Battery Size Calculator

The battery capacity calculator answers questions that can't be answered by the specifications alone. How much battery capacity is actually required? This tool has two purposes; it calculates the required battery capacity for individual devices or loads based on the set runtime, or it determines the total bank capacity for an off-grid solar system. By selecting a mode above, the fields shown will change accordingly.

Both approaches are based on the same concept: Ampere-hour capacity indicates how much electricity can be delivered over a given period of time. However, they differ in terms of which part of the battery's capacity is actually allowed to be used and how much buffer must be considered as system losses.

Calculation of required battery capacity for devices or loads:

Assuming you know the size of your load and desired run time, first convert the load to current. If the power (watts) is known, divide it by the battery voltage. If the current (amps) is already known, use that value directly.

If a 200 watt device is connected to a 12 volt battery the current draw will be approximately 16.7 amps, calculated as 200 divided by 12. The capacity required would be this current multiplied by the runtime and increased by some buffer to ensure that the bank of batteries does not fully discharge. How far you can let the battery discharge depends on how much energy you want left at the end.

To calculate a Lithium battery that will deliver 16.7 amps for 10 minutes and leave 50% energy remaining, you would multiply 100 by 16.7 then multiply it by one sixth before dividing the result by 50. The resulting figure is approximately 5.6 amp hours.

Lead-acid batteries have limited reserves. When power is drawn from the battery for a short period of time, some of its nominal capacity is lost. This phenomenon is named after Wilhelm Peukert, which means that a larger battery will be required to do the same work. A common correction method increases this value depending on the rate and speed at which it discharges.

If you put the exact same numbers into the formula for lead acid batteries, it comes out to about 9.2 amp hours not 5.6. Generally a single cell of 12 volts and 9 Ah will be able to run a load of 200 watts for 10 minutes leaving 50% capacity remaining. This difference is the reason why a Lithium Ion battery pack can be much smaller than a lead acid battery pack to do the same job.

How much capacity should you leave as a reserve?

Never fully discharge the battery, as this will reduce its life and in some cases significantly. The remaining capacity and depth of discharge are inversely proportional to each other. Depth of discharge is the percentage of actual capacity used.

As a rule of thumb, lead-acid batteries should be kept at about 50% and lithium-ion batteries at about 20%. A higher remaining charge protects the battery but also requires more capacity. Zero percent means that the battery is completely discharged. This may not be problematic mathematically, but it damages the hardware.

Discharge rate and C-rate.

Every battery has a maximum discharge rate at which it can safely be discharged. This is usually quoted as the C-rate, i.e. a multiple of its capacity per hour. At 1C the current drawn from the battery equals its capacity and it will be empty in one hour. At 2C it would take two hours. At 0.1C it would take twenty hours, which is a slow rate suitable for lead acid batteries. The tools show a rate that matches the load so this can be compared with the datasheet. A battery with a capacity of 10 Ah and a rating of 1C could supply 10A for one hour but not 20A for half an hour. If you force this, then power losses will increase and the actual amount of energy (in amp hours) that can be drawn from it will reduce.

Determination of capacity of a battery storage system for an island installation

The capacity of a battery storage system for an island application is determined based on the daily energy consumption and not individual loads. The first step is to determine the daily energy consumption, either in watt hours or kilowatt hours. This can be done by multiplying the power rating of each device by its operating time or by dividing the amount of energy shown on your electricity bill by 30.

Then multiply that by the number of days you need to run your battery storage system without sunlight (autonomy), and divide it by the allowable depth-of-discharge for this technology. Then add a small efficiency correction factor to account for losses in charging and wiring.

A load of 500 watts running for 3 hours consumes 1500 watt-hours. For a Lithium battery system with 24 volts the depth of discharge is 80%, and the efficiency loss factor is 1.05. To calculate actual energy content, multiply 1500 by 1.05, divide that result by 0.8, then divide again by 24, which gives approximately 82 amp hours.

As the capacity calculation for lead acid batteries and lithium batteries is very different, both options are shown side by side in this tool. Let's assume a household consumes 10kWh per day and wants to achieve one-day autonomy with a 48V battery storage system.

The capacity of the Li-ion battery pack is about half, due to a larger usable range and higher efficiency. Some capacity figures round up the factor for Li-ion batteries to around 12.6 kWh, but the stated 13.1 kWh here was calculated by accurately dividing the result by 0.8, not multiplying it with the rounded value of 1.2.

Autonomy in days, voltage, temperature

Three factors have the greatest impact on the outcome. The more days of autonomy required, the larger the capacity of the battery pack needs to be. The capacity should be chosen according to the longest expected duration of dim conditions, which in practice is often two or three days as a rule of thumb. The higher the system voltage (e.g. 48 V instead of 12 V), the lower the required amperage for the same amount of energy, reducing losses through thinner cables.

Cold is a factor that's often overlooked. At low temperatures, the battery performance falls below nominal values, so the actual required capacity of the battery pack depends on the lowest expected temperature. High temperatures and ageing also have an effect. The values given here are to be understood as minimum requirements and should be rounded up to the actually available battery capacity.