Battery Bank Sizing: From Daily Load to Nameplate Capacity

Sizing a battery bank is arithmetic, not opinion. The right size follows from a few measured quantities and a few equipment limits. This page walks through the calculation from daily energy use to nameplate capacity, and shows where the common mistakes hide.

The starting point is the daily load: the amount of energy the loads draw in one day, measured in kilowatt-hours (kWh). This is not the same as the power rating of the loads (watts) – it is the product of power and time. For example, a 100 W refrigerator that runs 8 hours per day consumes 0.8 kWh. The daily load is the sum over all loads that the battery must supply. If the system is grid-tied with backup, the load is the essential circuits during an outage; if off-grid, it is the entire household.

From daily load to usable capacity, three multipliers apply: depth of discharge (DoD), round-trip efficiency, and days of autonomy. Each is a separate factor, and each is often left out or double-counted.

The Three Multipliers

Depth of Discharge (DoD)

Depth of discharge is the percentage of the battery’s nameplate capacity that is actually withdrawn before recharging. No battery is meant to be fully discharged on a regular basis. Lead-acid batteries typically allow 50% DoD, meaning only half of the nameplate capacity is usable. Lithium-ion batteries allow 80–90%. The DoD is a property of the chemistry and the manufacturer’s specification, not a recommendation. Using a higher DoD than specified shortens cycle life.

Round-Trip Efficiency

Round-trip efficiency is the ratio of energy delivered to energy stored, expressed as a percentage. It accounts for losses during charging and discharging – heat, internal resistance, and conversion. Lead-acid is around 80–85%, lithium-ion around 90–95%. This factor increases the required battery size because the battery must store more energy than it delivers.

Days of Autonomy

Days of autonomy is the number of consecutive days the battery must supply the load without any input from the sun or grid. This is a design choice based on weather patterns and criticality of the load. Off-grid systems often use 2–3 days; grid-backup systems might use 1 day. More autonomy means a larger battery.

The Calculation

The formula is:

Nameplate capacity (kWh) = (Daily load × Days of autonomy) / (DoD × Round-trip efficiency)

Each factor is a decimal: 50% DoD is 0.5, 90% efficiency is 0.9.

Example: Daily load 10 kWh, 2 days autonomy, lead-acid (DoD 0.5, efficiency 0.85):

(10 × 2) / (0.5 × 0.85) = 20 / 0.425 = 47.1 kWh

That is the nameplate capacity. Notice it is more than twice the daily load times autonomy. The DoD and efficiency together multiply to 0.425, so the battery must be 2.35 times larger than the usable energy.

Temperature Derate

Battery capacity is rated at a standard temperature, usually 25°C (77°F). At lower temperatures, capacity decreases. For lead-acid, capacity at 0°C can be 70–80% of rated; at -20°C, 50%. Lithium-ion is less affected but still loses some capacity at freezing temperatures. The derate factor is applied to the nameplate capacity – or equivalently, the nameplate capacity is increased by the inverse of the derate.

If the battery will be installed in an unheated garage or outdoors, the derate must be included. The formula becomes:

Nameplate = (Daily load × Days) / (DoD × Efficiency × Temperature derate)

Where the temperature derate is a decimal (e.g., 0.8 for 80% capacity at the expected temperature).

Voltage and Configuration

The battery bank must also be configured at a voltage compatible with the inverter. Common voltages are 12 V, 24 V, and 48 V. The total energy (kWh) is the product of voltage and amp-hours (Ah). For a given energy, higher voltage means lower current, which reduces wire size and losses. The choice of voltage is determined by the inverter and the scale of the system.

To convert from kWh to Ah at a given voltage:

Ah = (kWh × 1000) / Voltage

For example, 47.1 kWh at 48 V is 981 Ah. That is the total amp-hour capacity. If using 200 Ah batteries, five in parallel would be needed (but see below for series-parallel considerations).

Series and Parallel

Batteries are connected in series to increase voltage, and in parallel to increase capacity. The number of series-connected batteries is the system voltage divided by the battery voltage. The number of parallel strings is the total Ah divided by the Ah of one battery, rounded up. However, parallel strings have issues with charge imbalance and require careful design. It is generally better to use larger batteries in a single string than to parallel many small ones.

Comparison Table

What People Get Wrong

A common mistake is to multiply the daily load by days of autonomy and then add a 20% buffer, ignoring the DoD and efficiency. That yields a battery that is far too small. For example, a 10 kWh daily load with 2 days autonomy would give 20 kWh, but the correct lead-acid size is 47 kWh. The mistake is natural because the DoD and efficiency are not obvious – they are hidden in the battery’s specifications. People assume a 10 kWh battery can deliver 10 kWh, but with lead-acid it can only deliver 5 kWh, and with efficiency losses, even less.

Another mistake is to use the DoD twice – for example, dividing by 0.5 and then multiplying by 2 for a 50% DoD, which is the same thing. Or to apply the temperature derate only to the daily load instead of the battery capacity. The derate affects the battery’s ability to deliver energy, so it must be applied to the capacity.

A third mistake is to size for the worst-case day of the year rather than the average. If the daily load is the highest day, the battery will be oversized for most of the year, which is costly. The correct approach is to use the average daily load, and let the days of autonomy cover the variability.

Practical Considerations

• The battery bank size calculator should be used as a starting point. The final size may be adjusted for available battery sizes and the inverter’s charge limits.
• The battery’s maximum charge and discharge rates (C-rate) also matter. A battery that is too small for the load will have a high C-rate, which reduces efficiency and lifespan. The C-rate is the current divided by the capacity; a 100 Ah battery discharging at 50 A is a 0.5C rate. Most batteries are rated for a specific C-rate, often C/20 for lead-acid.
• The inverter’s efficiency is separate from the battery’s round-trip efficiency. The inverter also has losses, but they are accounted for in the load calculation if the load is measured at the inverter’s output.
• The battery bank size calculator is not a substitute for a professional design. Local codes, safety, and warranty requirements may dictate specific practices.

Commands and Tools

There is no single command to size a battery bank. The calculation is simple enough for a spreadsheet. A typical spreadsheet formula might be:

=(daily_load*days_autonomy)/(DoD*efficiency*temp_derate)

The output is the nameplate capacity in kWh. To convert to Ah, divide by the system voltage and multiply by 1000.

For a quick estimate, an online battery bank size calculator can be used, but the user must input the correct parameters. The calculator is only as good as the inputs.

Local Rules

Net metering, interconnection, export compensation, permitting, and incentives are set by state and by utility, and they change. The sizing of a battery bank for a grid-tied system may be affected by these rules – for example, some utilities limit the size of a battery system or require specific interconnection agreements. Check with the local utility and authority having jurisdiction before finalizing a design. This page does not cover any specific jurisdiction’s rules.