Track 2 Β· Off-Grid

Off-Grid (Stand-Alone) System Design

Complete energy independence. With no grid to lean on, every watt-hour must be produced, stored and delivered by your system β€” so careful load analysis is everything.

Lesson 1

How it works

Panels charge a battery bank through a charge controller during the day. An off-grid inverter draws from the battery to supply AC loads around the clock. A backup generator is often added for long cloudy spells.

β˜€ PV ArrayDC MPPT Controllercharges battery πŸ”‹ Battery Bank12 / 24 / 48 V Off-Grid InverterDC β†’ AC, surge 🏠 AC Loads β›½ Generator (opt.)
PV DCBattery DCAC
No grid connection β€” the battery is the heart of the system.
Lesson 2

Design procedure, step by step

  1. Build a load table

    List every appliance with its power (W), quantity and daily hours of use. Prefer efficient appliances β€” every watt saved shrinks the battery and array.

    E_daily (Wh) = Ξ£ (Power Γ— Quantity Γ— Hours)
    Peak load (W) = Ξ£ (Power Γ— Quantity) of loads that can run together

  2. Choose system voltage

    Higher voltage = lower current, thinner cables, lower losses.

    Daily energy / inverter sizeSystem voltage
    < 1.5 kWh/day Β· up to ~1 kW12 V
    1.5–4 kWh/day Β· 1–3 kW24 V
    > 4 kWh/day Β· > 3 kW48 V

  3. Size the inverter

    Inverter continuous (W) β‰₯ Peak load Γ— 1.25

    Motor loads (pumps, fridges, compressors) draw 3–7Γ— their rating at start-up β€” check the inverter's surge rating. A pure sine-wave inverter is recommended for motors and electronics.

  4. Size the battery bank

    Battery (Ah) = (E_daily Γ— Days of autonomy) Γ· (V_sys Γ— DoD Γ— Ξ·_batt Γ— Ξ·_inv)

    Typical autonomy: 1–2 days for homes with a generator, 3–5 days for critical/remote sites. Batteries in series raise voltage; in parallel raise Ah.

  5. Size the PV array

    Use the worst-month PSH (usually winter), because the system must survive the darkest month.

    PV (Wp) = E_daily Γ· (PSH_worst Γ— Derating Γ— Ξ·_batt Γ— Ξ·_inv)

    Derating β‰ˆ 0.7–0.8 for temperature, dust, mismatch and cable losses.

  6. Size the charge controller

    MPPT current (A) = PV (Wp) Γ· V_battery Γ— 1.25
    PWM current (A) = Isc Γ— N_parallel strings Γ— 1.25

    Also check the array's cold Voc is below the controller's max PV input voltage.

  7. Cables, fuses & protection

    Battery cables carry the largest current β€” size them generously and fuse them close to the battery terminal (Class T or NH fuse for lithium). Add DC breakers between controller, battery and inverter, plus SPDs and earthing.

Lesson 3

Battery chemistry comparison

PropertyFlooded lead-acidAGM / GelLiFePO4 (LFP)
Usable DoD50%50–60%80–95%
Round-trip efficiency~80%~85%~95%
Cycle life (at rated DoD)1,200–1,500500–1,2003,000–6,000+
MaintenanceTop-up water, equaliseNoneNone (BMS)
Upfront costLowestMediumHighest β€” but lowest cost per cycle
πŸ’‘

Lithium iron phosphate (LFP) is now the default choice for most new off-grid systems: you need roughly 40% less nominal capacity than lead-acid for the same usable energy.

Lesson 4

Worked example

A remote cabin needs 3,000 Wh/day, peak load 1,200 W, 2 days autonomy, 48 V LFP battery (DoD 0.8, Ξ· 0.95), inverter Ξ· 0.9, worst-month PSH 5, derating 0.75, 550 Wp modules.

StepCalculationResult
Inverter1,200 Γ— 1.251.5 kW (choose 2 kW)
Battery3,000 Γ— 2 Γ· (48 Γ— 0.8 Γ— 0.95 Γ— 0.9)183 Ah β†’ 200 Ah @ 48 V
Battery energy200 Γ— 489.6 kWh
PV array3,000 Γ· (5 Γ— 0.75 Γ— 0.95 Γ— 0.9)936 Wp β†’ 2 Γ— 550 = 1,100 Wp
Charge controller1,100 Γ· 48 Γ— 1.2528.6 A β†’ 40 A MPPT
Practice

Off-grid load & sizing calculator

Edit the load table and system settings β€” everything recalculates live.

1 Β· Load table

ApplianceWattsQtyHours/dayWh/day

2 Β· System settings

h / day
%
%
Wp

Results

Daily energy
Peak load
System voltage
Inverter
Battery bank
Battery energy
PV array
Charge controller
Inverter surge needed
Before you install

Off-grid design checklist

Test yourself

Off-grid quiz

1. Which PSH value should an off-grid design use?

The system must keep running in the darkest month of the year.

2. 2,400 Wh/day, 1 day autonomy, 24 V lead-acid at 50% DoD (ignore efficiency). Battery size?

2,400 Γ· (24 Γ— 0.5) = 200 Ah.

3. Why choose 48 V instead of 12 V for a 5 kW system?

P = V Γ— I; quadrupling voltage cuts current to ΒΌ and IΒ²R losses to 1/16.

4. A 1,000 Wp array charges a 24 V battery. MPPT controller size?

1,000 Γ· 24 Γ— 1.25 β‰ˆ 52 A β†’ next standard size 60 A.

5. Why does a water pump need a larger inverter than its rating suggests?

Induction motors can draw 3–7Γ— rated current at start-up.

← On-Grid design Next: Hybrid design β†’