Solar Generator Sizing Guide: How to Choose the Right Capacity

Properly sizing a solar generator for emergency preparedness is one of the most critical yet frequently overlooked aspects of creating a resilient power backup system. Unlike conventional fuel generators where you can simply add more gas during extended outages, solar generators have fixed battery capacities that determine how long they can power your devices between recharging cycles. This guide will walk you through a systematic approach to calculating your emergency power needs and selecting an appropriately sized solar generator system.

The foundation of proper solar generator sizing begins with a critical load assessment—identifying which devices and appliances are absolutely essential during an emergency situation. Start by categorizing your needs into three tiers: life-sustaining (medical equipment, refrigeration for medications), high-priority (basic refrigeration, minimal lighting, communication devices), and comfort (additional lighting, fans, entertainment). For each device, you'll need to determine two key metrics: running watts (the power consumed during normal operation) and surge watts (the brief power spike when motors or compressors first start up).

Medical equipment deserves special consideration in your calculations. Devices like CPAP machines (typically 30-60W), oxygen concentrators (300-600W), and medication refrigeration can be life-critical during emergencies. For these devices, we recommend calculating their daily power consumption and then multiplying by a safety factor of 1.5 to ensure adequate power reserves. For example, a 50W CPAP machine used 8 hours per night consumes 400Wh daily (50W × 8h). With the safety factor, you should allocate 600Wh of your solar generator's capacity specifically for this device.

Refrigeration represents another critical consideration for many emergency scenarios. A modern energy-efficient refrigerator typically draws 100-200W while running but only operates about 30-50% of the time (cycling on and off to maintain temperature). This means a refrigerator might consume 600-1200Wh per day. Older, less efficient models can use significantly more power. During our testing, we found that opening the refrigerator door frequently during power outages can increase power consumption by 25-40%, so factor this into your calculations for emergency scenarios when normal usage patterns may be disrupted.

Once you've identified all critical loads, calculate your total daily power requirement by multiplying each device's wattage by its expected hours of use and summing these values. For example: Refrigerator (150W × 12h cycling) = 1800Wh + Lighting (20W × 6h) = 120Wh + Phone charging (10W × 3h) = 30Wh + CPAP machine (50W × 8h) = 400Wh = 2350Wh total daily requirement. We recommend adding a 20% buffer to this figure to account for inverter inefficiency and unexpected needs, bringing the example requirement to approximately 2800Wh per day.

With your daily power requirement established, the next consideration is how many days of autonomy you want—how long you need to power these devices without any solar recharging (during extended cloudy periods, for instance). For basic emergency preparedness, we recommend a minimum of 1-2 days of autonomy. This means multiplying your daily requirement by your desired days of autonomy. In our example, 2800Wh × 2 days = 5600Wh minimum battery capacity. This calculation provides the foundation for selecting an appropriately sized solar generator.