See what an hour of driving puts back, how long the alternator takes to refill your bank, and the DC-DC charger size your alternator can handle — power from the engine, not only the sun.
The designer sizes your DC-DC charger, solar and shore together and shows the combined recharge time for your real bank — every way your power comes in.
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A DC-DC (B2B) charger takes power from your alternator and pushes it into the house bank while the engine runs — so every hour of driving returns roughly its rated current in amp-hours (a 30 A charger ≈ 30 Ah an hour). For people who move often, this can be the main charge source, with solar topping up while parked.
Two limits keep it honest: the battery only accepts so much current (its C-rate), and the DC-DC should not pull more than about 25–30% of the alternator’s rating continuously, or it can overheat it. This calculator caps to both.
Divide the amp-hours you need back by your DC-DC charger’s current. A 30 A charger returns about 30 Ah per hour of driving, so refilling 120 Ah takes roughly four hours on the road — sooner if solar is feeding in too.
As a rule of thumb, keep the DC-DC continuous draw under ~25–30% of the alternator rating, so a 150 A alternator suits a 30–45 A charger. Bigger banks that want faster charging usually need a larger or second alternator.
Often much less. Drive most days and the alternator can carry the system, with solar as a tender for when you sit still. Park for long stretches and you will lean more on solar.
Wattonomy applies these standards in its calculations. It is not certified, sponsored or endorsed by ABYC, ISO, NFPA or Victron. Last reviewed June 2026 — see the methodology.
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