Engineering report · reviewed against NEC 2026 & BS 7671:2018+A2
Engineering Breakdown: Running a Vaillant aroTHERM Plus Air Source Heat Pump (7kW) on the Fox ESS EP5 Modular Hybrid
When pairing the Fox ESS EP5 Modular Hybrid (5,180 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 Fox ESS EP5 Modular Hybrid'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 | Fox ESS EP5 Modular Hybrid 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 Fox ESS EP5 Modular Hybrid 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 Fox ESS EP5 Modular Hybrid combines a hybrid solar inverter with stackable battery modules, wired permanently into the home's distribution board. Its 6,500 W PV input lets one unit handle both rooftop solar and storage.
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 5,180 Wh rating to about 4,403 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 | 7.7 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 Fox ESS EP5 Modular Hybrid 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 Fox ESS EP5 Modular Hybrid accepts up to 6,500 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% |
|---|---|---|---|---|
| 800 W | 640 W | 8.1 hrs | 1.8 | −510 W |
| 1,600 W | 1,280 W | 4.0 hrs | 0.9 | +130 W |
| 3,000 W | 2,400 W | 2.2 hrs | 0.5 | +1,250 W |
| 6,000 W | 4,800 W | 1.1 hrs | 0.2 | +3,650 W |
| 6,500 W | 5,200 W | 60 min | 0.2 | +4,050 W |
A typical 800 W portable array refills the battery in about 8.1 hrs of strong sun, while maxing out the 6,500 W input cuts that to 60 min. 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 Fox ESS EP5 Modular Hybrid run a Vaillant aroTHERM Plus Air Source Heat Pump (7kW)?
Yes. The Fox ESS EP5 Modular Hybrid 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 Fox ESS EP5 Modular Hybrid run a Vaillant aroTHERM Plus Air Source Heat Pump (7kW)?
With 5,180 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 Fox ESS EP5 Modular Hybrid?
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 Fox ESS EP5 Modular Hybrid with solar panels?
With an 800 W array and an 80% real-world harvest factor, a full recharge takes about 8.1 hrs of peak sun (1.8 days at 4.5 peak-sun-hours). At its maximum 6,500 W solar input it drops to 60 min.
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.