Daily Energy Use: Realistic Watt, Amp and Voltage Calculations for an Off-Grid Cabin

When you are trying to size a solar system for a small cabin or vineyard house, the most important step is understanding how much electricity you actually use in a full day. Device labels rarely show the real numbers. Temperature changes, night-time efficiency, inverter losses and battery limitations can all increase your total daily demand.

Below is a practical breakdown based on my own off-grid setup: LED lighting in several rooms and a full-size refrigerator that runs day and night during hot summer months.

Appliances Used in My Cabin

• 6 LED lamps — about 7W each
• 1 large refrigerator (140W rated)
• Lighting in the living room, kitchen, two bedrooms, bathroom and veranda
• All lights and the refrigerator run on AC power

Daily Lighting Consumption

LED lights are efficient, but when several of them stay on for hours, the total energy adds up. The calculation below assumes all lights stay on from 18:00 to midnight.

Realistic Refrigerator Consumption

Although the fridge is labeled as 140W, it does not draw that power for 24 hours straight. The compressor cycles on and off depending on room temperature. During hot days it runs more frequently, while at night it works with higher efficiency. In real-world conditions, most household refrigerators operate for roughly 8 hours per day in active mode.

Calculation:

140 W × 8 hours = 1120 Wh

On extremely hot days, a 10-hour cycle can be used as a safety estimate:
140 W × 10 hours = 1400 Wh

Inverter and MPPT Controller Usage

A commonly overlooked detail in off-grid systems is the self-consumption of the inverter and charge controller. Even when no appliances are running, these devices continuously draw power.

Total system overhead: 576 Wh

Total Daily Energy Demand

Why You Should Always Calculate for Maximum Load

Off-grid systems fail when the battery is pushed below a safe discharge level. To avoid deep discharge, sizing should always be based on the highest possible daily usage — not average use. This ensures your battery stays healthy even on the hottest or cloudiest days.

Gel Batteries

• Safe usable capacity is around 50%
• 12V 100Ah = 1200Wh on paper, but only ~600Wh usable

LiFePO4 Batteries

• Up to 80% of the capacity can be used safely
• 12V 100Ah = 1200Wh → roughly 960Wh usable
• Nearly twice as efficient as gel batteries in real use

Summary Table

Final Recommendations for an Off-Grid Cabin Setup

• Minimum solar panel capacity (summer): 400–600W
• For winter reliability: 800–1000W
• Recommended LiFePO4 battery: 12V 200Ah (≈ 2400Wh usable)
• If using gel batteries: At least 400Ah total

These calculations are based on everyday usage in a real off-grid environment, helping you build a reliable system that works year-round without damaging your batteries.