Useful information for understanding energy supply systems
What do watts, watt-hours and amp-hours actually mean? Why is battery capacity alone not enough? What determines real-world runtime, charging capability and whether larger loads can be operated?
The following overview explains fundamental technical relationships that are useful when selecting an EnergonX system or another battery-based energy supply solution.
Power, capacity and runtime
These are three different concepts. All three must be considered together when selecting the right system.
Watt (W)
It indicates instantaneous power: how much power a load requires from the system while operating.
Watt-hour (Wh)
It indicates an amount of energy. A 100 W load uses approximately 100 Wh of energy in one hour.
Amp-hour (Ah)
It indicates the battery's charge capacity at its stated nominal voltage. Ah alone does not show how much energy is stored.
Basic formulas
Approximate stored energy: voltage × capacity = watt-hours
Approximate theoretical runtime: available watt-hours ÷ average power consumption
Actual runtime may be shorter than the result of a simple division
The calculation is affected by inverter and electronic losses, the battery's actually usable capacity, temperature, load level, cabling, battery condition and the load's operating pattern.
Why is the Ah rating not enough?
The same Ah capacity represents a completely different amount of energy at different system voltages.
| Battery | Calculation | Nominal energy |
|---|---|---|
| 12.8 V / 100 Ah | 12.8 × 100 | 1280 Wh |
| 25.6 V / 100 Ah | 25.6 × 100 | 2560 Wh |
| 51.2 V / 100 Ah | 51.2 × 100 | 5120 Wh |
Wh is the comparable figure
The energy content of batteries with different voltages should be compared in watt-hours. The same Ah rating does not mean that two batteries can operate the same load for the same length of time.
Continuous and starting power
The rated power shown on a load's nameplate does not always reveal the highest demand that occurs at start-up.
Continuous power
This is the power that the inverter and the complete system must supply continuously. When sizing the system, the power of loads operating at the same time must be added together.
Starting or surge power
Motors, pumps, compressors, refrigerators, air conditioners and some power supplies may briefly require several times their rated power when switched on.
The inverter rating is not the only figure to check
The inverter, battery BMS, cabling, connectors and fuses must all be able to handle the starting demand. A high inverter surge rating is of no use if the battery or the DC side disconnects first.
Nominal and actually usable energy
The nominal Wh rating on a battery label is not the same as the amount of energy that ultimately reaches the load.
Discharge limit
To protect the cells, the BMS or system controller may disconnect before the full theoretical energy content has been used.
Conversion losses
The inverter, DC–DC converter, charge controller and other electronics consume energy and generate heat while operating.
Operating conditions
Temperature, battery age, load level and cabling losses all affect how much energy can be obtained.
Illustrative example
From a battery with a nominal energy content of 1280 Wh, if an assumed 85% reaches the load as usable energy through the complete system, approximately 1088 Wh can be used.
This is not a generally guaranteed ratio, only a calculation example. The actual value is determined by the specific battery and system.
How to choose a battery
The same nominal voltage and capacity do not automatically mean compatibility. The battery must be selected for the complete system and the actual task.
System voltage
A 12, 24 or 48 V system requires a battery, charger, inverter and other electronics designed for the corresponding voltage.
Physical fit
Dimensions, terminal positions, cable routing, mounting and the required installation space must all be checked.
BMS current capability
The continuous and short-term discharge current ratings must be sufficient for the current demand of the loads and inverter.
Charging requirements
The charging voltage, charge profile and maximum charging current must comply with the battery manufacturer's specifications.
Temperature limits
Charging around and below freezing requires particular attention. Self-heating or low-temperature charge cut-off may only be relied upon if the specific battery actually provides it.
Documentation and support
A reliable datasheet, an identifiable model, manufacturer limits, warranty and accessible sales or service support are important.
Capacity in Ah is not the only factor
Even a high-capacity battery may be unsuitable for a large inverter or a load with high starting current if the BMS current capability, cabling, connections or fusing are inadequate.
LiFePO₄ or AGM / lead-acid battery?
There is no single correct technology for every situation. The application, required cycle life, weight, environment and cost must be considered together.
| Criterion | LiFePO₄ | AGM / sealed lead-acid |
|---|---|---|
| Weight | Generally lower for the same nominal energy content. | Generally higher. |
| Usable capacity | A larger proportion can typically be used within the manufacturer's limits. | Usually requires shallower discharge to achieve a longer service life. |
| Cycle life | Generally longer when used correctly. | Generally shorter, especially with frequent deep discharges. |
| Voltage during discharge | Can remain relatively stable through a large part of the discharge. | Typically falls more gradually as the state of charge decreases. |
| Charging in cold conditions | Charge cut-off or self-heating may be required around and below freezing. | Different temperature and charging rules apply. |
| Charger | A LiFePO₄-compatible charging profile is required. | A charger matched to the specific lead-acid technology is required. |
| Self-discharge | Typically lower. | Typically higher. |
| Initial cost | Often higher. | Often lower. |
Why is LiFePO₄ popular?
Because of its favourable weight-to-energy ratio, typically long cycle life, stable discharge voltage, low self-discharge and the electronic protection that can be provided by a BMS. The extent of these benefits varies by model.
Inverter losses and self-consumption
A 230 V inverter does not convert battery energy without losses and may consume power even when switched on with no load.
Conversion efficiency
The inverter converts DC energy into AC energy. The difference appears mainly as heat, so less energy reaches the load than leaves the battery.
No-load consumption
The inverter may consume energy even when no load is connected or only a very low-power device is operating.
Direct DC supply can be more efficient for small loads
Operating a low-power 5 V, 12 V or 24 V device from a suitable DC output is often more efficient than first converting the energy to 230 V and then converting it back to low voltage with the device's mains power supply.
A direct DC connection may only be used with the correct voltage regulation, polarity, connection and protection.
Why can 24 or 48 V be advantageous at higher power?
At the same power, a higher system voltage means lower current. This is a significant technical advantage in higher-power systems.
| System voltage | Theoretical current at 1200 W | Typical implication |
|---|---|---|
| 12 V | approx. 100 A | Very high DC current; thick cables and high-current connections are required. |
| 24 V | approx. 50 A | Half the theoretical current at the same power. |
| 48 V | approx. 25 A | More manageable current, but components rated for the higher DC voltage are required. |
Higher voltage is not automatically better
System voltage is determined by power, compatible loads, batteries, chargers, inverter, safety, cost and future expandability together. A 12 V system can still be simple and practical at lower power.
Cable length, cross-section and voltage drop
In low-voltage, high-current systems, even a few metres of cable can be a significant technical factor.
Longer cable
At the same cross-section and current, it means higher resistance, greater voltage drop and more loss.
Higher current
It increases cable heating and power loss. High-current 12 V systems are therefore particularly sensitive to cabling.
Larger cross-section
It can reduce cable resistance and voltage drop, but connectors, terminals and fuses must also be suitable.
Illustrative example
For a copper cable with a total round-trip length of 4 metres and a cross-section of 25 mm², at 100 A, a simple calculation gives approximately 0.28 V of voltage drop and approximately 28 W of cable loss.
This is already significant in a system around 12 V. Actual sizing must also consider cable type, temperature, installation method, connections and the permitted voltage drop.
A fuse primarily protects the cable, not the load
The fuse rating must be selected so that, in a fault, it interrupts the current before the cable or connection can overheat dangerously.
Charging options and charging time
A suitable charger must match the battery not only in voltage, but also in chemistry, charging profile and maximum charging current.
From the 230 V mains
With an AC charger matched to the battery. This generally provides the most predictable charging performance.
From solar panels
With a suitable charge controller, the correct voltage range and a charging profile configured for the battery.
From a vehicle
Typically through a DC–DC charger that limits and regulates the energy drawn from the vehicle's electrical system.
From a generator
Usually by powering a mains charger matched to the battery. The generator must provide suitable power and output quality.
Simple charging-time estimate
Approximate formula: capacity to be replenished ÷ charging current. If 80 Ah must be replenished in a 100 Ah battery using a 20 A charger, the ideal lower estimate is 4 hours.
Actual charging may take longer because of charger losses, current limiting, the final charging stage, the BMS, temperature and power consumed during charging.
Charging current has limits
Maximum charging current is limited jointly by the battery manufacturer, BMS, cabling, connectors, fuses and heat dissipation. An oversized charger does not necessarily provide faster or safer charging.
What does a solar panel's rated power mean?
A panel rated at 400 W can deliver 400 W only under defined laboratory conditions. Actual output changes continuously.
Sunlight conditions
Time of day, season, cloud cover, atmospheric conditions and shade directly affect instantaneous output.
Orientation and tilt angle
A panel produces the most when correctly oriented towards the incoming sunlight. Fixed and adjustable installations can deliver different results.
System losses
Panel temperature, the charge controller, cabling, connections and the battery's state of charge can all reduce the usable energy.
Approximate daily energy yield
panel rated power × equivalent full-sun hours × system efficiency
For example, 400 W × 4 hours × 0.75 ≈ 1200 Wh of energy per day. This is only an illustrative example, not a guaranteed yield.