only when the battery is empty. Size battery for evening/night consumption.
Time-of-use (TOU)
Charge when tariffs are cheap, discharge in peak-tariff hours. Size battery for peak-period energy.
AC-coupled vs DC-coupled
| DC-coupled | AC-coupled | |
|---|---|---|
| Architecture | PV and battery share one hybrid inverter on the DC side | Existing grid-tie inverter + separate battery inverter on the AC side |
| Efficiency (PV → battery) | Higher (~95%) — one conversion | Lower (~90%) — DC→AC→DC |
| Best for | New installations | Retrofitting batteries to an existing on-grid system |
| Cost | Lower (one inverter) | Higher (two inverters) |
| Flexibility | Limited by inverter PV input | Very flexible, modular |
Design procedure, step by step
Analyse consumption and its timing
From bills or a smart-meter export, find total daily kWh and how much is used after sunset. A typical home uses 30–50% at night.
E_daily = Monthly kWh ÷ 30 E_night = E_daily × night shareIdentify critical loads
List loads that must run during an outage (lights, fridge, router, fans, medical devices, pumps). Wire them to a dedicated backup sub-panel.
P_critical (kW) = Σ critical loads running together E_backup = P_critical × outage hoursSize the PV array
Same method as on-grid. Add 10–20% if the battery must be fully recharged from solar each day.
PV (kWp) = E_daily ÷ (PSH × PR)Size the battery
Usable (kWh) = E_backup (backup) or E_night (self-consumption) — or the larger of the twoNominal (kWh) = Usable ÷ (DoD × η_round-trip)Check the battery's continuous discharge power ≥ critical load, and that its voltage (low-voltage 48 V or high-voltage 150–500 V) matches the inverter.
Select the hybrid inverter
Inverter (kW) ≥ max( P_critical × 1.25 , PV kWp ÷ 1.25 )Also verify: PV input voltage/current per MPPT (string sizing as on-grid), max battery charge/discharge current, backup (EPS) output rating, transfer time (<20 ms for UPS-like behaviour), and export-limit (zero-export) capability via CT sensor.
Protection & wiring
Everything from on-grid (DC isolator, SPD, MCB, RCD, earthing) plus battery fuse/breaker, battery communication cable (CAN/RS485 to BMS), CT clamp at the grid entry, and a manual bypass switch for the backup panel.
Worked example
A home uses 400 kWh/month, 40% at night. Critical loads 1.5 kW, outages up to 4 h. PSH 5, PR 0.78, LFP battery (DoD 0.9, η 0.92), 550 Wp modules.
| Step | Calculation | Result |
|---|---|---|
| Daily energy | 400 ÷ 30 | 13.3 kWh/day |
| PV array | 13.3 ÷ (5 × 0.78) = 3.42 → 7 × 550 | 3.85 kWp |
| Backup energy | 1.5 kW × 4 h | 6.0 kWh |
| Evening energy | 13.3 × 0.40 | 5.3 kWh |
| Battery (nominal) | max(6.0, 5.3) ÷ (0.9 × 0.92) | 7.2 kWh → 51.2 V 150 Ah (7.7 kWh) |
| Hybrid inverter | max(1.5 × 1.25, 3.85 ÷ 1.25) | 3.1 kW → choose 3.6–5 kW |
Choosing a slightly larger inverter (e.g. 5 kW) leaves room to add panels and an EV charger later — but check that your utility's export limit allows it.
Hybrid sizing calculator
Explore how operating mode and critical loads change the battery size.
Hybrid design checklist
Hybrid quiz
1. What is the main advantage of hybrid over on-grid?
2. Which architecture is best for adding a battery to an existing on-grid system?
3. Critical load 2 kW, outage 3 h, LFP DoD 0.9, η 0.92. Nominal battery?
4. What does the CT clamp at the grid entry do?
5. In self-consumption mode, the battery is mainly sized for…