How large should my home battery be?
The right home battery is not determined solely by your annual electricity consumption or the number of solar panels. Three values are essential for selecting the right system: how much energy you want to store in kWh, how much power the system must be able to deliver in kW and how much capacity you deliberately want to keep in reserve.
Only then can you determine which battery, inverter, backup solution and other system components can technically support these requirements.
A home battery is an investment for many years. Your current energy profile is therefore the starting point, but not the complete answer. How you intend to use the battery, both now and in the future, also affects the required capacity and system configuration.
Do you mainly want to shift your own solar energy from daytime to the evening? Do you also want to charge when dynamic electricity prices are low? Do you want to keep capacity available for backup power, reduce grid peaks or take future loads such as an electric vehicle or heat pump into account?
These applications place different requirements on energy capacity, power, reserve and energy management. This is why there is no universal battery size that is right for every household.
1. Calculate: first determine an indicative energy requirement based on your current energy profile.
2. Size: then determine which functions, power levels and reserve capacity are important to you.
3. Validate: finally check whether the battery, inverter, BMS, meter, communication and any backup equipment are technically compatible as a complete system.
Choose a home battery based on kWh, kW and reserve
The term battery size is often used as if only storage capacity in kWh matters. However, a properly sized battery system requires three separate parameters to be assessed.
| 1. Energy capacity, kWh | Indicates how much energy the battery can store. This determines how much energy you can shift from daytime to evening, use for price optimisation or reserve for backup power. |
|---|---|
| 2. Power, kW | Indicates how much power the system can charge or deliver at a given moment. This is relevant for high loads, peak shaving and backup operation. |
| 3. Reserve, % SOC or kWh | Indicates how much of the available battery capacity you deliberately do not want to use for normal daily operation, for example to keep energy available during a grid outage. |
A high number of kWh does not automatically mean sufficient power. A high number of kW does not automatically mean sufficient autonomy. Capacity protected as reserve is not simultaneously fully available for normal daily energy use.
Which factors determine the right battery size?
- How much energy do you want to shift each day?
- Do you want to use dynamic tariffs and smart energy management?
- How much power must the system be able to deliver?
- How much energy do you want to reserve for backup power?
- Which large electrical loads do you want to include?
- How do you allocate the available battery capacity?
- What will your energy profile look like in 5 to 10 years?
- Which three values result from your assessment?
1. How much energy do you want to shift each day?
For many households, battery sizing starts with storing solar energy that is not used directly during the day. Do not look only at the total PV capacity or annual solar production.
For the battery, the key question is how much energy is actually available for storage on representative days and how much of that energy can be used later.
Start with your actual energy flows
- How much PV surplus do you have on representative days?
- How much electricity do you use during the evening and night?
- How much do summer, winter and the transitional seasons differ?
- How often can the battery actually be charged and discharged?
Suppose you have approximately 8 kWh of PV surplus on a representative sunny day and use approximately 6 kWh of electricity between sunset and the following morning.
If the battery is used solely for this daily energy shift, a 20 kWh battery is not automatically better than a smaller battery. A large proportion of that capacity may remain unused on such days.
Additional capacity can still be relevant if the same battery is also used for price optimisation, peak shaving, backup power or future electrification.
The relevant question is therefore not: "How many solar panels do I have?" but: "How much energy do I actually want and have the ability to shift between different times?"
Relevant daily PV surplus: _____ kWh
Evening and night consumption: _____ kWh
2. Do you want to use dynamic tariffs and smart energy management?
When a battery is used exclusively to store your own solar energy, the usable capacity is strongly influenced by the available PV surplus.
With a suitable and correctly configured system, the battery may under certain conditions also be charged from the grid, for example during low-price periods. The energy can then be used later for self-consumption or other permitted applications.
This means that additional battery capacity may be more useful than would be expected based solely on your PV surplus.
Available storage space also has value
Smart energy management is not only about how much energy is stored in the battery. Sometimes the remaining free storage capacity is equally important.
If significant solar production is expected while the battery is already fully charged, the battery cannot absorb additional PV surplus. Depending on the installation, settings and contract, that energy must then be used directly, exported or curtailed.
The optimal SOC is not automatically 100% under all circumstances. The desired state of charge depends on what you expect from the battery later: PV storage, price optimisation, peak shaving, backup power or a combination of these functions.
A larger battery is not automatically financially better
Additional storage capacity only creates additional economic value when that capacity can be used sufficiently. The actual result depends on factors such as price differences, the energy contract, taxes and surcharges, system losses, charging and discharging behaviour and future market conditions.
For supported configurations, Solis documents energy management functions that can take into account realtime data, forecasts, dynamic tariffs and import/export settings.
The functions actually available must be checked for each region, inverter model, firmware version, battery and system configuration.
Technical sources: Solis, Inverter Control: Solis AI & More and Smart Energy at Home: Making the Most of Dynamic Tariffs .
- Store only or mainly your own solar energy.
- Also charge from the grid during favourable price periods.
- Use automatic energy optimisation.
- Keep battery capacity available for expected PV surplus.
- Limit export during unfavourable price periods where possible.
- Use energy at favourable times or export it where permitted.
3. How much power must the battery system be able to deliver?
A battery is not only limited by the amount of energy stored. The available charging and discharging power is also a determining factor.
This becomes particularly important when the battery must support high loads, reduce grid peaks or supply multiple or heavier loads during a grid outage.
Peak shaving clearly illustrates the difference between kW and kWh
With peak shaving, the battery system temporarily supplies additional power when grid consumption would otherwise exceed a defined limit.
Current electrical load:
12 kW
Desired maximum grid consumption:
8 kW
Required battery support:
approximately 4 kW
If this support is required for 15 minutes, the theoretical energy requirement is approximately 1 kWh.
If the same support is required for three hours, the theoretical energy requirement is approximately 12 kWh.
In an actual installation, system losses, battery limits, SOC limits and available inverter power must also be taken into account.
A battery can contain sufficient energy in kWh and still be unsuitable for the intended application if the battery and inverter cannot deliver sufficient power in kW.
Solis documents peak shaving for supported systems and distinguishes, among other settings, between Peak Shaving Baseline SOC and Reserve SOC. These settings serve different purposes and should not be treated as the same reserve limit.
Technical sources: Peak Shaving Baseline SOC and Reserve SOC Explained and Solis Cloud Remote Control Settings, Desktop Version .
Highest expected load: _____ kW
Desired maximum grid consumption: _____ kW
Expected duration of relevant peaks: _____ minutes / hours
4. How much energy do you want to keep available for backup power?
Simply stating that you want backup power is not sufficient to size a battery system. You first need to determine which equipment must continue operating during a grid outage, how much power it requires and how long you want it to operate.
- Only essential equipment, such as the refrigerator, lighting and internet.
- A number of selected circuits.
- A large part of the home.
- As much of the home as possible.
- Critical business equipment.
Initial calculation of the energy requirement
As a first indication:
Average backup power in kW × desired autonomy in hours = theoretical energy requirement in kWh
Example: 0.8 kW average backup power × 8 hours = 6.4 kWh theoretical energy requirement.
This is not yet the final battery size. For a technically correct backup configuration, the following factors must also be checked:
- the inverter's continuous backup power;
- short-term peak and starting power;
- the battery's maximum discharge power;
- the actual usable battery capacity;
- the permitted SOC limits;
- system losses;
- single-phase or three-phase configuration;
- the selected backup circuits;
- the electrical installation and protective devices.
Reserve capacity reduces the capacity available for daily use
If you deliberately want to retain a minimum part of the battery for a possible grid outage, that part cannot simultaneously be fully used for normal daily discharge.
Every kWh that is structurally protected as backup reserve reduces the capacity available for other battery functions at that moment.
For supported systems, Solis documents Reserve SOC as a setting that can protect part of the battery capacity from normal discharge. Solis distinguishes between Reserve SOC and Peak Shaving Baseline SOC.
Technical source: Solis, Peak Shaving Baseline SOC and Reserve SOC Explained .
Average desired backup load: _____ kW
Desired autonomy: _____ hours
Desired reserve: _____ % / _____ kWh
Equipment with high starting or peak power: ____________________________
5. Do you want to include an EV, heat pump or other large electrical load?
Electrification can significantly change your energy profile. Examples include an electric vehicle, EV charger, heat pump, electric boiler, air conditioning, electric heating, pumps or business equipment.
A large load does not necessarily have to be supplied entirely by the battery
An electric vehicle can require tens of kWh during a single charging session. This does not automatically mean that you need the same amount of additional home battery capacity.
First determine the role of the home battery:
- supply the load directly from the battery;
- only support high power peaks;
- shift consumption to periods with high solar production;
- shift consumption to low-cost grid periods;
- optimise PV, battery and large loads together.
These strategies place different requirements on kWh, kW and energy management.
Within the SolisAI platform, Solis describes functions for coordinating solar energy, battery storage and EV charging.
Platform functionality does not mean that every EV charger, battery, external controller or other component is automatically compatible with every Solis configuration. Always check the exact model, firmware, communication interface, accessories and regional support.
Technical source: Solis, SolisAI Platform Updates .
6. How do you allocate the available battery capacity?
One battery can support multiple functions, but the same stored energy cannot be used without limit for several different purposes at the same time.
Depending on settings and circumstances, this capacity can be used for:
- daily self-consumption;
- dynamic price optimisation;
- peak shaving;
- smart control of electrical loads;
- backup reserve.
If, for example, 30% of the available capacity is deliberately protected for backup power, that reserve is not simultaneously fully available for daily price optimisation.
Conversely, intensive daily discharge may mean that less energy is available if a grid outage occurs shortly afterwards, unless the system protects sufficient reserve capacity.
One kWh can only be used once at the same moment.
Capacity that has already been discharged, is fully occupied by stored energy or is deliberately protected as reserve cannot simultaneously be used without limit for another function.
Determine your priorities first
| Priority 1 | ____________________________ |
|---|---|
| Priority 2 | ____________________________ |
| Priority 3 | ____________________________ |
7. What will your energy profile look like in 5 to 10 years?
A home battery will usually remain in use longer than your current energy situation remains completely unchanged. A system that is suitable today may therefore be used differently as your home or business becomes more electrified.
- a first or second electric vehicle;
- a heat pump;
- expansion of the PV system;
- structurally higher electricity consumption;
- greater need for backup power;
- new business equipment or other high-power loads.
Buying a larger battery is not the only way to prepare for the future
Future readiness can also mean deliberately choosing a battery system that can later be expanded in accordance with the manufacturer's requirements.
Check, among other things:
- the maximum number of battery modules;
- the maximum number of battery strings;
- BMS architecture and configuration;
- LV or HV architecture;
- firmware requirements;
- conditions for expanding an existing system;
- permitted combinations of modules and system components.
Do not automatically assume that additional battery modules can be added later without restrictions. Expansion options must be checked for the exact battery system.
Your own battery assessment
Use the assessment below to bring together the most important starting points before selecting a final battery size or system configuration.
| Subject | My situation or choice |
|---|---|
| Indication from the Rebor battery calculator | _____ kWh |
| Relevant daily PV surplus | _____ kWh |
| Evening and night consumption | _____ kWh |
| Dynamic energy contract | Yes / No / Maybe |
| Charging from the grid desired | Yes / No |
| Automatic energy optimisation desired | Yes / No |
| Peak shaving desired | Yes / No |
| Desired maximum grid consumption | _____ kW |
| Highest expected electrical load | _____ kW |
| Backup power desired | Yes / No |
| Average backup power | _____ kW |
| Desired autonomy | _____ hours |
| Desired backup reserve | _____ % / _____ kWh |
| EV now or in the future | Yes / No |
| Heat pump now or in the future | Yes / No |
| PV system expansion expected | Yes / No |
| Other future large electrical loads | ____________________________ |
| Priority 1 | ____________________________ |
| Priority 2 | ____________________________ |
| Priority 3 | ____________________________ |
What battery size follows from this assessment?
The result is not a single number. For a suitable system selection, three results must be considered together.
| 1. Required energy capacity, kWh | How much energy do you want to store, shift and/or deliberately reserve? |
|---|---|
| 2. Required power, kW | How much power must the system be able to charge or deliver during normal operation, peak shaving and/or backup operation? |
| 3. Desired reserve, % SOC or kWh | How much stored energy do you deliberately want to keep outside normal daily use? |
The right home battery does not simply have "enough kWh". Capacity, power and reserve must collectively match the functions you expect from the system.
The battery calculator is the starting point, not automatically the final choice
The Rebor battery calculator provides an initial indication based on your current energy profile. The final system selection may differ if, for example, you:
- want to retain capacity as backup reserve;
- want to actively use dynamic tariffs;
- want to apply peak shaving;
- have a higher power requirement;
- add an EV, heat pump or other large electrical load;
- expect to install more solar panels;
- consider future battery expansion important.
From battery size to a technically correct energy system
Once you know how much capacity, power and reserve you need, the next step is to determine which hardware can actually support these requirements.
The following factors are relevant for this technical validation:
- the exact battery model;
- nominal and usable battery capacity;
- maximum charging and discharging power;
- the exact inverter model;
- LV or HV battery architecture;
- single-phase or three-phase configuration;
- BMS compatibility;
- supported communication;
- firmware requirements;
- Smart Meter and metering configuration;
- EPS or backup capabilities;
- required accessories;
- official manufacturer compatibility approval.
The fact that a battery and inverter use the same type of communication interface does not prove that both products form a supported system combination.
Battery and inverter compatibility must be checked at exact model level using current official manufacturer documentation.
Summary: how to determine the right size for your home battery
The right battery size cannot be reliably determined using only annual consumption, household size or the number of solar panels.
Start with your actual energy flows. Then determine which functions you expect from the battery and how important each of those functions is.
- sufficient kWh for the energy you want to store, shift or reserve;
- sufficient kW for the power you want to charge or deliver;
- a deliberate reserve for functions such as backup power;
- a battery, inverter, BMS and other components that technically support these functions together.
The practical sequence is therefore: understand, calculate, size and check compatibility.
Frequently asked questions about home battery size
How many kWh of home battery capacity does an average household need?
This cannot be reliably determined based solely on household size or annual electricity consumption. Two households with the same annual consumption can require different battery capacities because of differences in PV surplus, consumption patterns, backup requirements, dynamic tariffs, peak loads and future electrification.
Should my home battery be as large as my daily electricity consumption?
No. What matters more is how much energy you want to shift or reserve between different times. For PV self-consumption, the actual PV surplus and consumption outside solar production hours are generally more relevant than total daily consumption alone.
Is a larger home battery always better?
No. Additional capacity mainly has value when it can be used regularly or is deliberately required as reserve. Battery capacity that remains structurally unused does not automatically make a system technically or financially better.
What is more important for a home battery: kWh or kW?
Both. kWh determines how much energy is available; kW determines how much power the system can charge or deliver at a given moment. A system can therefore have sufficient kWh while still being unable to deliver enough kW for the intended application.
Do I need additional battery capacity for backup power?
Possibly. If you deliberately reserve capacity for a potential grid outage, that part is normally not fully available for daily use. The required reserve depends on factors including the desired backup load, autonomy, usable battery capacity and system configuration.
How do I calculate the battery capacity required for backup power?
As a first approximation, multiply the average desired backup power in kW by the desired autonomy in hours. An average load of 1 kW for 6 hours theoretically requires 6 kWh of energy.
Final sizing must also take into account usable capacity, system losses, SOC limits, battery power, inverter power, peak loads and phase configuration.
Does a dynamic energy contract affect the required battery size?
It can. If a suitable system can also charge from the grid at favourable times, the usefulness of the battery is not determined solely by the available PV surplus. However, more capacity does not automatically result in a higher financial return.
Can I add additional battery modules later?
This depends on the exact battery system and the manufacturer's requirements. The maximum number of modules, BMS configuration, voltage architecture, firmware, battery strings and conditions for expanding existing batteries may all impose restrictions. Check this before purchase if future expansion is important.
Can a battery have sufficient kWh but still be too small?
Yes. If the battery or inverter cannot deliver sufficient power in kW, a system may be unsuitable for high loads, peak shaving or the desired backup supply despite having sufficient energy capacity.
Can smart energy management automatically determine when a battery charges or discharges?
With suitable systems, energy management can automatically control charging and discharging based on available system data and configured rules. The available options vary by manufacturer, inverter model, battery, firmware, region, energy contract and connected components.
Technical principles used in this battery selection guide
When sizing battery systems, Rebor deliberately distinguishes between energy capacity in kWh, power in kW and available or protected reserve capacity.
General platform functionality or the use of the same communication protocol is not evidence that two specific products form a supported system configuration.
For a specific technical assessment, depending on the subject, the current installation manual, datasheet, official compatibility list, firmware documentation, technical manufacturer bulletins and safety documentation are the leading sources.
Solis is used in this guide as a specific example of manufacturer-dependent functions such as dynamic energy management, peak shaving and SOC management. This does not mean that every function described is available with every Solis product, nor that comparable functions are available exclusively from Solis.
Availability and operation must always be checked for a specific installation based on the exact inverter model, battery model, firmware, accessories, configuration and regional support.
Sources checked: 11 September 2026
Technical basis: official manufacturer documentation, installation manuals, product datasheets and current compatibility documentation.
Maintenance: manufacturer-dependent functions and compatibility may change as a result of firmware, software and product updates. Therefore, always check the current documentation for the exact system components when making a specific system selection.
Technical and financial information
This selection guide is intended to clarify the relevant intended uses, sizing questions and technical considerations for a home battery before purchase.
The calculations and examples on this page are indicative and do not guarantee a specific battery capacity, autonomy, savings, payback period, electricity price or financial return.
Energy prices, energy contracts, regulations, taxes, usage patterns, weather conditions, system losses, battery behaviour and software functionality may change.
A final system configuration must therefore be validated based on the exact battery model, inverter model, BMS, firmware, communication, meter, backup configuration and current official manufacturer documentation.
Prepared by Rene Borro, Managing Director and Technical Advisor at Rebor B.V.