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SOLARCALCKIT SUITE / Solar Battery Sizing Instrument

Solar Battery Sizing Instrument

IEEE 485 & NEC 706 REFERENCE METHODOLOGY

Deterministic battery bank sizing from daily energy, autonomy, system voltage, and chemistry — with live SVG topological schematic and transparent loss accounting.

TOOLS: Solar Battery Battery Bank MPPT Sizing Wire Gauge Inverters Load Sizing
Input Parameters
Client-Side Engine
01 Load Demand & Autonomy

Total energy all loads consume in 24 hours, in kWh/day.

Days the bank sustains loads with no solar recharge (fractions allowed).

02 System & Chemistry

Nominal DC bus voltage.

Sets recommended depth of discharge default.

03 Individual Battery Specs

Must divide system voltage exactly.

Rated nominal capacity of one unit, in Ah.

04 Losses & Depth of Discharge

Usable % of nominal capacity. Left blank uses chemistry default.

DC→AC path efficiency, %. 100 for DC-coupled.

What solar battery sizing means

Solar battery sizing answers two questions at once: how much energy your battery bank must store, and how that energy must be arranged as physical batteries. The energy side is set by your daily consumption, your autonomy requirement, your inverter's efficiency, and the depth of discharge your chemistry tolerates. The physical side is a series-and-parallel arrangement that reaches your system voltage and amp-hour target using whole batteries.

Methodology

The four-step sizing math

Every number this calculator produces follows from these four transparent steps — no hidden margins.

Step 1 · Autonomy energy

E_autonomy = daily energy × autonomy days

Total load energy consumed over the autonomy window with no solar recharge.

Step 2 · Terminal Energy

E_usable = E_autonomy ÷ inverter efficiency

Accounts for DC→AC conversion losses in the inverter (typically 90%).

Step 3 · Nominal Capacity

E_nominal = E_usable ÷ depth of discharge

DoD is the usable fraction (80% LiFePO4, 50% Lead-Acid) to protect battery cycle life.

Step 4 · Series / Parallel Topology

series = V_sys ÷ V_bat · strings = ⌈req Ah ÷ bat Ah⌉

Series multiplies voltage; parallel multiplies capacity. Strings round up to prevent undersizing.

Assumptions

What is — and is not — modeled

Modeled explicitly

Daily load energy, autonomy, inverter (DC→AC) path efficiency, and usable depth of discharge. Nominal energy, usable energy, and required capacity are reported separately.

Deliberately not modeled

Battery round-trip losses and aging margins are not applied as hidden multipliers. Temperature derating is not assumed — keep margin for extreme cold.

Frequently asked questions

How many batteries do I need for a 24V solar system?

It depends on your daily energy use and how many days of autonomy you need. For 12 V batteries on a 24 V system, two are wired in series per string, and the number of parallel strings is the required amp-hours divided by one battery's amp-hours, rounded up. Example: 5 kWh/day with 1 day of autonomy on LiFePO4 needs about 289 Ah at 24 V, so three 100 Ah parallel strings — 6 × 12 V 100 Ah batteries (2S3P) in total.

What is the difference between wiring batteries in series and in parallel?

Series connections add voltage: two 12 V 100 Ah batteries in series give 24 V at 100 Ah. Parallel connections add capacity: the same two batteries in parallel give 12 V at 200 Ah. Solar battery banks use identical batteries in series to reach the system voltage, then identical series strings in parallel to reach the required amp-hours.

What depth of discharge should I use for battery sizing?

Use the value your battery chemistry can sustain regularly: 80% is the conservative design value for LiFePO4 and NMC lithium, and 50% is the standard conservative value for AGM, gel, and flooded lead-acid. Deeper discharge means more usable capacity per cycle but shorter cycle life — check your battery datasheet.

Should I choose a 12V, 24V, or 48V system voltage?

Higher system voltage means lower current for the same power, which allows thinner, cheaper wiring and fewer parallel strings. 12 V suits small systems, 24 V is common for mid-size off-grid systems, and 48 V is typical for larger home systems. The battery voltage must divide the system voltage exactly — four 12 V batteries in series give 48 V.

Why does the calculator round parallel strings up?

A partial string cannot be built — you cannot install 2.4 strings. The requirement is divided by one battery's amp-hours and the result is rounded up to the next whole string, so the bank is never under-sized. The surplus capacity is reported so you can see exactly how much headroom the rounded design provides.

Related Calculators

Continue your system design with the other tools in the SolarCalcKit suite — in the order the rest of your design flows: load audit → battery → bank wiring → MPPT → wire size → inverter.

Solar Load Calculator

Start by auditing daily appliance energy use (Wh & kWh) and connected load.

Open Tool

Battery Bank Wiring

Turn a target kWh or Ah into an exact series–parallel battery configuration.

Open Tool

MPPT Charge Controller

Size the charge controller from PV array wattage and the battery system voltage.

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Solar Wire Size / Drop

Size DC conductors from current, run length and allowable voltage drop.

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Inverter Sizing

Size an inverter from continuous and surge loads, with DC battery current.

Open Tool