To step up from a 12V nominal source to a 24V output at 5A using a DC-DC voltage converter with a realistic 90% efficiency, your input current will be exactly 11.11 Amps. This calculation assumes a steady 12.0V input and a continuous 120W output load. Unlike AC power calculations, DC-DC conversions rely strictly on voltage, current, and converter efficiency—power factor and phase angles do not apply to the DC stage.
The Core DC-DC Conversion Formula
When sizing wires or selecting a fuse for the input side of a boost converter, you cannot simply use the output current. The converter must draw extra power from the source to cover its own internal switching and conduction losses. The governing formula for input current ($I_{in}$) is:
$$I_{in} = \frac{V_{out} \times I_{out}}{V_{in} \times \eta}$$
Substituting our target values (24V out, 5A out, 12V in, 90% efficiency):
$$I_{in} = \frac{24V \times 5A}{12V \times 0.90} = \frac{120W}{10.8V} = 11.11A$$
The primary assumption that fixes this answer is the efficiency rating ($\eta$) and the exact input voltage. While we used a nominal 12V, a real-world 12V lead-acid or LiFePO4 battery swings from 11.5V (discharged) to 14.4V (under alternator/charger charge). Because power factor (PF) and phase are strictly AC concepts, they are replaced by the converter's switching efficiency in DC calculations. A high-quality synchronous boost controller like the Texas Instruments LM5122 might hit 94% efficiency at half-load, but drop to 88% at maximum thermal limits, which would push your input current closer to 11.36A.
Neighboring Values: Input Current vs. Output Load
Designing for a single exact number is a common bench mistake. Loads fluctuate, and battery voltage sags under heavy draw. The table below maps a ±20% range around our 5A target output, factoring in both a nominal 12.0V source and a sagging 11.0V source (typical of a deeply discharged lead-acid battery under heavy load).
| Output Current (A) | Output Power (W) | Input Current @ 12.0V (A) (90% eff) |
Input Current @ 11.0V (A) (90% eff, low battery) |
|---|---|---|---|
| 4.0 (−20%) | 96W | 8.89A | 9.70A |
| 4.5 (−10%) | 108W | 10.00A | 10.91A |
| 5.0 (Base) | 120W | 11.11A | 12.12A |
| 5.5 (+10%) | 132W | 12.22A | 13.33A |
| 6.0 (+20%) | 144W | 13.33A | 14.55A |
Wiring Note: If your input current can hit 14.55A during a low-voltage sag, standard 14 AWG wire (rated 15A in free air) is dangerously close to its thermal limit. Step up to 12 AWG THHN or automotive SXL wire, and protect the input feed with a 15A or 20A inline fuse located as close to the battery positive terminal as possible.
Why AC Concepts (Power Factor, 3-Phase) Are Meaningless Here
Attempting to apply a power factor (PF) correction to a pure DC-DC conversion is meaningless. In DC circuits, the power factor is exactly 1.0 because voltage and current do not alternate and therefore cannot be out of phase. If your PF is unknown or measured at less than 1.0, you are measuring an AC circuit, not a DC-DC stage, and using DC-DC formulas will yield incorrect results. For deeper insights into measuring true efficiency across different topologies, refer to this Analog Devices design note on DC-DC efficiency measurement.
Furthermore, because DC-DC converters operate strictly on direct current, concepts like 120V vs 230V mains or 3-phase power do not shift the DC-DC conversion math itself. However, if your DC-DC converter is fed by an upstream AC-DC power supply (like a 24V DIN-rail supply feeding a 24V-to-12V buck converter), the AC mains draw shifts dramatically based on the supply type:
- 120V AC Single-Phase: Assuming a 0.9 PF and 85% AC-DC efficiency, a 120W DC load will draw about 1.30A AC from the wall.
- 230V AC Single-Phase: That same 120W load draws just 0.68A AC.
- 3-Phase Industrial: The current per leg drops even further, balancing the load across three hot conductors.
But at the DC-DC stage itself, these upstream AC variables are entirely isolated and irrelevant to your input current calculations.
Frequently Asked Questions
How do I choose a DC-DC voltage converter for a 12V to 24V solar application?
For solar applications, you need a converter that can handle wide input voltage swings. A 12V "nominal" solar battery bank can see voltages as high as 15V during equalization charging and as low as 10.5V during deep discharge. Choose a boost converter with an input voltage range of at least 9V to 18V, and ensure it features a synchronous rectifier (using a MOSFET instead of a catch diode) to maintain high efficiency when the solar array is producing limited wattage. Additionally, look for conformal coating on the PCB if the unit will be mounted in an unsealed outdoor enclosure to prevent humidity-induced tracking and short circuits.
What is the difference between isolated and non-isolated DC-DC voltage converters?
A non-isolated converter (like a standard boost, buck, or buck-boost topology) shares a common ground between the input and output. They are smaller, cheaper, and generally more efficient, making them ideal for automotive and battery-powered electronics. An isolated converter uses a high-frequency transformer to physically separate the input and output circuits (galvanic isolation). This is critical in medical devices, industrial PLC I/O, and high-voltage battery management systems (BMS) where a ground loop or a short-to-chassis on the output side must not energize the input source or pose a shock hazard.
Can a single DC-DC voltage converter automatically step up and step down voltage?
Yes, but you must specifically look for a buck-boost or SEPIC (Single-Ended Primary-Inductor Converter) topology. A standard boost converter can only step up, and a standard buck converter can only step down. If you are running a 24V device off a 12V battery bank that is being charged at 14.4V, a standard buck converter won't work. A true 4-switch buck-boost DC-DC voltage converter will seamlessly transition between stepping up and stepping down as the battery voltage crosses the 24V threshold, maintaining a stable output without interrupting the load.
