500A for almost any rig with an inverter, 300A for one without. That is the amperage answer and it takes one paragraph, which is why every other page on this question stops there.
If you have not settled the prior question — whether you need a separate shunt at all when the battery already has Bluetooth — that one turns on what your battery actually measures, and it is answered here.
A shunt has two ratings. The second is the millivolts it produces at its full rated current, and it is the one that quietly breaks systems — because a mismatched shunt does not fail, error, or warn. It reports plausible numbers that are wrong by a fixed multiple, forever.
The two numbers, and why only one gets discussed
A shunt is a precision resistor of known, tiny value, wired into the battery negative so that everything entering or leaving the bank goes through it. The monitor does not measure current directly. It measures the small voltage that appears across the shunt and calculates the current from Ohm’s law.
That gives every shunt two ratings:
- Full-scale amps — the most it is built to carry. 300A, 500A, 1000A, 2000A.
- Full-scale millivolts — what it produces at that current. Almost always 50mV, 75mV or 100mV.
So a 500A/50mV shunt puts out exactly 50 millivolts when 500 amps flow, 25 millivolts at 250 amps, 10 millivolts at 100 amps. The monitor is built around one of those combinations and divides by the constant it expects.
Amperage is the rating everyone asks about. Millivolts is the rating that decides whether the number on the display means anything.
Sizing the amps: the inverter decides, and nothing else is close
Work out the largest current that will ever pass through the shunt in either direction. In practice one load dominates so heavily that the rest is noise.
| What is drawing | Current at 12V |
|---|---|
| Xantrex Freedom XC 2000 at full load | 192A published, up to 400A starting heavy loads |
| Renogy 3000W inverter at full output | about 294A |
| Renogy 2000W inverter at full output | about 196A |
| 80A converter on shore power | 80A, charging |
| 50A DC-DC charger from the alternator | 50A, charging |
| 400W of solar in full sun | about 22A, charging |
The charging side tops out around 150A even with everything running at once. The discharge side runs to three or four hundred the moment a large inverter is fitted — and the Xantrex figure is the one to design against, because it is the manufacturer’s own published starting draw rather than an average.
That gives the two-line answer. With an inverter of any real size, 500A. Without one, nothing in a 12V RV approaches 300A and the smaller shunt is plenty.
Why the biggest shunt is not the safe choice
The common advice is “pick the smallest shunt that meets your needs”, and the reasoning is sound: a shunt’s zero offset and its resolution both scale with its full-scale rating, so an oversized shunt measures small currents worse.
Victron is the only manufacturer here that publishes the numbers, so it is the only one where the advice can be checked rather than repeated:
| SmartShunt rating | Published zero offset |
|---|---|
| 300A | under 10 mA |
| 500A | under 10 mA |
| 1000A | 20 mA |
| 2000A | 40 mA |
The advice is right at the top of the range and wrong at the bottom. A 2000A shunt carries four times the offset of a 500A. Forty milliamps of phantom current is roughly 1Ah a day of drift — on a 200Ah bank that is half a percent of state of charge accumulating every day you are away from hookups, always in the same direction, until you next charge to full and it resets.
But 300A and 500A have the same published offset, so choosing 300A buys no precision at all. It only removes the headroom for an inverter you might fit later. That distinction is worth having, and it is the part the general advice misses.
The millivolt trap
Here is what actually happens when the second rating is wrong. One hundred amps of real current, three different shunts, one monitor built for 500A/50mV:
The 75mV case is the dangerous one. A tenfold error announces itself — nobody believes their fridge is pulling a thousand amps. A 1.5× error does not. The display looks normal, the numbers are plausible, and the bank appears to be draining half again as fast as it really is. You would replace the battery before you suspected the shunt.
Three practical consequences:
- Buy the monitor and shunt as a matched pair unless you know both ratings.
- If you are replacing a shunt, match the millivolts first and the amps second. A 1000A/50mV in place of a 500A/50mV reads correctly; a 500A/75mV in place of a 500A/50mV does not.
- Victron is the easy case — every SmartShunt and BMV shunt is 50mV across all four current ratings, so within that range only the amps change. Which of its two monitors to put on the end of it is a separate and smaller question: SmartShunt vs BMV-712 — they are the same measurement engine, and the decision comes down to a relay.
Not every monitor includes a shunt, and not every maker publishes the rating
Two things worth checking before ordering, both real:
Bogart’s TriMetric TM-2030 ships without a shunt and asks you to choose one: an SH-500-50 or an SH-100-100. Those differ in both ratings — 500 amps at 50 millivolts against 100 amps at 100 millivolts — so this is a live version of the decision above, from a single manufacturer, for a single monitor.
Renogy’s 500A monitor does not publish its millivolt rating at all. We checked the US product page, the FAQ page and the EU product page in August 2026; none of them states whether the shunt is 50mV, 75mV or 100mV, and Renogy’s own manual PDF returns a server error. It is a perfectly reasonable budget monitor bought as a sealed pair. It is not something you can confidently mix with other hardware, and on this specific question that is the whole point.
Victron publishes 50mV plainly, along with the offset table above. On a page about matching two ratings, which manufacturers tell you both is not a detail.
Five ways this goes wrong
- Treating it as an amperage question. It is two ratings and the second one fails silently.
- Buying the biggest shunt available. Past 500A you are paying for offset you did not want.
- Sizing to average draw. Size to the peak, and on an inverter rig the peak is the starting surge, not the running load.
- Leaving a negative connection on the battery post. Anything that bypasses the shunt is current the monitor never sees, and it always drifts one way.
- Assuming a shunt is in the box. Sometimes it is not, and sometimes the choice of which one is yours to get right.
The monitors this points at
The 500A/50mV default, no display
- Bluetooth
- Yes
Victron SmartShunt 500A
Covers any RV inverter, and Victron publishes the offset and accuracy figures nobody else does.
The same shunt with a panel display
- Bluetooth
- Yes
Victron BMV-712 Smart
Identical measurement engine to the SmartShunt — you are buying the display, an alarm and a relay.
Budget, with a caveat this page is about
- Bluetooth
- Not published for this model
Renogy 500A Monitor
500A and a 20 ft cable for less, but the millivolt rating is not published anywhere we could find.
All four, plus the one that ships without a shunt at all, are compared on what each maker actually publishes in best RV battery monitors.
Related guides
- What size inverter do I need for my RV? — the load that decides your shunt rating, sized properly.
- RV battery calculator — how big the bank needs to be before you start measuring what it is doing.
- Best RV lithium batteries — the flat discharge curve that makes voltage-based monitoring nearly useless.
- 12V wire size calculator — the cable the shunt sits in, sized for the same peak current.
- Do I need a DC-DC charger? — charging current, which passes through the same shunt in the other direction.
- The electrical section collects everything published so far.
For how products on this site are selected and what we do not claim about them, see the editorial policy.
Questions worth answering
What size shunt do I need for an RV battery monitor?
500A for almost any rig with an inverter, and 300A for one without. The inverter is what decides it: Xantrex publishes a 192A full-load draw for its 2000W Freedom XC and up to 400A when starting heavy loads, and a 3000W inverter pulls close to 300A continuously. Charging never comes close — an 80A converter, a 50A DC-DC charger and 400W of solar together are about 150A. If you have no inverter, nothing in a 12V RV gets near 300A.
What is the millivolt rating on a shunt?
The voltage the shunt produces across itself at its full rated current. A 500A/50mV shunt puts out exactly 50 millivolts when 500 amps flow through it, and proportionally less below that. The monitor reads those millivolts and multiplies by a constant it was built around, which is why the rating has to match what the monitor expects.
What happens if I use a 75mV shunt with a 50mV monitor?
Every reading comes out 1.5 times too high, and nothing warns you. At 100 real amps a 500A/75mV shunt produces 15mV; a monitor built for 500A/50mV divides by its own constant and reports 150 amps. It is not an error state — the number is plausible, the display looks normal, and your state of charge drains 50% faster than reality. Mismatches in the other direction under-report by the same logic.
Is a bigger shunt better?
No, and past a point it is measurably worse. A shunt's zero offset scales with its rating: Victron publishes under 10mA for both its 300A and 500A SmartShunts, 20mA for the 1000A and 40mA for the 2000A. A 40mA offset is about 1Ah of phantom drift a day, which on a 200Ah bank is half a percent of state of charge accumulating every day you are off hookups. Buy the rating your peak load needs, not the biggest one sold.
Is there any reason to choose 300A over 500A?
Not with Victron, and this is where the common advice is slightly wrong. "Pick the smallest shunt that meets your needs" is sound in general, but Victron publishes the same under-10mA offset for the 300A and the 500A, so the 500A costs you nothing in precision and leaves room for an inverter later. The advice starts to bite at 1000A and above.
Does the shunt go on the positive or negative side?
Negative, and it has to be the only path. Every negative connection in the rig — chassis grounds, the converter, solar, the inverter — must land on the load side of the shunt rather than directly on the battery post. Anything measuring at the terminals instead of counting current has the separate problem described in the [state of charge calculator](/electrical/rv-battery-state-of-charge-calculator/). Anything that bypasses it is current the monitor cannot see, which produces slow, one-directional drift that looks like a failing battery.
Do all battery monitors come with a shunt?
No. Victron's SmartShunt and BMV-712 both include a 500A/50mV shunt. Bogart's TriMetric TM-2030 includes none and asks you to choose between an SH-500-50 and an SH-100-100 — which are different in both ratings, 500 amps at 50 millivolts against 100 amps at 100 millivolts. Check what is in the box before assuming.
Can I replace just the shunt and keep my monitor?
Only if you know both ratings and can match the millivolts. That is straightforward with Victron, which publishes 50mV across the range, and not possible with confidence on hardware that does not publish the figure at all — the Renogy 500A monitor being the example on this page. Buy that kind as a sealed pair and replace it as a pair.
Does a shunt work with lithium?
Yes, and it matters more on lithium than on lead-acid. A LiFePO4 bank holds almost the same terminal voltage from full to nearly empty, so voltage tells you very little about state of charge. Counting amps in and out is the only method that resolves it, which is precisely what a shunt exists to do.