What size fuse between battery and inverter?
The fuse between your battery bank and inverter is one of the most important safety devices in an off-grid system. If a short circuit happens, a properly sized fuse opens before your wiring melts. Many DIY builders treat it as an afterthought; it is not.
The right rating starts from your inverter's continuous current draw at full load, with the standard continuous-load allowance on top, and it must coordinate with your wire: rated above normal operating current and no higher than the wire's ampacity, so the fuse always gives up before the copper does.
How it works
The core relationship is fuse rating = continuous current x a safety factor, where continuous current is inverter watts divided by battery voltage. The factor is the continuous-load allowance from electrical code: fuses and conductors are not meant to run at their full rating for hours on end, so continuous circuits are sized with headroom. Brief motor-start surges are absorbed by the fuse's time-current curve, which tolerates short overloads well above the rating, not by the factor.
System voltage matters enormously: the same inverter wattage draws half the current at 24V and a quarter at 48V compared to 12V, which changes the fuse class, the wire, and the cost. Inverter manuals usually publish a recommended fuse or breaker rating - when the manufacturer speaks, follow them. The one rule that never bends: wire ampacity must be at least the fuse rating.
The rating is only half the spec. A fuse also has an interrupt rating, the largest fault current it can safely open. A lithium bank can dump thousands of amps into a dead short, and a fuse with a low interrupt rating can arc across instead of clearing, which is exactly the fault it exists to stop. That is why battery-main fuses on lithium banks are usually Class T or terminal-mount MRBF types rather than automotive blade or ANL styles; the guidance names the construction for each segment.
Common questions
What happens if the fuse is too large?
A dead short will still open it, but an overload that sits above the wire's ampacity and below the fuse rating never will, and that band is where the wire cooks. An oversized fuse leaves the wire as the weakest link, and wires fail by burning.
Can I use a circuit breaker instead?
Yes, if it is rated for DC at your system voltage. DC arcs are harder to interrupt than AC; an AC-only breaker may fail to break the circuit. DC-rated breakers add a resettable disconnect, which is genuinely convenient.
Do both the positive and negative wires need a fuse?
Typical DIY systems fuse the positive conductor only. Some installations add a disconnect on the negative side for full isolation during service, which is a different job than overcurrent protection.
Why does battery voltage change the fuse size?
Power is voltage times current, so the same inverter pulls proportionally less current at higher system voltage. That is a major reason larger systems run 24V or 48V.
Does the fuse type matter, or only the rating?
Both. The rating decides when it opens; the interrupt rating decides whether it can open safely against the current your bank can deliver into a short. Lithium banks have very low internal resistance, so a correctly rated fuse of the wrong construction can fail to clear the fault. Class T fuses cover the large banks and higher voltages; MRBF terminal fuses are widely accepted on small 12V banks.
Pick your battery and inverter in the builder and the fuse and wire guidance is computed for your exact combination, math shown.
Open the builderThese guides are educational and qualitative on purpose - the numeric answers depend on your exact components, and the builder computes them with every formula shown. Always verify a final design with a licensed professional.