How to Design an Off-Grid System from Scratch? Complete Calculation Process: Load List → Battery Capacity → Inverter Power (Lanyu / Torchn)

I. Overview of the Four-Step Method

II. Step 1: List Loads and Calculate Daily Consumption

List each appliance: Power (W) × Daily usage hours (h) = Daily consumption (Wh).

Example: LED light 10W × 5h = 50Wh; refrigerator 45W × 24h = 1080Wh; laptop 60W × 4h = 240Wh; water pump 200W × 0.5h = 100Wh.

Total daily consumption = 50 + 1080 + 240 + 100 = 1470Wh ≈ 1.47 kWh.

III. Step 2: Battery Capacity and Chemistry Selection

Battery capacity (Ah) = Daily consumption (Wh) × Backup days ÷ (System voltage × DoD × Inverter efficiency)

The usable depth of discharge (DoD) varies greatly among chemistries, directly affecting the required capacity:

Example: 48V system, 2 backup days, choose lithium with DoD 0.8, efficiency 0.9:

- Capacity = 1470 × 2 ÷ (48 × 0.8 × 0.9) ≈ 85.1Ah

- Choose the nearest Torchn 51.2V platform: 51.2V 100Ah wall-mounted (id=20) or Lanyu 12V200Ah × 4 in series (id=13)

If switching to gel lead-acid with DoD 0.5, the capacity would double, reflecting the capacity cost of gel under deep cycling.

IV. Step 3: How to Determine Inverter Power

Inverter rated power ≥ Maximum simultaneous load × Surge factor (1.2–1.5, including motor starting).

Example: The maximum simultaneous load at the above site is about refrigerator 45W + water pump 200W + others 100W = 345W. With a surge factor of 1.5 → about 518W. A Torchn 24V/48V 1.5kW inverter (US standard id=31, EU standard id=37) is sufficient; if a larger pump or air conditioner is needed, go up to 3.2kW (US standard id=32).

V. Step 4: PV Module Power

PV power (W) = Daily consumption (Wh) ÷ (Peak sun hours × System efficiency 0.8)

Example: Daily average 1470Wh, peak sun hours 4.5h: 1470 ÷ (4.5 × 0.8) ≈ 408W, so 500W of PV modules is sufficient.

VI. Voltage Standard Decision Table

VII. Frequently Asked Questions (FAQ)

Q1: The calculated battery capacity differs a lot from the seller's recommendation. Which one should I follow?

A: Follow your real load list. The formula in this article is based on daily consumption and backup days, more reliable than guesswork. If you have motor loads, do not forget the surge factor.

Q2: How to choose among gel, lithium, and sodium-ion for off-grid?

A: For high-frequency deep cycling, choose Torchn LFP or sodium-ion (long life); for extreme cold or no heating, choose gel or sodium-ion; for very low budget and low usage frequency, choose Lanyu gel lead-acid.

Q3: Is a bigger inverter always better?

A: No. Oversizing wastes cost and reduces light-load efficiency. Choose based on "maximum simultaneous load × surge factor" with about 20% margin.

Q4: What happens if the PV array is undersized?

A: If undersized, the battery may not be fully recharged, and consecutive cloudy days could lead to outages. Use the Step 4 formula with local peak sun hours and allow margin for seasonal differences.

Q5: Can Yangzhou Dongtai Solar Energy provide the entire system as a one-stop solution?

A: Yes. Lanyu LANYU (batteries) and Torchn (inverters/lithium/sodium) are both under Yangzhou Dongtai Solar Energy, which can provide a complete off-grid system from batteries to inverters, with UN38.3 / MSDS / CE certification baselines.

Related products: Torchn 48V residential energy storage inverter (EU standard id=37) | Torchn 51.2V 100Ah wall-mounted lithium battery (id=20) | Lanyu 12V200Ah gel battery (id=13)

Off-grid system solution: official website solutions