DC-to-DC Voltage Step-Up/Down Converter Sizer
Calculates the primary input current draw, thermal heat dissipation, and wire/fuse specifications when transforming DC voltages between 12V, 24V, and 48V power networks.
Enter the values above and click Calculate to view recommendations and engineering specifications.
The Input Current Multiplier in Step-Up Converters
Calculation Assumptions
- Conservation of energy with 88%–92% typical electronic buck/boost conversion efficiency.
- Input current $I_{\text{in}} = \frac{V_{\text{out}} \times I_{\text{out}}}{V_{\text{in}} \times \eta}$.
Engineering Limitations
- Step-up converters (e.g. 12V to 24V) draw double the amperage on the input side; wiring must be sized accordingly.
Delivering 24V at 10A (240W) from a 12V battery at 90% efficiency requires 240W / (12V × 0.90) = 22.2A input draw, demanding 10 AWG input wire.
When converting DC voltages, electrical power is conserved minus internal switching losses:
• Pin = Primary battery input power in Watts.
• Iin = Primary battery input current in Amperes.
• η = Converter efficiency (typically 0.88 to 0.95).
When stepping voltage UP (for example, stepping 12V up to 24V to run an air conditioner, or stepping up to 48V for a Starlink/communications dish), the input current is more than double the output current:
- To output 10A at 24V (240W), the 12V battery feed must supply 21.5 Amps.
- The input wire must be sized significantly heavier than the output wire. Undersizing the input wire causes voltage sag that drops input voltage below the converter's low-voltage protection limit, causing erratic reboot cycles.
DC-DC Voltage Conversion & Thermal Dissipation Matrix
The table below demonstrates input current multiplication and continuous waste heat dissipation across common step-up and step-down configurations operating at a typical 90% switch-mode efficiency:
| Voltage Conversion Domain | Secondary Load Output | Primary Battery Input Current | Continuous Waste Heat | Rec. Input Wire (10 ft) | Input Fuse Rating |
|---|---|---|---|---|---|
| 12V to 24V Step-Up | 240W (10A @ 24V) | 22.2 Amps @ 12.0V | 26.7 Watts Heat | 10 AWG Copper | 30A ATO Blade |
| 12V to 48V Step-Up | 240W (5A @ 48V) | 22.2 Amps @ 12.0V | 26.7 Watts Heat | 10 AWG Copper | 30A ATO Blade |
| 12V to 48V Step-Up | 480W (10A @ 48V) | 44.4 Amps @ 12.0V | 53.3 Watts Heat | 6 AWG Copper | 60A Maxi / MIDI |
| 24V to 12V Step-Down | 240W (20A @ 12V) | 11.1 Amps @ 24.0V | 26.7 Watts Heat | 14 AWG Copper | 15A ATO Blade |
| 24V to 12V Step-Down | 480W (40A @ 12V) | 22.2 Amps @ 24.0V | 53.3 Watts Heat | 10 AWG Copper | 30A ATO Blade |
| 48V to 12V Step-Down | 360W (30A @ 12V) | 8.3 Amps @ 48.0V | 40.0 Watts Heat | 14 AWG Copper | 15A ATO Blade |
Frequently Asked Questions
What size 24V to 12V step-down converter do I need for RV accessories?
Sum the continuous amperage of all 12V DC branch accessories (water pump 8A, lights 5A, diesel heater 10A surge, USB ports 6A = 29A peak). Add a 25% engineering headroom factor (29A * 1.25 = 36A), requiring a 40A continuous step-down DC-DC buck converter (such as Victron Orion 24/12-40).
How do you calculate input current for a step-down DC-DC converter?
Input Current $I_{\text{in}} = \frac{V_{\text{out}} \times I_{\text{out}}}{V_{\text{in}} \times \eta}$. For example, a 24V to 12V converter delivering 30A at 12V (360W) with 92% efficiency requires: $\frac{12\text{V} \times 30\text{A}}{24\text{V} \times 0.92} = 16.3\text{ Amperes}$ of 24V input current.
Why do DC-DC converters generate heat under load?
While modern switch-mode buck-boost converters achieve high efficiency (88% to 95%), the remaining 5% to 12% of converted energy is dissipated as waste heat through internal switching MOSFETs and inductor coils. A 500W converter operating at 90% efficiency generates 50W of continuous heat, requiring adequate ventilation.
What is the difference between isolated and non-isolated DC-DC converters?
Non-isolated converters share a common negative ground between input and output, making them compact, inexpensive, and ideal for standard vehicle automotive circuits. Isolated converters provide complete galvanic barrier isolation between input and output grounds, preventing ground loops, marine electrolysis, and audio interference in sensitive communication gear.