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 depends on your inverter's continuous current draw at full load plus margin for surges, and it must coordinate with your wire: rated above normal operating current, below 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 safety factor covers startup surges and prevents nuisance blowing; the exact value depends on fuse type and installation.
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 exceed the fuse rating.
Common questions
What happens if the fuse is too large?
It may never open during a fault, letting fault current cook the wire instead. An oversized fuse protects nothing - the wire becomes the fuse, 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.
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.