How do you size a 12V LiFePO4 battery for an RV fridge, lights and inverter?

Direct answer: Add up daily amp-hours: each load’s amps × hours, plus inverter losses. Divide by the share of capacity you will use and choose the smallest pack that covers it with margin. The example below comes to about 114Ah per day, which a 200Ah pack covers for roughly 1.4 days.

Know the sizes on offer. The 12V 100Ah is listed at 12.8V and 1.28 kWh, about 10 kg. The 12V 200Ah is 2.5 kWh as printed on its page and about 18 kg. The 12V 300Ah is 3.84 kWh and about 26 kg. The 200Ah and 300Ah pages list 100A, 150A or 200A continuous options, which matters if you add an inverter.

Measure the loads instead of copying labels. A compressor fridge cycles on and off, so clamp a DC meter on it for a full day, ideally in warm weather, and record the amp-hours it used. For lights, multiply amps by hours. For an inverter, DC current is roughly watts ÷ (battery volts × inverter efficiency), and an inverter left on draws power even with no load, so check its idle consumption on its datasheet.

Here is a worked example with hypothetical numbers, not Redway data. Fridge: 72Ah per day (3A average over 24 h). Lights: 5Ah (1A for 5 h). Laptop through an inverter: 100W for 4 h is 400Wh, which at 85% inverter efficiency is about 470Wh from the battery, or 470 ÷ 12.8 = 37Ah. Total is about 114Ah per day. If you plan to use 80% of capacity (the pages quote cycle life at 80% DOD), a 200Ah pack gives 160Ah, which covers roughly 1.4 days without charging. A 100Ah pack covers less than one day.

Add margin for cloudy days, cold weather and ageing, and think about how the battery will be charged. The Redway pages list a charge temperature of 0 to 60 °C, so charging below freezing needs a plan, as described in our post on charging a 12V 200Ah battery in cold RV weather. Shore power, solar and the vehicle’s alternator each need their own checks.

Pick the smallest pack that meets the daily need with margin, because capacity costs money and weight. If the inverter is large, compare 12V with 24V, as in our post on two 12V batteries in series versus one 24V pack. Finally, check that the inverter’s peak current stays inside the pack’s continuous and peak ratings, and confirm the pack and wiring with Redway for your layout.