Methodology
How the numbers are made.
Every equation, unit, default, boundary and rounding rule behind the BatteryCaliber estimates—with a worked example and sources.
BatteryCaliber publishes every equation, unit, default and boundary it uses so that you, your installer or a reviewer can check the arithmetic. The calculator is a first-order planning model. It does not size conductors, select fuses or certify compliance with any code or standard.
Scope. Results are estimates for preparing a conversation with a qualified installer. Cable ampacity, fuse coordination, fault current and equipment compatibility require installation-specific professional review.
What the calculator estimates
- Battery energy for reserve — the energy your loads draw over the no-charge reserve period (Wh / kWh).
- Estimated nameplate battery capacity — the rated capacity needed once usable depth of discharge and capacity derating are added back (Wh / kWh, and Ah at your actual nominal voltage).
- Estimated continuous and surge DC current — under separate load assumptions (A).
- First-order copper cable estimates — loop resistance, continuous voltage drop and percentage, continuous cable heat loss and surge drop.
- Explicit unresolved status for conductor ampacity and overcurrent protection.
Not included: solar array or charge-controller sizing, charging time, AC branch design, fault-current study, automatic wire or fuse sizing, complete BMS validation, equipment compatibility, earthing/bonding design, thermal simulation or installation approval.
Reserve energy vs. nameplate capacity, amps vs. amp-hours
Reserve energy is what your loads consume during the period with no charging. Nameplate capacity is larger: you normally do not plan to use 100% of a battery, and available capacity falls with age and temperature. The calculator divides reserve energy by both allowances.
Amps (A) describe the rate of current flow at a moment; amp-hours (Ah) describe stored charge. A battery’s Ah rating does not tell you whether it can deliver a given continuous or surge current—check the manufacturer’s discharge ratings.
LiFePO4 banks are often described by a familiar class (12 V, 24 V, 48 V), but their actual nominal voltage is usually different (for example 12.8, 25.6 or 51.2 V). Always enter the manufacturer’s rated nominal voltage.
Inputs, units and accepted ranges
Ranges are software bounds that keep the arithmetic meaningful. They are not safe operating ratings. Percentages are entered as percent and converted to fractions once inside the model.
| Input | Definition | Unit | Example | Range |
|---|---|---|---|---|
| Daily AC energy | Appliance energy measured downstream of the inverter | Wh/day | 3,500 | 0–1,000,000 |
| Daily DC energy | Direct DC demand at the battery bus, excluding the inverter | Wh/day | 0 | 0–1,000,000 |
| Reserve duration | Days with no charging | days | 2 | 0.5–7 |
| Nominal voltage | Actual manufacturer-rated nominal bank voltage | V | 51.2 | 10–64 |
| Inverter efficiency | Used for energy and continuous load | % | 90 | 50–100 |
| Depth of discharge | Usable fraction of available capacity | % | 80 | 10–100 |
| Capacity after derating | Available fraction of nameplate after assumed derating | % | 90 | 50–100 |
| Inverter overhead | Extra overhead not already counted in efficiency or demand | W | 10 | 0–1,000 |
| Overhead hours | Hours per day the overhead is present | h/day | 24 | 0–24 |
| Continuous AC power | Simultaneous continuous real power | W | 2,000 | 0–100,000 |
| Surge AC power | Total simultaneous real power during starting surge (not incremental) | W | 3,000 | 0–200,000 |
| Minimum inverter voltage | Minimum voltage at inverter terminals under load | V | 44.8 | 8–64 |
| Surge efficiency | Estimated inverter efficiency during surge | % | 85 | 50–100 |
| Direct DC current | Concurrent DC current through the modelled main cable | A | 0 | 0–2,000 |
| Cable length | One-way copper length; equal positive and return leads | m | 3 | 0.01–100 |
| Cable resistance | Actual datasheet resistance at 20 °C | Ω/km | 0.5 | 0.001–100 |
| Conductor temperature | Assumed copper operating temperature (not ambient) | °C | 75 | −20–90 |
| Voltage-drop target | Your selected continuous goal | % | 2.5 | 0.1–10 |
The example resistance is hypothetical—not an AWG conversion or a cable recommendation. The temperature is an assumed conductor temperature, not ambient, not a predicted temperature and not an approved terminal rating. Watts are real power, not VA; motor start behaviour depends on power factor, VA limits and duration, so consult your inverter and motor specifications.
Voltage-class selector
| Class | Illustrative nominal V | Illustrative minimum inverter-terminal V |
|---|---|---|
| 12V | 12.8 | 11.2 |
| 24V | 25.6 | 22.4 |
| 48V | 51.2 | 44.8 |
These are example values only—replace them with your equipment specifications. They are not BMS cutoff recommendations, and choosing a class does not enforce compatibility.
Equations (formula version 1.0.0)
All intermediate values are kept unrounded.
η = eff / 100 ηs = surgeEff / 100
D = dod / 100 F = retention / 100
Eday = ac / η + dc + idle × hours [Wh/day]
Ereserve = Eday × days [Wh]
Enameplate = Ereserve / (D × F) [Wh]
Ah_exact = Enameplate / nominal [Ah]
Ah_display = ceil(Ah_exact) [whole Ah]
Icontinuous = watts / (η × minv) + dcamps + idle / minv [A]
Isurge = surge / (ηs × minv) + dcamps + idle / minv [A]
Rloop = 2 × length × resistance / 1000
× [1 + 0.00393 × (temp − 20)] [Ω]
Vdrop_cont = Icontinuous × Rloop [V]
Drop_% = 100 × Vdrop_cont / minv [%]
Cable_heat = Icontinuous² × Rloop [W]
Vdrop_surge = Isurge × Rloop [V]Model conventions
- Overhead: enter idle overhead only if it is not already included in your efficiency or demand figures. It is added to continuous and surge current as a conservative concurrent assumption, even though its daily duration is entered separately.
- Minimum voltage: the minimum inverter-terminal voltage already represents loaded voltage. The computed cable drop is not subtracted again—this is a first-order estimate, not a solved constant-power circuit.
- Cable current: make sure the direct DC current you enter actually flows through the modelled cable segment. This is not a branching-circuit solver.
- Temperature correction is a linear copper approximation referenced to about 20 °C; alloy, stranding and connectors vary. Use your supplier’s data.
- Heat loss is peak continuous I²R loss, not a predicted cable temperature, and it is not added to daily energy (duty cycle is unknown). Budget distribution losses separately.
- No charging is credited during the reserve period. No self-discharge, contact resistance, dynamic voltage sag or temperature-capacity curve is modelled.
- Depth of discharge is a planning assumption. An 80% figure does not guarantee any cycle life.
Validation, rounding and status
- Blank, non-numeric, partial (e.g. “12abc”), infinite and out-of-range values are rejected with a field-specific message. Results are cleared—never left stale—and the report cannot be printed until inputs are valid.
- Cross-field rules: minimum inverter voltage ≤ nominal voltage; surge power ≥ continuous power; AC + DC daily energy > 0. Advisories appear when AC energy is entered with zero continuous power, or when DC energy and DC current disagree.
- Capacity is rounded upward to whole Ah (a tiny 1×10⁻⁹ Ah allowance absorbs floating-point noise only). Other results are rounded for display; very small non-zero values show as “<0.01”.
- Voltage-drop status compares unrounded values: within, at (exact equality) or above your target. If continuous drop reaches 25% of minimum voltage, or surge drop reaches 50%, the result reads “Outside useful range of this simplified model; professional analysis required.”
- A within-target result means only that the stated numerical criterion was met.
Worked example (default inputs)
| Step | Calculation | Result |
|---|---|---|
| Daily energy | 3,500 / 0.90 + 0 + 10 × 24 | 4,128.89 Wh/day |
| Reserve energy | 4,128.89 × 2 | 8,257.78 Wh |
| Nameplate energy | 8,257.78 / (0.80 × 0.90) | 11,469.14 Wh |
| Capacity | 11,469.14 / 51.2 = 224.007 Ah → rounded up | 225 Ah |
| Continuous current | 2,000 / (0.90 × 44.8) + 0 + 10 / 44.8 | 49.8 A |
| Surge current | 3,000 / (0.85 × 44.8) + 0 + 10 / 44.8 | 79.0 A |
| Loop resistance | 2 × 3 × 0.5 / 1000 × 1.21615 | 0.003648 Ω |
| Continuous drop | 49.83 × 0.003648 | 0.18 V (0.41%) |
| Heat loss | 49.83² × 0.003648 | 9.1 W |
| Surge drop | 79.00 × 0.003648 | 0.29 V |
Why there is no automatic wire or fuse size
Safe conductor ampacity depends on the applicable installation standard and edition, conductor construction and insulation, terminal ratings, wiring method, ambient conditions, bundling, duty cycle and more. Overcurrent protection additionally depends on maximum DC voltage, available fault current, interrupt capability at that voltage, time-current behaviour, surge magnitude and duration, and placement. A handful of calculator inputs cannot determine these, and adding a disclaimer to an arbitrary table would not make it safe. Any future sizing module would require independent qualified electrical review and versioned acceptance tests before release.
Testing
Calculator v1.0.0 is checked by automated tests with independently hand-calculated expected values (for example: 3,500 Wh/day for 2 days at 100% efficiency, 80% DoD and 48 V must display 183 Ah, not 182), boundary and invalid-input tests for every field, and browser checks of result clearing and recovery. Automated tests are not a professional engineering review; independent electrical review is pending.
Sources consulted
These references support the principles and cautionary wording on this site. They do not certify BatteryCaliber, and no source organization sponsors or has approved these formulas.
| Source | Used for | Limitation |
|---|---|---|
| Victron Energy — Wiring Unlimited: DC wiring | Cable voltage drop, wiring and protection concepts | Manufacturer guidance, not universal installation approval |
| Victron Energy — Lithium Battery Smart technical data | Example of product-specific limits (including a +5 °C minimum charging temperature for that range) | Applies to the described products only |
| NFPA — NEC ampacity resources | Why ampacity tables and correction factors are required | Not a complete code assessment |
| ABYC — Standards list | Identifies marine electrical standards E-11 and E-13 | Not the standards text |
| ABYC — 2025 standards update | Shows standards are revised; editions must be checked | Locally applicable edition must be confirmed |
| Cirris — Temperature coefficient of copper | Basis for the linear copper resistance correction | Approximation near the reference temperature |
Version history
- Calculator 1.0.0 / formula 1.0.0 — 24 October 2026. Initial public version. Separate continuous and surge current; minimum-voltage current basis with explicit efficiency; capacity rounded upward; strict validation with no stale results; “at target” and “outside model range” statuses; no automatic conductor or fuse sizing.
Found a possible error? Email contact@batterycaliber.com with your inputs and what you expected. Formula changes are versioned, tested and recorded here.