DC Power Supply Calculator

A DC power supply converts AC mains voltage to a regulated DC output and powers loads directly. A battery charger converts AC to DC but its purpose is storing energy in a battery, not powering loads in real time. In a solar off-grid system, a charge controller (MPPT or PWM) acts as the DC supply between the panels and battery, while the battery then powers loads directly or through an inverter. This calculator sizes the power supply (or charge controller output capacity) to match the total continuous DC load demand.

It depends on your daily watt-hour consumption and local peak sun hours. The basic formula is: daily Wh ÷ (peak sun hours × system efficiency). For example, 500 Wh/day of DC loads at 4.5 peak sun hours with 80% system efficiency requires: 500 ÷ (4.5 × 0.80) = 139W of panel capacity - roughly one 150W or 200W panel. For critical loads, size for your worst-month peak sun hours rather than the annual average. Use this calculator's Solar settings section to get a panel count specific to your load profile and location.

12V is the most common voltage for small off-grid systems under 1,000W and for RV/van applications - most 12V appliances and DC-DC converters are widely available at low cost. 24V systems reduce conductor current by half for the same power level, cutting wire size and losses - suitable for systems of 1,000–3,000W. 48V is standard for larger off-grid homes and commercial off-grid systems above 3,000W, where the lower current at high voltage makes long wire runs practical and charge controllers more efficient. Match your system voltage to your load voltage and the available battery bank configurations.

Switching mode power supplies (SMPS) regulate output voltage by rapidly switching transistors, achieving 85–95% efficiency. Linear regulators dissipate the voltage drop as heat - a 24V supply running from 48V at 5A loses 120W as heat, requiring a large heatsink. For solar and off-grid applications, wasted power directly increases battery capacity and panel count requirements. The calculator recommends SMPS for most applications above 50W and only suggests linear supplies for specialized low-noise or low-power scenarios. The topology comparison card shows the efficiency and heat output difference for your specific load.

Most DC power supplies are rated at 25°C ambient. As temperature rises, the supply's ability to dissipate heat decreases, forcing it to reduce output to avoid thermal shutdown. A supply rated for 100W at 25°C may only safely deliver 70–80W at 50°C. Enclosures in vans, outdoor equipment cabinets, and desert installations regularly see 40–60°C internal temperatures. The temperature derating input adjusts the required supply rating upward to compensate - and recommends adding active cooling (fan or forced-air ventilation) if heat dissipation exceeds the passive threshold.

LFP (Lithium Iron Phosphate) is the preferred chemistry for new off-grid solar installations: 80% usable depth of discharge, 3,000–6,000 cycle life, no maintenance, and safe thermal behavior. Lead-Acid/AGM works for budget builds or short-term backup - 50% usable depth of discharge and 500–1,000 cycle life make it significantly less cost-effective over the system lifetime despite lower upfront cost. Standard Li-Ion (NMC/NCA) offers high energy density but is less thermally stable than LFP and not typically recommended for permanent outdoor off-grid installations. Select your chemistry in the battery settings - the calculator adjusts the required Ah figure based on the chemistry's depth of discharge characteristic.