The consumption balance is the number from which battery size, charging power and, in the end, also the cross-sections follow. It results from a single calculation per consumer: current consumption multiplied by daily operating time.
The Calculation
FormulaCurrent consumption in amperes × operating time in hours per day = consumption in ampere-hours per day. The sum of all consumers is the daily balance.
If a power rating instead of a current is indicated on the type plate, it is divided by the on-board voltage: 90 watts at 12 volts are 7.5 amperes. For devices with converters or power supplies, the actual value is slightly higher than the calculated value, because the converter itself has losses.
Calculate travel and layover time separately
Two columns instead of one, and the balance becomes usable. While underway, the engine runs and the alternator charges; the only question then is whether the charge is sufficient. During layover, only what is in the battery bank and what solar or shore power supplies count.
Consumers are distributed quite differently: navigation, autopilot, and radar run while underway, cooling and lighting always, water pump and entertainment rather during layover. Anyone who uses only one column plans for a condition that does not exist.
An example
| Consumer | Current | Hours/Day | Ah/Day |
|---|---|---|---|
| Cooling compressor | 7.5 A | 8 (cycled) | 60 |
| LED lighting | 3 A | 5 | 15 |
| Water pump | 8 A | 0.5 | 4 |
| Navigation and plotter | 2.5 A | 6 | 15 |
| Autopilot | 4 A | 6 | 24 |
| Radio on receive | 0.7 A | 10 | 7 |
| Charging small devices | 3 A | 3 | 9 |
| Quiescent current of the system | 0.4 A | 24 | 10 |
Total 144 Ah per day. With a 25 percent surcharge, approx. 180 Ah. For lead with 50 percent depth of discharge, this results in a battery bank of 360 Ah, for lithium with 80 percent approx. 225 Ah.
What is remarkable about such lists is usually the same: cooling is the largest single item, and the quiescent current — the item nobody plans for — is in the order of magnitude of the lighting.
The regularly underestimated items
- Quiescent current. Monitors, radio systems in standby, alarm systems, gas detectors, chargers without load. Individually milliamperes, together a relevant number — multiplied by 24 hours.
- Cooling in summer. The cycled operating time increases significantly with the outside temperature; datasheet values apply to moderate conditions.
- Converter losses. An inverter also draws current in idle mode, and its conversion costs ten to fifteen percent.
- Starting currents. Incidental for the balance, crucial for the design of fuses and converters.
- New consumers. What is missing in the planning is added during operation — hence the surcharge.
Better than calculating: measuring
A calculated balance is an estimation with structure. It only becomes more accurate through measurement: a shunt in the negative line of the consumer battery bank counts ampere-hours, and a week of normal operation provides a value that no table can achieve.
Therefore, anyone rebuilding an existing system should start with measurement technology and not with the battery. Anyone building new should plan for the measuring point right away — it costs little and answers questions from day one that would otherwise remain open.
WarningThe charge state of a battery bank can only be roughly estimated from the voltage; with lithium, due to the flat discharge curve, practically not at all. Without ampere-hour counting, any statement about remaining runtime remains a guess.
From the balance to the battery bank
The size of the battery bank is derived from the daily balance using two factors: the desired autonomy in days and the permissible depth of discharge of the technology.
FormulaNominal capacity = (Daily balance × Autonomy days) ÷ Permissible depth of discharge
With a daily requirement of 180 ampere-hours, one day of autonomy, and lead with 50 percent, that's 360 ampere-hours. If two days without charging are desired, the value doubles. At this point, it usually becomes clear why solar or a powerful charging method are more economical than a battery bank twice as large.
And to the charging side
The same number determines what needs to be replenished. The charging sources must replace the daily balance within the available time — and not just mathematically: A battery bank draws little current in the final phase, so the last few percent take disproportionately long.
In practical terms, this means budgeting for a surcharge and choosing a rather generous charging power — limited by the charge acceptance of the battery technology, which must not be exceeded.
Typical errors
- Quiescent current not accounted for. Multiplied by 24 hours, it becomes one of the largest items.
- Only one operating state calculated. Travel and layover time place different demands.
- Planned without surcharge. Twenty to thirty percent reserve is the norm, not a luxury.
- Depth of discharge forgotten. Nominal capacity is not usable capacity.
- Converter losses overlooked. Inverters cost money even in idle mode.
- Never re-measured. A week with a shunt beats any table.
