Engineering report · reviewed against NEC 2026 & BS 7671:2018+A2
Engineering Breakdown: Running a Vaillant aroTHERM Plus Air Source Heat Pump (7kW) on the myenergi Libbi (5kW Inverter + 5kWh Battery)
When pairing the myenergi Libbi (5kW Inverter + 5kWh Battery) (5,120 Wh LiFePO4 capacity, 5,000 W continuous, 7,500 W surge) with a Vaillant aroTHERM Plus Air Source Heat Pump (7kW) (2,300 W running, 3,500 W starting surge), the system operates at 46% of continuous inverter capacity on a 230 V UK/EU/AU circuit. At a 100% duty cycle and 85% inverter efficiency, a full battery delivers roughly 1.9 hrs of off-grid runtime. Wiring the connection over a 7.5 m run calls for a 10 A MCB and 1.5 mm² copper, terminated at a Hardwired via DP isolator · IEC 60309 32A connection.
1. Compatibility Verdict & Inverter Load Utilization
In UK/EU/AU 230 V mode the Vaillant aroTHERM Plus Air Source Heat Pump (7kW) loads the myenergi Libbi (5kW Inverter + 5kWh Battery)'s inverter to 46% of its 5,000 W continuous rating and 47% of its 7,500 W surge rating. Both figures are under the 80% sustained-load guideline, so this is a comfortable, verified match with 2,700 W of spare capacity for lights, a router or a refrigerator on the same unit.
| Check | Load requires | myenergi Libbi provides | UK/EU/AU |
|---|---|---|---|
| Continuous output | 2,300 W | 5,000 W | Pass |
| Start-up surge | 3,500 W | 7,500 W | Pass |
| Voltage | 230 V | 230 V | Pass |
| Connection | Hardwired via DP isolator · IEC 60309 32A | Hardwired consumer unit (RCBO protected) |
Air source heat pumps such as this one use inverter-driven compressors that ramp up gradually, so their start-up demand (3,500 W, 1.5× running) is far gentler than a fixed-speed air conditioner. The real challenge is energy, not surge: in a cold snap a heat pump can run for most of the day, and defrost cycles add short peaks. At 2,300 W, the myenergi Libbi (5kW Inverter + 5kWh Battery) would supply about 1.9 hrs of continuous heating from a full battery — so in practice batteries are used to shift cheap off-peak or solar energy into the evening, not to heat a home through a multi-day outage. UK installations are made under BS 7671 and MCS standards, and the local network operator (DNO) must be notified.
The myenergi Libbi (5kW Inverter + 5kWh Battery) is a UK smart home storage system, installed on a dedicated circuit from the consumer unit under BS 7671 and typically paired with a time-of-use tariff to charge overnight and discharge at peak. Backup operation for specific circuits depends on the installer fitting an EPS/backup switch.
2. Real-World Runtime by Duty Cycle
Nameplate capacity is not usable capacity. DC-to-AC conversion, battery management overhead and cell protection reduce the 5,120 Wh rating to about 4,352 Wh of deliverable AC energy (85% system efficiency). Duty cycle is the share of each hour the load actually draws its running power — thermostats, pressure switches and intermittent use all lower it.
| Duty cycle | Average draw | Runtime (no solar) | Energy per 24 h |
|---|---|---|---|
| 25% | 575 W | 7.6 hrs | 13.8 kWh |
| 50% | 1,150 W | 3.8 hrs | 27.6 kWh |
| 65% | 1,495 W | 2.9 hrs | 35.9 kWh |
| 100% | 2,300 W | 1.9 hrs | 55.2 kWh |
Even at a 50% duty cycle the battery lasts 3.8 hrs, so plan on solar, grid or generator recharging for anything longer than an evening outage.
3. BS 7671 / IEC 60364 Breaker & Cable Sizing
IEC-based codes size protection by design current rather than a blanket 125% multiplier: Ib ≤ In ≤ Iz. At 230 V the load draws 10.0 A, so a 10 A Type B/C MCB with RCD protection applies. In the UK, BS 7671 Section 722 additionally requires a dedicated circuit and Type A or Type B RCD for EV charge points.
| One-way distance | MCB | Min. cable | Voltage drop |
|---|---|---|---|
| 7.5 m (25 ft) | 10 A | 1.5 mm² | 0.79% |
| 15 m (49 ft) | 10 A | 1.5 mm² | 1.58% |
| 22.5 m (74 ft) | 10 A | 1.5 mm² | 2.37% |
| 30 m (98 ft) | 10 A | 2.5 mm² | 1.93% |
Moving the myenergi Libbi (5kW Inverter + 5kWh Battery) from 7.5 m to 30 m away forces the conductor up from 1.5 mm² to 2.5 mm² — park the unit as close to the load as ventilation and weather protection allow.
4. Solar Array Recharge Time Breakdown
The myenergi Libbi (5kW Inverter + 5kWh Battery) accepts up to 5,000 W of photovoltaic input. Real arrays rarely deliver nameplate output, so we apply an 80% harvest factor for MPPT conversion, cell temperature and soiling, and convert hours to days using 4.5 peak-sun-hours — a conservative annual average for the US, southern Europe and Australia (UK winters can fall below 1.5).
| PV array | Real harvest | 0–100% recharge | Sun-days @ 4.5 PSH | Net with load @ 50% |
|---|---|---|---|---|
| 400 W | 320 W | 16.0 hrs | 3.6 | −830 W |
| 800 W | 640 W | 8.0 hrs | 1.8 | −510 W |
| 1,600 W | 1,280 W | 4.0 hrs | 0.9 | +130 W |
| 3,000 W | 2,400 W | 2.1 hrs | 0.5 | +1,250 W |
| 5,000 W | 4,000 W | 1.3 hrs | 0.3 | +2,850 W |
A typical 800 W portable array refills the battery in about 8.0 hrs of strong sun, while maxing out the 5,000 W input cuts that to 1.3 hrs. To fully offset the Vaillant aroTHERM Plus Air Source Heat Pump (7kW) at a 50% duty cycle (1,150 W average) during daylight you need roughly 1,450 W of panels, which this unit can accept.
5. Frequently Asked Questions
Can the myenergi Libbi (5kW Inverter + 5kWh Battery) run a Vaillant aroTHERM Plus Air Source Heat Pump (7kW)?
Yes. The myenergi Libbi (5kW Inverter + 5kWh Battery) delivers 5,000 W continuous and 7,500 W surge, covering the Vaillant aroTHERM Plus Air Source Heat Pump (7kW)'s 2,300 W running and 3,500 W start-up draw at 46% inverter utilization.
How long will the myenergi Libbi (5kW Inverter + 5kWh Battery) run a Vaillant aroTHERM Plus Air Source Heat Pump (7kW)?
With 5,120 Wh of capacity and 85% inverter efficiency, expect about 1.9 hrs of continuous operation and 3.8 hrs at a 50% duty cycle. Adding solar input during the day extends this further.
What breaker and wire size do I need to connect a Vaillant aroTHERM Plus Air Source Heat Pump (7kW) to the myenergi Libbi (5kW Inverter + 5kWh Battery)?
At 230 V the load draws 10.0 A. Under BS 7671 / IEC 60364 the protective device must be at least the design current (Ib ≤ In ≤ Iz), so use a 10 A MCB with RCD protection on 1.5 mm² copper cable for runs up to 7.5 m (0.8% voltage drop). Connection: Hardwired via DP isolator · IEC 60309 32A.
How long does it take to recharge the myenergi Libbi (5kW Inverter + 5kWh Battery) with solar panels?
With an 800 W array and an 80% real-world harvest factor, a full recharge takes about 8.0 hrs of peak sun (1.8 days at 4.5 peak-sun-hours). At its maximum 5,000 W solar input it drops to 1.3 hrs.
Methodology & safety note
Specifications come from manufacturer datasheets in our open dataset; ampacity and resistance values follow NEC Table 310.16 and Chapter 9 Table 8, and BS 7671 Appendix 4. Calculations assume copper conductors, 30 °C ambient and no bundling derating. Inverter-to-panel connections require a listed transfer switch or interlock to prevent back-feeding the grid. These results are an engineering estimate, not a substitute for a licensed electrician and local permit. See our methodology and safety disclaimer.