how big of a battery backup do i need

The size of the battery pack depends on the energy required to run the appliance. The energy capacity of the battery system should always be greater than the energy requirements.

But how do you calculate the power demand?

Step 1: Estimate Energy Requirements

What do you want to supply to the battery pack?

All appliances have specifications for voltage (in volts) and current (measured in amps). Applying higher values than the recommended voltage and current will damage the device.

Another way to express these ratings is in watts. In physics, power is the product of voltage and current.

Power (W) = Voltage (V) x Current (A)

Once you have calculated the power required to operate each device, you need to determine how long (in hours) each load will run.

For example, a refrigerator typically runs 24 hours a day. Considering that an RV refrigerator requires 70W on average, it would require 70W x 24h = 1680 Wh = 1,68 kWh to power that refrigerator for 24 hours.

The above calculation determines the energy requirement of the refrigerator expressed in watt-hours (Wh). Using the battery pack, you can calculate each load you wish to power. Simply check the load specifications and use

Energy requirement (Wh) = Power (W) x Time (h) = Voltage (V) x Current (A) x Time (h)

We also need to estimate the required autonomy time

Continuing with the refrigerator example, if we want to run this refrigerator for 2 days before recharging the batteries, the total energy required would be 1,68 kWh x 2 days = 3,36 kWh.

Knowing this, we need to find a battery with compatible specifications.

Step 2: Determine the size of the battery

As shown in the figure below, batteries are rated in terms of voltage and ampere-hours (Ah).

A 12V LiFePO4 battery with a capacity of 100 Ah.

12V LiFePO4 battery used as part of the battery sizing chart.

There are two ways to express battery capacity: charge capacity (Ah) or energy capacity (Wh).

The charge capacity represents the amount of current the battery can supply in one hour until the voltage drops to the point where the battery can no longer "push" (produce current) enough electrons. This value can be calculated using the following equation

Charging capacity (A) = current (A) x time (h)

It is probably not necessary to calculate ampere-hours, since the battery case always shows this value.

The energy capacity then represents the amount of energy the battery can store/supply in one hour before it is exhausted. This value can be calculated using the following equation

Energy Capacity (Wh) = Voltage (V) x Amp Hours (Amp)

After calculating the total energy requirement (Wh), the energy capacity of the battery (in terms of operating voltage and amp-hours) can be calculated to find a battery that meets the energy requirement.

Step 3: Consider the available energy of the battery

LiFePO4 batteries can be discharged up to 100% and AGM and gel batteries up to approximately 80% without significant damage. However, this will shorten the life of the battery.

Manufacturers typically recommend 80% discharge (20% state of charge)4 cells for LiFePO. AGM and GEL batteries, 50% depth of discharge (DoD). Following these recommendations will maximize the number of cycles the battery will perform.