By Boat Battery editorial team · 2 minute read · Editorial policy
Size a low-solar scenario explicitly. Choose which loads must keep running, how long you need them and the charging option available before the reserve is exhausted.
Essential loads during low-solar weather
Start with an essential-load list. Navigation electronics, communications, lighting and other necessary equipment depend on the vessel and trip; a coffee machine usually belongs in a different group. Give each load an honest operating time. Reserve is a plan for continued operation, not an arbitrary percentage that happens to look reassuring on a display.
Calculate reserve without expected solar
Run the calculation with little or no solar contribution for the period you want to cover. For example, an essential demand of 500 Wh per day requires 1,000 Wh over two days before adding allowances. At 12.8 V that is roughly 78 Ah of energy demand. The installed capacity must be higher if only part of the bank is allocated to this reserve.
Plan reserve recovery and charging
Then plan how the reserve will be restored. An engine charger does not necessarily return its full labelled output throughout a trip, and an engine should not be run under unsuitable conditions simply to satisfy a spreadsheet. Consider the actual charger limit, other loads, operating restrictions and access to shore power. A reserve without a realistic recovery option postpones the same problem.
Set a crew action threshold
Choose an early action point with the crew. Reduce discretionary loads, change destination or arrange charging before the low-voltage cutoff. Keep the plan visible and test whether the monitor alarms can be heard or seen where people spend time. Review the assumptions when cruising in a different season or adding equipment.
Write the reserve plan
- Essential loads and hours of use
- Low-solar endurance target
- Early warning and action point
- Practical charging or destination fallback
Frequently asked questions
Should I count a panel’s rated watts all day?
No. Panel ratings are test-condition output. Day length, shade, orientation, cloud and system losses all affect useful energy aboard.
Related installation planning resources
Use these notes to prepare your own measurements and questions. Check the exact equipment manuals and have the proposed system assessed for your vessel before buying or changing electrical components.
Calculate your boat battery capacity
The house bank calculator takes your loads, your nights on the hook, your solar and your alternator, and returns the bank, the DC-DC charger, the AGM equivalent and the weight you take off the boat.
Open the calculator