Getting the battery bank size right is one of the most important planning steps for any standalone solar system. A bank that is too small leaves you without power during cloudy stretches. One that is too large wastes money and space. This guide walks through the key concepts: days of autonomy, converting between amp-hours and kilowatt-hours, usable capacity, system voltage, and how series and parallel wiring affect balance. You will also find a simple formula to estimate the bank size you need.
Understanding days of autonomy
The Hong Kong Electrical and Mechanical Services Department (EMSD) explains that “days of autonomy” is the number of days a battery system can support the electrical loads without being recharged during cloudy and rainy weather. This figure directly multiplies the size of the bank. If your daily consumption is 1,000 Wh and you want three days of autonomy, the bank must store at least 3,000 Wh of usable energy. The right number depends on your local climate and how often you can run a backup generator, but the concept is the same: more autonomy means a larger, heavier and more expensive battery bank.
Battery capacity: amp-hours and kilowatt-hours
Battery capacity is usually given in ampere-hours (Ah). According to the EMSD, a battery rated at 50 Ah can theoretically deliver 1 ampere for 50 hours or 2 amperes for 25 hours. To relate this to the energy your appliances consume, you need watt-hours (Wh) or kilowatt-hours (kWh). The EMSD states that the storage capacity in kWh equals the Ah rating multiplied by the voltage rating, divided by 1,000. For a battery bank, the total kWh capacity is the sum of the individual battery capacities.
Victron Energy’s Wiring Unlimited book describes the power formula P = V × I, where power in watts equals voltage times current. Because power is independent of voltage, a 2,400 W load draws 200 A from a 12 V battery but only 100 A from a 24 V battery. This relationship is central to choosing a system voltage and converting between Ah and Wh when sizing a bank.
Usable capacity and depth of discharge
You rarely use the full rated capacity of a battery. How deeply you discharge it affects both the usable energy and the battery’s service life. Victron’s 2013 book Energy Unlimited, written mainly for boats and vehicles with lead-acid batteries, notes that intensive cycling (repeated deep discharging and recharging) is a main cause of active material shedding and aging. The book describes how traction or deep-cycle lead-acid batteries are often discharged down to 60 to 80% of their capacity every day and then recharged overnight. While these batteries are built for deep cycling, regularly taking them to a very low state of charge still shortens their life. Many system designers therefore limit the depth of discharge (DoD) to a fraction that balances usable capacity against longevity.
Lithium batteries behave differently. The Victron Lithium Smart Battery manual explains that the battery management system (BMS) monitors cell voltages and temperature. If a cell voltage drops too low, the BMS sends a signal to disconnect the loads. If a cell voltage rises too high, it disconnects the chargers. This protection effectively defines the usable window. Because the BMS prevents extreme states, a lithium bank can safely use a larger share of its rated capacity than a lead-acid bank, though the exact usable range is set by the BMS thresholds.
Choosing a system voltage
System voltage has a direct effect on current and wiring. Victron’s Wiring Unlimited shows that for a given power, doubling the voltage halves the current. Lower current means less voltage drop in cables and less heat, so higher-voltage systems can use thinner, more manageable wiring. Common DC system voltages for small off-grid setups are 12 V, 24 V and 48 V. The choice often depends on the voltage of your largest loads, the inverter you plan to use, and whether you will connect batteries in series to reach a higher voltage. A higher voltage also reduces the Ah figure you need for the same energy storage, which can simplify the battery bank layout.
Wiring batteries in series and parallel
To build a battery bank, you connect individual batteries together. Victron’s Wiring Unlimited explains the basic rules: connecting batteries in series increases the voltage while keeping the capacity the same; connecting them in parallel increases the capacity while keeping the voltage the same. A series/parallel arrangement raises both voltage and capacity.
When wiring multiple batteries in parallel, the way you connect the main cables matters. The book warns that a common mistake is to connect the load or charger to just one end of a parallel bank. This forces the battery closest to the connection to work harder, while batteries farther away see less current. Over time, the hardest-worked battery fails early. The correct method is to make the total cable path to each battery equal, for example by using busbars, connecting halfway, or using a diagonal connection. For large lead-acid banks, Victron recommends limiting the number of parallel strings to about three or four, because small differences in internal resistance and wiring make it difficult to keep many strings balanced.
Balancing lead-acid battery banks
When lead-acid batteries are connected in series to create a higher voltage, slight differences in internal resistance can cause their individual voltages to drift apart during charging. Victron’s Wiring Unlimited describes how this imbalance grows over time: one battery gets overcharged while another remains undercharged, leading to premature failure. To check balance, you measure the voltage of each battery near the end of the bulk charge stage. A noticeable difference indicates an unbalanced bank.
For ongoing balance, Victron describes a battery balancer. The device activates when the charge voltage reaches a set level and compares the voltages of the series batteries. If it detects a difference of more than 0.1 V, it draws a small current from the higher battery until the voltages equalise. If the difference exceeds 0.2 V and cannot be corrected, the balancer triggers an alarm, which may indicate a faulty battery. For a 24 V system one balancer is needed; for 48 V, three balancers are placed between each pair of batteries.
Putting it together: a sizing formula
You can estimate the required battery bank size with a simple two-step calculation. First, find the total energy the bank must supply:
Bank energy (Wh) = Daily energy use (Wh) × Days of autonomy ÷ Usable fraction
The usable fraction is the portion of the rated capacity you plan to use, expressed as a decimal. For example, if you decide to use only 50% of the capacity to extend life, the usable fraction is 0.5. Next, convert the bank energy to amp-hours at your chosen system voltage:
Bank capacity (Ah) = Bank energy (Wh) ÷ System voltage (V)
Example: Suppose your daily consumption is 1,000 Wh, you want 3 days of autonomy, and you plan to use 50% of the rated capacity. The bank energy needed is 1,000 × 3 ÷ 0.5 = 6,000 Wh. At 12 V, that is 6,000 ÷ 12 = 500 Ah. At 24 V, it would be 250 Ah. This example uses round numbers to illustrate the method; your own figures will come from a load audit and your local weather.
Monitoring your battery bank
Once the bank is installed, a battery monitor helps you track the state of charge. The Victron Lithium Smart Battery manual mentions that some BMS models, such as the Lynx Smart BMS, include a built-in battery monitor. A monitor measures the current flowing in and out of the bank and calculates the remaining energy, giving you a fuel-gauge-like reading. For lead-acid banks, a monitor can also help you avoid discharging below your chosen depth of discharge. Energy Unlimited (2013, lead-acid) describes battery voltage as only a rough indication of state of charge, which is one reason it recommends a battery monitor.
Key inputs that affect battery bank size
| Input | Typical source | Effect on bank size |
|---|---|---|
| Daily energy consumption (Wh) | Load audit or appliance labels | Directly scales the bank size |
| Days of autonomy | Local weather data or user preference | Multiplies the bank size |
| Depth of discharge (usable fraction) | Battery chemistry and lifespan goals | Inversely scales the bank size |
| System voltage | Component selection and wiring practicalities | Changes the Ah rating for the same Wh |
Frequently asked questions
How many days of autonomy should I plan for?
The EMSD defines days of autonomy as the period a battery bank can support loads without recharging during cloudy or rainy days. The right number depends on your location and how much backup you are comfortable with. In areas with long overcast periods, you may want more days; if you have a generator, you might choose fewer.
What is the difference between series and parallel battery connections?
According to Victron’s Wiring Unlimited, series connections increase the voltage while keeping the capacity the same. Parallel connections increase the capacity while keeping the voltage the same. A series/parallel arrangement raises both. The choice affects the system voltage, the current in the cables, and how you manage battery balance.
Why does system voltage matter when sizing a battery bank?
Victron’s Wiring Unlimited shows that for a given power, a higher voltage results in a lower current. Lower current reduces voltage drop and heat in cables. It also means that the same energy storage requires fewer amp-hours, which can simplify the bank layout and reduce the number of parallel strings.
For a deeper look at load audits, autonomy choices and worked examples, visit our battery sizing guide. You may also find the DIY Solar Planning section and the how to plan a DIY solar system article helpful as you work through the full design.
