What a kWh From Your Alternator Actually Costs

There is a recurring argument in van forums that never resolves, because both sides are right about different situations. One camp says alternator charging is basically free and you should size your build around it. The other says it is a fuel-burning scam and you should buy solar. The thing that decides which one is correct is whether the engine was going to be running anyway.

One honesty note before the numbers, per How We Test: VanTested has not put a clamp meter on an alternator yet. Everything below is arithmetic from published fuel-economy figures, manufacturer claims, and the efficiency assumptions van and solar forums have converged on, labeled as exactly that. Our own charging chain appears here as an example of a specific architecture, not as a measurement.

Two prices, and why the gap is so wide

The physics is not complicated. An alternator is an engine-driven generator, so its electricity costs fuel. What changes between the two cases is the accounting.

Driving. The engine is already burning fuel to move the van. Adding a DC-DC charger to the load adds a small increment on top of that. You pay only for the extra.

Idling. The engine is running for the purpose of charging. Every drop of fuel is charged to the battery, including all the fuel the engine burns just to keep turning over. That base burn dwarfs the electricity you get out of it.

Then a second effect stacks on top: alternators produce substantially less at idle than at road speed, and forum reports of alternators struggling past about 20A at idle are common. So the idle case pays more fuel for less charge, at both ends.

The estimated math

Here is the arithmetic, with each assumption stated so you can substitute your own.

A gallon of gasoline carries roughly 33.7kWh of chemical energy. Almost none of it reaches your battery: the engine wastes most of it as heat, the alternator is lossy, and the DC-DC charger takes its own cut. The figure DIY-solar and van forums generally work from is that about 5kWh per gallon arrives at the battery, which is roughly 15% end to end. That number is an assumption, not a measurement, and it is the single biggest lever in everything below.

At the AAA national average of about $4.12 a gallon as this publishes:

Source	Estimated cost per kWh	What the number assumes
Alternator, while driving	~$0.82	~5kWh delivered per gallon; you pay only the marginal fuel
Alternator, at idle	~$4.80	~0.5 gal/hr idle burn on a large gas van; ~430W delivered by a 30A charger
Shore power	~$0.19	US average residential rate, ~18.8 cents/kWh (EIA, April 2026)
Solar, after payback	~$0.00	Marginal cost only; ignores the panels, controller, and roof work

The idle row is the one worth sitting with. Charging at idle is running a large engine for an hour to produce less energy than a hair dryer uses in half of one. It costs about twenty-five times what the same kilowatt-hour costs from a campground pedestal.

And the driving row is the one that surprises people the other direction. Eighty cents a kilowatt-hour sounds expensive next to grid power, until you notice that a full 100Ah lithium bank is only about 1.3kWh. Refilling it from empty on a travel day costs roughly a dollar of fuel. For most builds, alternator charging while driving is not a cost centre worth optimising. It is capacity you already own.

Why you cannot skip the DC-DC charger

The old approach was a battery isolator: a relay that connects the house bank to the starter battery once the engine is running. On a lead-acid house bank behind an old-fashioned alternator, that worked well enough. Two changes broke it.

Lithium changed what the bank wants. LiFePO4 needs a held charge voltage in the region of 14.2 to 14.6V to actually reach full. Feed it whatever the alternator happens to be producing and it will take a charge, then stall short of capacity, and you will spend a year wondering why a 100Ah bank behaves like a 70Ah one.

Smart alternators changed what the alternator gives. Emissions-era vans run regulated, variable-output alternators that deliberately back off once the starter battery is satisfied, and use the alternator as a load for regenerative braking rather than a constant supply. A house bank hanging off that sees a voltage that rises and falls for reasons that have nothing to do with its own state of charge.

A DC-DC charger sits between the two and decouples them: it accepts whatever the alternator is doing, and outputs a proper multi-stage profile for the chemistry you selected. It also limits current, which protects the alternator from a large lithium bank’s willingness to accept everything on offer.

If your van is older, carbureted-era simple, and your bank is AGM, an isolator is still a defensible cheap answer. For anything modern with lithium, the charger is not an upgrade, it is the part that makes the system work.

Sizing it, in two constraints and a derate

The size question has a tidy answer and an untidy caveat.

Constraint one, the battery. Common guidance is up to 50% of a lithium bank’s amp-hour rating, and around 20% for AGM. A 100Ah lithium bank tolerates up to a 50A charger; a 200Ah bank comfortably takes 40 to 80A.

Constraint two, the alternator. Size to roughly half the alternator’s rating so the vehicle’s own systems keep their headroom. A 100A alternator points at a 50A ceiling. Note also that chargers can draw meaningfully more than their rated current at startup — vendor guidance describes surges around 50% above rating — so the alternator needs to absorb that spike too.

Take the lower of the two. Then apply the caveat: heat. Manufacturer derating notes describe chargers delivering well under their rated output as ambient temperature climbs, which matters because these units usually get mounted in exactly the hot, unventilated places that trigger it. This is not a theoretical concern in a van — the loudest thread in the most recent climate listening cycle was a builder whose charge controllers were cooking under his bed on the way to West Texas. If your charger lives in a sealed cabinet, buy a size up and run it below its limit rather than buying to the number and running it at the ceiling.

The architecture question the spec sheets skip

There are two ways engine power reaches a house bank, and they behave differently.

Direct. Alternator to DC-DC charger to house bank. One conversion, one device, sized to the constraints above. This is the standard van build and it is the one the sizing math describes.

Through a power station. An alternator charger feeds a portable power station, and a second DC-DC charger takes power back out of the station into the 12V bank. This is the chain we run in our own van, and it is worth being explicit about its cost: energy converts twice, and the bank’s charge rate is set by whatever the second charger can pass, not by what the alternator can produce. In our chain that exit is a claimed 20A, which we worked through in the 12V output ceiling guide. A 1,200W alternator charger upstream of a 270W exit is a 270W system.

Neither is wrong. The station-in-the-middle version buys you clean AC output and a battery you can carry out of the van, and it costs you charge rate at the bank. The direct version is the faster and simpler path to a full house battery, and it does none of the other jobs. What you should not do is buy a large alternator charger, route it through a station, and expect the bank to see the big number.

The one place we have first-hand experience rather than claims is what happens when the loads outrun all of it: in our festival field log, a 100Ah bank running a 12V mini split went flat in about three hours with the DC-DC charger assisting the whole time. Charge rate, not battery size, is the thing that decides whether a build survives a hot afternoon.

What we would need to measure to make this a real number

Every figure in the table above rests on that ~5kWh-per-gallon assumption, and nobody we can find has measured it on a modern van. The test is not exotic: a clamp meter on the charger output, a fuel-flow reading from the vehicle’s own data, and a pair of runs — one at road speed, one at idle — with the bank at a known state of charge. That yields real delivered watt-hours against real burned fuel, for both cases, on a specific vehicle.

That is a measurement job and it is on our bench list for the ProMaster. Until it happens, treat the numbers here as a well-constructed estimate that tells you the shape of the answer, which is the part that matters: driving-time charging is cheap, idle charging is not, and the gap between them is large enough that no plausible correction to the efficiency assumption closes it.

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Changelog

• July 30, 2026: First publication. Cost figures are estimates from published fuel data and community efficiency assumptions; a clamp-meter measurement on the ProMaster is queued to replace them.