How to choose the right battery based on daily kWh needs?

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Calculating Daily Energy Demand To select the right battery, you need to calculate your daily energy demand. For a cabin, this can be done by estimating the energy consumption of appliances, lighting, and other loads. A typical 1-bedroom cabin might consume 1-2 kWh of energy per day. Consider the following energy usage estimates for common appliances: 100W LED lights (0.1-0.2 kWh/day), 500W refrigerator (2-4 kWh/day), and 1 kW electric water heater (8-10 kWh/day).
Battery Selection When selecting a battery, consider the depth of discharge (DOD), which is the percentage of battery capacity that can be used before it needs to be recharged. A typical DOD for a deep cycle battery is 50-80%. You’ll also want to consider the round-trip efficiency (RTE) of the battery, which is the ratio of the energy put into the battery to the energy extracted. A higher RTE means less energy loss. For example, a lithium iron phosphate (LiFePO4) battery might have an RTE of 90-95%. Finally, consider the battery type: lead-acid, lithium-ion, or AGM. Each type has its own strengths and weaknesses.
Battery Sizing Example Let’s say you’ve calculated a daily energy demand of 2 kWh for your cabin. To determine the battery size, you’ll want to select a battery that matches or slightly exceeds this demand. Assuming a DOD of 50% and an RTE of 90%, you can calculate the required battery capacity: 2 kWh / (0.5 x 0.9) = 4.44 kWh. This means you’ll need a battery with a minimum capacity of 4.44 kWh. For a lead-acid battery, you might choose a 6 kWh Trojan T105 deep cycle battery. For a lithium-ion battery, you might choose a 5 kWh LiFePO4 battery from a reputable manufacturer.