DC to AC Inverter Load Calculator

DC to AC inverter load calculator for choosing the right inverter size

This calculator is for one decision: choosing an inverter that can reliably run your AC appliances from a DC source such as a 12V, 24V, or 48V battery system. People typically search for this when building a solar setup, outfitting a camper or boat, running backup power at home, or powering tools on-site. The risk is always the same: buying an inverter that is too small (it trips, overheats, or shuts down) or sizing it wildly too large (it costs more, wastes space, and can be less efficient at very light loads).

The starting point is your total running watts, which is what your appliances draw continuously once they are already operating. Many loads, especially those with motors and compressors, also have a short surge at startup. If you know the surge wattage, include it. If you do not, you can still get a useful recommendation based on running watts plus sensible headroom, and you can treat the surge result as a prompt to verify the worst-case starting load for your biggest motor-driven device.

The calculator returns more than a single number because inverter selection is not just “watts.” It estimates the inverter’s recommended continuous rating (with headroom), a surge rating target, the DC input current you should expect, and a practical fuse size for the DC side. The DC current estimate matters because most real failures come from undersized DC wiring, loose connections, or inadequate protection. Even a modest AC load can translate into very high DC current at 12V, especially during surges.

Assumptions and how to use this calculator

• Running watts are the steady-state AC power draw of everything you expect to run at the same time.
• Surge watts are optional and represent the highest short-term starting or peak demand; if omitted, only continuous sizing is emphasized.
• Inverter efficiency defaults to 90% to reflect typical real-world conversion losses; you can adjust it if you have a manufacturer figure.
• Power factor defaults to 0.90 to estimate apparent power (VA), which can matter for certain loads and inverter limits.
• Fuse sizing is estimated as a practical step-up above expected maximum DC current; it is not a substitute for local electrical standards or a qualified installer.

Common questions

Why does a 12V system show such high current compared to 24V or 48V?

Power is roughly voltage times current. For the same AC load, a lower DC voltage requires more DC current to deliver the same power after losses. That higher current is harder on cables, connectors, and fuses, and it is a common reason 12V inverter installs fail when people move beyond small loads. If you can choose the system voltage, higher voltage usually makes wiring and protection easier.

Do I size the inverter based on watts or VA?

Most people think in watts, but some loads (and some inverter limits) relate to apparent power (VA). This calculator uses power factor to estimate VA from watts. If you are running mostly resistive loads (heaters, incandescent lighting), power factor is close to 1.0 and watts and VA are similar. If you are running motors or certain electronics, VA can be meaningfully higher than watts, and extra headroom reduces nuisance shutdowns.

What if I do not know the surge watts for my appliance?

Use the calculator with running watts first, then identify your most surge-heavy device (typically a fridge compressor, pump, or power tool). Check the appliance label, manual, or a typical starting watt estimate. If you cannot find it, treat your surge requirement as uncertain and bias toward a larger inverter or avoid starting multiple motor loads simultaneously.

Is the recommended fuse size always correct?

It is a practical estimate, not a guarantee. Real fuse selection depends on the inverter’s maximum DC input specification, cable gauge, cable length, insulation rating, installation method, and local code. The safest approach is to confirm the inverter manufacturer’s recommended DC fuse or breaker and ensure your cabling is sized for both continuous current and surge current without excessive voltage drop.

How accurate is the runtime estimate if I add battery capacity?

It is a rough planning number. It assumes a usable percentage of the battery, ignores Peukert effects at high discharge currents, and assumes the load is steady. Real runtime can be shorter when current is high, temperature is low, batteries are older, or loads cycle on and off. Use it to sanity-check expectations, not to design mission-critical backup time.