A 10 kWh Battery May Have Plenty of Energy and Still Fail to Start the Heat Pump
To start a heat pump from a home battery, size the system from power first, not battery capacity. Check the heat pump's running power, compressor starting or locked-rotor current, supply voltage, and the other loads that may be operating at the same time. Battery kWh determines how long the heat pump can run; battery and inverter kW—and especially short-duration surge capability—determine whether it can start.
That distinction becomes important during a blackout.
A homeowner sees a 10 kWh battery and a heat pump drawing perhaps 2–3 kW while running. On paper, the combination looks comfortable.
Then the compressor starts.
The inverter trips.
The problem was never the 10 kWh.
Start With the Heat Pump Nameplate
For an existing home, I would collect the heat pump's actual electrical data before selecting the battery–inverter package.
Useful values include:
Supply voltage and phase
Rated or running current
Rated input power
MCA — Minimum Circuit Ampacity
MOCP — Maximum Overcurrent Protection
LRA — Locked Rotor Amps , where provided
Compressor/inverter type
Do not estimate starting power simply from heating capacity in BTU/h or kW thermal.
A 12 kW heating-output heat pump does not continuously consume 12 kW of electricity. Heating output and electrical input are different quantities.
Likewise, the U.S. Department of Energy notes that heat pumps move heat rather than converting electricity directly into heat, which is why their heating output can exceed their electrical input. DOE Heat Pump Systems
Running the Compressor and Starting It Are Different Jobs
Consider a simplified 230 V heat pump.
Assume measured running power is:
2.5 kW
But manufacturer data indicates a substantially higher short-duration starting requirement.
Now compare two hypothetical home ESS configurations:
| Home ESS | Continuous Output | Surge Capability |
|---|---|---|
| System A | 3 kW | 4 kW |
| System B | 6 kW | 9 kW |
Both could use the same 10 kWh battery .
System A may contain enough stored energy to run the heat pump for hours, yet fail when the compressor starts.
System B may handle the startup successfully.
This is why distributors should never turn:
10 kWh home battery
into:
Suitable for heat pumps
without checking the inverter and battery power limits.
Do Not Turn LRA Directly Into Battery kW
There is an important complication.
Suppose a compressor nameplate states:
LRA = 40 A
At 230 V, simply calculating:
230 × 40 = 9.2 kVA
does not automatically mean you should specify a 9.2 kW inverter.
Locked-rotor current is not the same thing as normal starting power under every real operating condition. Power factor, compressor technology, startup duration, inverter behavior, voltage drop, and any factory-installed starting electronics matter.
Modern inverter-driven variable-speed heat pumps can also behave very differently from traditional fixed-speed compressors.
So I use LRA as a warning that startup needs investigation—not as a universal battery-sizing formula.
For an actual project, the strongest evidence is the heat-pump manufacturer's electrical/startup data combined with the home ESS manufacturer's approved motor/compressor-load capability.
The Battery Has Its Own Power Limit
The inverter is not the only gatekeeper.
Imagine a 10 kWh battery rated for only:
5 kW continuous discharge
paired with an inverter capable of:
8 kW output
The inverter cannot continuously create 8 kW from a battery that is permitted to provide only 5 kW.
For a modular Ruibit/Dawnice Home ESS package, this is why I would check the complete battery–inverter combination rather than quoting battery capacity independently.
The relevant chain is:
Battery discharge limit → inverter continuous/surge capability → heat-pump startup requirement
The weakest limit wins.
Other Household Loads Do Not Disappear During Startup
Now add a refrigerator, lights and household electronics.
Assume:
Heat pump running: 2.5 kW
Other simultaneous loads: 1.2 kW
Normal backup demand becomes:
3.7 kW
During compressor startup, the heat pump temporarily asks for more.
The home ESS has to support that event while the other 1.2 kW is still present .
This is why whole-home backup sizing differs from testing a heat pump by itself.
An installer can sometimes improve the design by separating loads or using load management so high-power appliances do not operate simultaneously during an outage.
Starting Is Only Half the Heat-Pump Question
Once the compressor starts, battery kWh becomes important.
Suppose the heat pump averages 2.0 kW over an evening after cycling, while other essential loads average 0.5 kW.
Average backup load:
2.5 kW
If the system provides 9 kWh of usable AC energy :
9 ÷ 2.5 = 3.6 hours
That is only a simplified runtime estimate.
Outdoor temperature, thermostat setting, building insulation, defrost cycles, heat-pump modulation, auxiliary resistance heat, battery SoC and other household loads can all change the result.
Auxiliary electric heat deserves particular attention. If resistance heating activates during cold conditions, electrical demand can rise substantially and completely change both inverter-power and battery-energy requirements.
The Five Numbers I Want Before Quoting
For a heat-pump backup project, I would ask the installer for:
1. Heat-pump supply voltage and phase
2. Actual/rated running power
3. Manufacturer startup/LRA information where applicable
4. Maximum simultaneous household backup load
5. Required backup duration
Then I match those against:
battery continuous and peak discharge capability
inverter continuous output
inverter surge power and permitted duration
usable AC battery energy
battery–inverter–heat-pump compatibility
That produces a much more defensible quotation than saying:
"A 10 kWh battery can run a heat pump."
It might.
But that statement answers only the energy question.
For heat-pump backup, kW gets the compressor started. kWh keeps it running. A correctly sized Home ESS has to satisfy both.
FAQs
1. How much battery power is needed to start a heat pump?
It depends on the heat pump's running power, compressor starting requirement, voltage, inverter surge capability, and other simultaneous household loads . Battery capacity in kWh alone cannot determine whether a heat pump will start.
2. Can a 10 kWh home battery start a heat pump?
Possibly, but 10 kWh describes energy capacity, not maximum power output . The battery and inverter must provide enough continuous and short-duration surge power for the heat pump's compressor and other loads operating at the same time.
3. What heat pump specifications should be checked before choosing a home battery?
Check the supply voltage and phase, rated or measured running power, running current, compressor type, and manufacturer startup or LRA data where applicable . These should be compared with the battery and inverter power limits.
4. Can locked-rotor amps (LRA) be converted directly into the required inverter kW?
Not reliably. LRA is useful for identifying a potentially demanding compressor startup, but actual requirements also depend on power factor, compressor technology, startup duration, voltage behavior, and inverter surge characteristics .
5. What is the difference between kW and kWh when backing up a heat pump?
kW determines whether the Home ESS can start and operate the heat pump. kWh determines approximately how long it can keep the heat pump and other household loads running.
