Start with actual use
Create a table listing equipment, average power, daily operating time and energy. E = P × t gives Wh when P is in watts and t in hours. A measurement at the battery already includes losses in downstream equipment: do not add them twice. A measurement at the inverter output instead requires allowance for its efficiency and idle consumption.
Separate three profiles: at anchor, daytime sailing and overnight sailing. Autopilot, radar and lighting can change the result substantially. A refrigerator cycles; multiplying its rated power by 24 hours does not necessarily give daily consumption. Measure several cycles under expected conditions.
A transparent example, not a typical consumption claim
For an exercise only: refrigeration averaging 30 W for 24 h = 720 Wh; instruments at 25 W for 12 h = 300 Wh; lights at 15 W for 4 h = 60 Wh; miscellaneous uses at 40 W for 3 h = 120 Wh. Total: 1,200 Wh/day. Replace every assumption with your own measurements.
An additional autopilot averaging 35 W for 18 h would add 630 Wh, bringing the scenario to 1,830 Wh. This difference explains why a marina energy budget is insufficient for an ocean passage. Always specify where measurements were made and whether losses are already included.
Calculate usable reserve
Our tool uses: available energy = nominal voltage × capacity Ah × (initial SOC − reserve SOC)/100 × capacity factor × efficiency. It estimates endurance without recharge by dividing this energy by average power. Capacity factor and efficiency are adjustable assumptions, not automatic diagnoses.
Example: 12 V, 400 Ah, starting at 90%, reserve at 50%, retained capacity of 90% of nominal and efficiency of 90% produce 1,555.2 Wh. At an average 100 W, that is 15.55 hours. If consumption is measured at the batteries, use 100% efficiency to avoid counting losses twice.
Solar watts are not harvested watt-hours
The peak power printed on a panel is not daily production. Compare the Wh actually recorded by the controller over several days. Manufacturer documentation helps distinguish insufficient generation, charging limitations and faults.
Link harvest figures to conditions: rig shading, season, orientation, anchorage and state of charge. Low harvest when batteries are full does not prove a fault. For an unfavourable scenario, use a measured low-yield day and also test a period without solar input.
References for this section : Victron Energy — BlueSolar MPPT 150/60 & 150/70, Troubleshooting
Instantaneous power is a separate constraint
Enough stored energy does not guarantee the system can support a large current demand. Battery, BMS, cables, protection and inverter must be compatible. Voltage drop and circuit protection belong in electrical design, not just the daily energy budget.
List appliances that might run simultaneously: electric cooking, watermaker, water heater and laptop charging, for example. Keep this list separate from the Wh table. Changes to the bank or installed power warrant an installation assessment by a competent person.
References for this section : Victron Energy — Wiring Unlimited, DC wiring
Use two scenarios and a fallback
Keep a normal and a degraded scenario: lower generation, more autopilot use, a longer day or a weakened bank. Decide which uses can be reduced and which are required for safety. Plan recharge before a protective shutdown is reached.
The simplified calculation does not model Peukert effects, temperature, transients or actual cutoff voltage. It must not be the sole basis for a passage decision. Once aboard, compare predictions with readings and revise the budget. That feedback loop makes the estimate useful.
Size uses and reserve first, then charging. Calculated endurance remains an assumption to compare with measurements.
Your practical checklist
Sources & documents
- Victron Energy — BlueSolar MPPT 150/60 & 150/70, Troubleshooting
Accessed 2026-11-02 - Victron Energy — Wiring Unlimited, DC wiring
Accessed 2026-11-02
