On-Grid (Grid-Tied) System Design
The simplest and cheapest PV system: no batteries. Solar powers your home first, surplus is exported, and the grid fills any gap — day or night.
How it works
The grid-tie inverter synchronises with the grid's voltage and frequency, pushing solar current into your distribution board. Your loads draw from solar first; the net meter records whatever flows in or out.
Advantages
- Lowest cost per kWp — no batteries
- Fastest payback (often 3–6 years with net metering)
- Almost no maintenance
- The grid acts as an unlimited "virtual battery"
Limitations
- No power during outages — anti-islanding shuts the inverter down
- Needs utility approval and net-metering agreement
- Savings depend on export tariffs
- Export limits may cap system size
Design procedure, step by step
Find your energy consumption
Take 12 months of bills and average the units (kWh). Use the annual average for net-metered systems.
E_daily (kWh/day) = Monthly units (kWh) ÷ 30Get Peak Sun Hours and losses
Look up PSH for your city (e.g. NASA POWER, PVGIS, NREL). Apply a Performance Ratio (PR) of 0.75–0.80 to cover temperature, dust, mismatch, cable and inverter losses.
Size the PV array
PV (kWp) = E_daily ÷ (PSH × PR)No. of modules = PV (W) ÷ Module rating (Wp) → round upCheck roof area: about 5–6 m² per kWp for modern 20%+ efficient modules.
Choose the inverter
Pick an inverter whose AC rating gives a DC/AC ratio of 1.1–1.3. Slight oversizing of the array increases morning/evening output with minimal clipping.
Inverter (kW AC) ≈ PV (kWp) ÷ 1.1…1.3Design the strings
Modules in series add voltage. The string must stay below the inverter's max DC voltage on the coldest morning and above the MPPT minimum on the hottest afternoon.
Voc_cold = Voc × [1 + β_Voc × (T_min − 25)] → N_max = ⌊V_max,inv ÷ Voc_cold⌋Vmp_hot = Vmp × [1 + β_Vmp × (T_cell,max − 25)] → N_min = ⌈V_MPPT,min ÷ Vmp_hot⌉Also check: string Isc × 1.25 ≤ inverter MPPT max input current; parallel strings on one MPPT must be identical.
Size cables and protection
Voltage drop (%) = (2 × L × I × ρ) ÷ (A × V) × 100 — keep DC ≤ 2%, AC ≤ 3%DC side: string fuses (if ≥3 strings in parallel), DC isolator, Type 2 DC SPD. AC side: MCB rated ≥ 1.25 × inverter output current, RCD (Type A/B per inverter), AC SPD, earthing of frames and inverter.
Grid approval & net metering
Submit single-line diagram, datasheets and inverter certification (e.g. IEC 62109, IEC 61727 / IEEE 1547 / UL 1741) to your utility. Install the bidirectional meter after approval.
Worked example
A home uses 300 kWh/month, PSH = 5, PR = 0.78, using 550 Wp modules (Voc 49.6 V, Vmp 41.6 V, β_Voc −0.27 %/°C, β_Vmp −0.35 %/°C). Inverter: 3 kW, max 600 V DC, MPPT 120–550 V.
| Step | Calculation | Result |
|---|---|---|
| Daily energy | 300 ÷ 30 | 10 kWh/day |
| Array size | 10 ÷ (5 × 0.78) | 2.56 kWp |
| Modules | 2560 ÷ 550 = 4.66 → 5 (we choose 6 for a 1.1 DC/AC ratio) | 6 × 550 W = 3.3 kWp |
| Inverter | 3.3 ÷ 1.1 | 3 kW |
| Voc at 5 °C | 49.6 × (1 + 0.0027 × 20) | 52.3 V |
| Max in series | ⌊600 ÷ 52.3⌋ | 11 |
| Vmp at 70 °C cell | 41.6 × (1 − 0.0035 × 45) | 35.0 V |
| Min in series | ⌈120 ÷ 35.0⌉ | 4 |
| String layout | 1 string of 6 → Voc_cold 314 V, Vmp_hot 210 V ✔ | 1 × 6 |
| Annual yield | 3.3 × 5 × 0.78 × 365 | ≈ 4,700 kWh |
On-grid sizing calculator
Change any value — results and the full working update instantly.
On-grid design checklist
On-grid quiz
1. What happens to a standard on-grid system during a power cut?
2. A home uses 450 kWh/month with PSH 5 and PR 0.75. Required array?
3. Which temperature determines the maximum number of modules in a string?
4. A typical DC/AC ratio for grid-tied systems is…
5. Recommended maximum DC cable voltage drop?