To build a 100% self-sufficient (off-grid) system, you must design for the "worst-case" scenario: the shortest day of the year and the highest simultaneous power draw.

I will base this guide on a real example, my own home. I want to make the same system I have right now with my electrical company but with solar 100% self-sufficient. I have a contract of 4900W and I use on average 18KWh a day. I know that because I've installed an IOT meter like this one at the main input. Is very useful. So these will be my values:

Target Power: 4900W (matches your current utility contract).

Daily Consumption (\(E_{day}\)): 18,000 Wh (18kWh).

Battery Bank: 3750Ah at 12V (Total energy: 45,000 Wh).

Autonomy: At 18kWh/day, you have 2.5 days of storage (at 100% discharge) or ~1.2 days (at 50% discharge for lead-acid safety).

1. Energy Audit & Load Limits

Before buying parts, you must define your two power limits to avoid system failure or fire hazards.

1. Daily Consumption (E_day): Total Watt-hours used in 24 hours.
2. Calculation: 18,000Wh (Base) + 1,800Wh (Inverter Idle Load) =19,800Wh/day.
3. Peak Power (P_peak): The maximum Watts drawn if all appliances run at once.
4. Your Goal: Match your 4900W utility contract.
5. Inrush Current: Note that motors (fridges, pumps) need 3x–7x their rated power to start.

2. Inverter Selection

The inverter converts DC to AC. It must handle the continuous load and the "kick" of starting motors.

1. Continuous Rating: P_inv = P_load x 1.25 (Safety Buffer).
2. Calculation: 4900W x 1.25 = 6,125W.
3. Strategy: Since 12V handles high amps poorly, split the load. Three 2400W inverters 7200W total is safer and provides redundancy.
4. Requirement: Must be Pure Sine Wave to protect home electronics.

3. Battery Bank (Storage)

Storage is calculated based on "Days of Autonomy" to ensure power during cloudy weather.

1. Formula: Ah_Capacity = (E_day * Days_of_Autonomy) / (System_Voltage * Depth_of_Discharge)
2. Example: (18,000Wh * 2.5) / (12V * 1.0) = 3,750 Ah.
3. Depth of Discharge (DoD): Use 0.5 for Lead-Acid (to protect life) or 0.9 for Lithium (LiFePO4).

4. Solar Array (Panels)

You need to harvest enough energy during Peak Sun Hours (PSH) to cover 100% of your daily use.

1. Formula: W_array = E_day / (PSH * 0.8_efficiency_factor)
2. Example: 19,800Wh / (5 * 0.8) = 4,950 Watts of solar panels.
3. Material Selection: Use Monocrystalline PERC panels for the best efficiency in low-light conditions.

5. Copper Busbar & Wire Thickness

In a 12V system, high amperage creates extreme heat. You must size your copper to stay cool.

1. Max Current (I): I = Total_Watts / System_Voltage
2. Example: 4,900W / 12V = 408 Amps.
3. Busbar Cross-Section: Cross_Section_mm2 = Max_Current / 1.2_Amps_per_mm2
4. Example: 408A / 1.2 = 340 mm2 minimum.
5. Material: Use a 40mm x 10mm copper bar (400 mm2) to safely handle the current.

6. Charge Controllers (MPPT)

The MPPT converts high-voltage solar energy into 12V battery charging current.

1. Charging Current: I_charge = W_array / Battery_Charging_Voltage
2. Example: 4,950W / 14.4V = 343 Amps.

Strategy: Use 4 separate 100A MPPT controllers in parallel (400A total capacity) to handle the 4,950W of solar panels.

7. Heat Dissipation

Inverters and controllers release energy as heat based on their efficiency.

1. Heat Output: Watts_of_Heat = Operating_Watts * (1 - Efficiency_Decimal)
2. Example: 4,900W * (1 - 0.90) = 490 Watts of heat.
3. Tip: You must treat your equipment room like a small server room. Use active fans (ventilation) to move the hot air out.

8. Fuses and Breakers

In a high-current system, fuses are not optional; they are the only thing preventing a battery explosion or fire during a short circuit.

1. Battery Main Fuse: Protect the entire bank at the source.
2. Formula: Main_Fuse_Amps = Total_System_Current * 1.25
3. Example: 408A * 1.25 = 510A. (Use a 500A or 600A Class-T fuse).
4. Individual Inverter Fuses: Each of your three 2400W inverters needs its own protection.
5. Formula: Inverter_Fuse = (Inverter_Watts / System_Voltage) * 1.25
6. Example: (2400W / 12V) * 1.25 = 250A.
7. Solar DC Breakers: Place a breaker between each panel group and its MPPT.
8. Formula: Breaker_Amps = Panel_Short_Circuit_Current (Isc) * 1.25
9. ???? Selection: Use Class-T fuses for the main battery. They are the only ones rated to handle the massive "interrupt capacity" of a large battery bank.

9. Wiring and Insulation

Voltage drop is your biggest enemy at 12V. Even a small drop makes inverters shut down under load.

1. Wire Gauge (Size): For your 2400W inverters, use 0000 AWG (4/0) copper wire.
2. Voltage Drop Formula: Voltage_Drop = (2 * Length_in_feet * Current * Resistance_per_ft) / 1000
3. Goal: Keep voltage drop under 2% between the battery and the inverter.
4. Insulation Type: Use THHN or Battery Cable (UL 1426) with a 105°C temperature rating.

???? Tip: At 12V, never make the battery-to-inverter cables longer than 1.5 meters (5 feet).

10. Series vs. Parallel Panel Wiring

You are using 4 groups of 3 panels in series (150V per group). This is an "Over-Voltage" strategy to save on wire costs.

1. The Logic: High voltage (150V) travels through thinner wires with less loss than low voltage.
2. Series Calculation: V_total = V_panel_1 + V_panel_2 + V_panel_3.
3. Parallel Calculation: I_total = I_group_1 + I_group_2 + I_group_3 + I_group_4.

Wire Selection: Since each group carries low current (usually under 10A), you can use standard 10 AWG (6mm2) MC4 solar wire for the long run from the roof.

11. Grounding and Lightning Protection

A 100% self-sufficient system is an island; it needs its own "earth."

1. Equipment Ground: Connect the metal cases of all 3 inverters and 4 MPPTs to a common ground busbar.
2. Earth Ground: Drive a 2.5-meter copper rod into the earth and connect the ground busbar using 6 AWG wire.
3. Surge Arrestors: Install a DC Surge Protective Device (SPD) on each 150V line coming from the roof to protect your MPPTs from nearby lightning strikes

12. Final Torque and Inspection

In a 400A system, a connection that is "hand tight" will melt.

1. The Torque Rule: Use a torque wrench on all busbar bolts. Usually, 12-15 Nm (Check your terminal specs).
2. Thermal Check: After 1 hour of heavy use, use an Infrared Thermal Camera or a laser thermometer to check every joint.
3. ???? Warning: If any connection is 10°C hotter than the wire itself, it is loose or dirty. Clean it and tighten it immediately.

• 6 x 2V Solar Batteries - Link Leroymerlin.es
• 20 100A plate connector - Link Es.aliexpress.com
• Copper bar (40x10mm) - Link Bricometal.com
• 3 x 2400W Phoenix 12/3000 Inverter - Link Tienda-solar.es
• 3 x 100A/250V Smart Solar MPPT - Link Leroymerlin.es

1. VICTRON SMARTSOLAR MPPT 250/100-TR (Solar Charge Controllers) - 4 units
2. VICTRON PHOENIX INVERTER 12/3000 SMART (VE.Bus Parallel Capable) - 2 units
3. 500W MONOCRYSTALLINE SOLAR PANELS - 12 units
4. HAWKER POWERSAFE 20 OPZS 2500 (Used 2V 3753Ah Cells) - 6 units
5. VICTRON SMARTSHUNT 500A/50MV (Battery Monitor with Bluetooth) - 1 unit
6. BATTERY CABLE 120MM2 (H01N2-D Extra Flexible) - 10 meters
7. COPPER BUSBARS (E-Cu 40mm x 10mm x 1000mm) - 2 units
8. BATTERY CABLE 50MM2 (For MPPT to Busbar) - 15 meters
9. SOLAR CABLE 6MM2 (UV Resistant Red/Black) - 100 meters
10. VICTRON INTERFACE MK3-USB (VE.Bus to USB for Parallel Config) - 1 unit
11. ALUMINUM PLATE (5mm thickness for MPPT heat dissipation) - 1 unit
12. FUSE HOLDERS FOR CLASS T OR NH1 FUSES (For Inverters) - 2 units
13. FUSE HOLDERS FOR MEGA FUSES (For MPPTs and Shunt) - 5 units
14. CLASS T FUSES 400A (High Rupture Capacity) - 2 units
15. DC CIRCUIT BREAKERS (2-Pole 250V 20A for PV Strings) - 4 units
16. COPPER LUGS (M8/M10 Heavy Duty for 120mm2 and 50mm2) - 30 units
17. MEGA FUSES 125A (For MPPT protection) - 5 units
18. BUSBAR INSULATORS (Standoff Drum type M8/M10) - 6 units
19. STAINLESS STEEL BOLTS/NUTS/WASHERS (A4 Grade M8 and M10) - 1 pack
20. MC4 CONNECTOR PAIRS (Waterproof) - 10 units
21. DISTILLED WATER (5L Jugs for battery maintenance) - 6 units
22. THERMAL PASTE (For MPPT to Aluminum plate mounting) - 1 tube