Balcony solar battery: how many kWh do you need?

To work out how many kWh of battery storage you need for balcony solar panels, compare the solar surplus actually available with the energy your home can use when generation falls. A battery of around 1 kWh may suit a modest surplus; 2 kWh is worth considering when surplus generation regularly exceeds 1 kWh; and 4 kWh requires more available energy or specific needs. These are starting points, not standard sizes: usable capacity, losses and charging and discharging power can change the decision.

The rated power of balcony solar panels alone is not enough. Two balconies with the same panels can generate very different surpluses because their orientation, shading and simultaneous household demand differ. Even substantial generation does not justify a large battery if the home cannot use the stored energy.

Before comparing battery sizes, establish whether adding storage makes sense for your consumption pattern. Once you know there is energy available to shift into the evening, the next step is to determine how much storage you actually need.

Four things to know before choosing a battery

Reasonable battery sizing starts with four pieces of information:

• how much energy the small solar system generates during the day;

• how much of that energy the home uses directly at the same time;

• the demand you want to cover when generation falls;

• the battery’s specifications, including usable capacity and power.

These figures should cover several days and, ideally, different seasons. The best summer day shows the system’s potential, but does not represent its normal operation.

Generation and surplus are not the same

If the panels generate 2 kWh and the home uses 1.4 kWh of that energy directly, the theoretical surplus is 0.6 kWh. The battery will not receive the full 2 kWh generated.

For a system that gives priority to household demand, daily surplus can be expressed as:

Surplus = solar generation − solar energy used directly.

Direct self-consumption must be measured when generation and demand coincide. Subtracting all consumption recorded between morning and evening from solar generation, without checking when it occurred, can produce an incorrect result.

kWh measures energy; kW measures power

A 2 kWh battery stores a certain amount of energy. It cannot necessarily deliver 2 kW of power.

For example, a 2 kWh battery with a maximum output of 600 W can contribute to household demand up to that limit. If the home needs 1,500 W and the system is connected to the grid, the remaining 900 W can come from the electricity network.

Capacity determines how long that contribution can continue; power determines how much demand can be covered at any moment. Both affect the energy that can actually be used during the evening.

Nominal capacity, usable capacity and delivered energy

To understand how a storage battery works, distinguish the advertised energy capacity from the energy that is actually available. Nominal capacity is the specification on the datasheet. Usable capacity is the portion accessible within the manufacturer’s operating limits.

A battery with a nominal capacity of 2 kWh might allow 1.8 kWh of stored energy to be used if its usable fraction were 90%. This is not a universal percentage: some datasheets state usable capacity directly, while others specify nominal capacity and depth of discharge.

Do not apply the same adjustment twice. If the manufacturer already specifies 1.8 kWh of usable capacity, reducing that figure by another 10% would underestimate the available energy.

Charging and discharging losses must be distinguished

The energy available to the home also depends on the conversions between the battery, converter and household electrical installation.

To estimate what an already charged battery can deliver, account for discharging and conversion losses. To calculate how much energy is recovered from an initial solar surplus, use the efficiency of the whole charging and discharging process instead.

Round-trip efficiency compares the energy delivered with the energy that entered the system. It should not automatically be used as the efficiency of discharging alone.

The examples below use these illustrative assumptions, without attributing them to any particular product:

• usable capacity: 90% of nominal capacity;

• charging efficiency: 95%;

• discharging and conversion efficiency to the home: 95%.

The combined efficiency is therefore approximately 90.25%. Real results can vary with system architecture, temperature, power, reserve settings and internal consumption. When comparing products, also check where and how their efficiencies are measured.

How to measure surplus without overestimating it

The most useful information is a time-based profile of generation and consumption. Hourly data provides a starting point; shorter intervals can better capture appliance operation and changing shadows.

An example over two intervals

Imagine two consecutive hours:

• first hour: 0.6 kWh generated and 0.2 kWh consumed;

• second hour: 0.2 kWh generated and 0.6 kWh consumed.

Total generation and total consumption are both 0.8 kWh. Subtracting only the totals would suggest there is no surplus.

In fact, 0.4 kWh of surplus is available during the first hour. The battery could store some of it and deliver it during the second hour, within the system’s limits. This shows why daily totals do not always describe the potential of battery storage.

When household metering is useful

The microinverter’s app may show generation without knowing all the home’s consumption. Identifying surplus also requires measurement of electricity flows, or an estimate of demand aligned with generation.

A smart plug can measure a single appliance. A meter correctly installed at the appropriate point in the electrical installation can provide an overall picture. Compatibility, location and configuration must follow the system’s instructions.

Dynamic measurement may also allow the battery to adjust its output to actual demand. In systems with scheduled output, a fixed power setting could exceed the current demand or leave part of it uncovered.

How to estimate generation before installation

If the solar system is not installed yet, PVGIS, the European Commission Joint Research Centre’s tool, can provide an initial estimate based on location, power, orientation and tilt.

For a home in Europe, enter the balcony’s actual location rather than using a single generation figure for the whole continent. The estimate should reflect local conditions and monthly variation.

The orientation, tilt and loss settings described in the official PVGIS user manual help produce a consistent estimate. The simulation should represent the real balcony. A vertical panel on a railing should not be modelled as a rooftop panel at an optimum angle. Nearby shading also needs attention: parapets, awnings and balconies above can have effects that a general simulation does not fully describe.

Panel watt-peak ratings and inverter AC power

The panels’ nominal power, expressed in Wp, is rated under standard test conditions. The inverter’s AC power rating describes its limit on the alternating-current side.

A system may have panels with a combined nominal rating higher than the inverter’s output power. The relationship between these values depends on the design and permitted configurations.

When a kit is described as an “800 W system”, clarify which rating is being quoted. Generation depends on the combination of panels, conversion and real conditions; connection depends on the system characteristics and applicable local requirements. A commercial power rating does not establish that a kit can be connected to any household socket.

Summer, winter and orientation

In European locations, a south-facing balcony may concentrate generation around midday; an east-facing balcony shifts it towards the morning, while a west-facing balcony shifts it towards the afternoon. Storage needs depend on how these profiles match consumption.

It is not correct to assume that one orientation always requires a larger battery than another. A household working from home may use much of a south-facing system’s generation directly. If the occupants return late, even a west-facing balcony may produce a surplus.

To choose a capacity, compare representative months and examine both days with surplus generation and days when the battery is barely used.

A 1 kWh battery: when can it be enough?

Around 1 kWh of storage is a plausible starting point when daily surplus is modest and the aim is to cover moderate consumption after solar generation falls.

For example, it may fit a surplus frequently in the region of 0.5–0.8 kWh, provided the home can use the delivered energy. This range is indicative and does not replace calculations based on the product’s specifications.

Example with a 0.7 kWh surplus

Suppose 0.7 kWh is available at the input to the storage pathway. At 95% charging efficiency, approximately 0.665 kWh is stored. At 95% discharging efficiency, around 0.632 kWh reaches the home.

A battery with a nominal capacity of 1 kWh and 0.9 kWh of usable capacity could accept this energy if it has enough free capacity and sufficient charging power.

A larger capacity would not automatically increase the energy recovered from that surplus. It might offer other advantages, but these should be assessed separately.

When 1 kWh may be limiting

This size may be too small when surplus generation frequently exceeds the available capacity and the home has enough later demand to use the additional energy.

Power can also limit its use: if the surplus is concentrated in a short interval, the battery may not absorb all of it before generation falls.

A 2 kWh battery: when does it offer a good balance?

A capacity of around 2 kWh is worth considering when a smaller battery often reaches its charging limit, leaving more generation available, and the home can use more energy during the evening or overnight.

The number of panels is not a sufficient criterion. Two persistently shaded panels can produce less surplus than one well-exposed panel in a home with little simultaneous demand.

Example with a 1.6 kWh surplus

With 1.6 kWh available and the efficiency assumptions used here:

• stored energy: 1.6 × 0.95 = 1.52 kWh;

• delivered energy: 1.52 × 0.95 = approximately 1.44 kWh.

A battery with a nominal capacity of 2 kWh and 1.8 kWh of usable capacity could contain this energy. To make use of it, however, the home needs later demand compatible with the battery’s output profile.

If only 0.5 kWh is consumed after sunset, some charge may remain available for the next day. This is not necessarily a problem, but it reduces the free capacity for new surplus generation.

When 2 kWh may be too much

If almost all generation is already used directly, even a 2 kWh battery may see little use. Before buying, compare its cost with the additional energy it can recover relative to a smaller battery or a system without storage.

A 4 kWh battery: when is the extra capacity useful?

Four kWh can make sense when substantial surpluses are available reasonably regularly, later demand is sufficient, or a definite and compatible expansion is planned.

The battery does not need to reach 100% every day. Partial cycles are normal. The question is how much additional energy it recovers over the year compared with a 2 kWh battery.

Example with a 3.2 kWh surplus

With 3.2 kWh available:

• stored energy: 3.2 × 0.95 = 3.04 kWh;

• delivered energy: 3.04 × 0.95 = approximately 2.89 kWh.

A battery with a nominal capacity of 4 kWh and 3.6 kWh of usable capacity could accept this energy. A 2 kWh battery with 1.8 kWh of usable capacity could not store all of it during the same interval unless discharging also freed up capacity.

The larger size becomes interesting if this situation occurs often enough and the home uses the extra energy. A few exceptional days do not, by themselves, establish that the investment is worthwhile.

How much energy is needed to fill it?

To fill 3.6 kWh of usable capacity at 95% charging efficiency requires approximately:

3.6 ÷ 0.95 = 3.79 kWh available at the storage input.

This energy must be available after direct self-consumption and with upstream conversions accounted for. It is not the same as total panel generation.

A practical method for sizing the battery

Battery sizing can be organised into five steps.

1. Collect generation and consumption profiles

Record different days, keeping the time intervals aligned. If the system is already installed, use actual data; otherwise start with a simulation and measured household consumption.

2. Identify the available surplus

For each interval, assess the generation that is not used directly. In systems that prioritise the home, consider the positive portion of the difference between generation and demand.

Systems that prioritise charging or use particular schedules may behave differently: the method must reflect the chosen operating strategy.

3. Estimate later demand

Consider the energy consumed after generation falls, as well as the power required over time. A battery can contribute to a high-power load up to its own limit, while the grid supplies the remainder.

The useful question is: how many kWh can it actually deliver before demand falls or solar generation resumes?

4. Convert the data into usable capacity

For an initial estimate based on a simple daily cycle:

Energy to store inside the battery ≈ the smaller of surplus × charging efficiency and later demand ÷ discharging efficiency.

All quantities must refer to consistent measurement points. The formula does not fully describe daytime discharging, power limits, internal consumption, seasonal variations or remaining charge.

If the result is 1.5 kWh of internal stored energy and the usable fraction is 90%, the indicative nominal capacity is:

1.5 ÷ 0.90 = approximately 1.67 kWh.

You can then compare a nearby commercially available size, checking whether the extra margin and cost are justified.

5. Compare sizes across the year

Repeat the comparison over representative periods. Additional capacity should be assessed by the kWh it allows you to recover, rather than simply the number of days on which the battery is full.

A time-based simulation can also account for remaining charge: the battery does not necessarily start empty each morning.

How much runtime do 1, 2 and 4 kWh provide?

Under the stated assumptions, a full battery delivers approximately 90% of its nominal capacity multiplied by the 95% discharging efficiency. These figures describe an energy example, not backup capability during an outage.

Theoretical runtime at an average load of 100 W

• 1 kWh nominal: approximately 0.855 kWh delivered, equivalent to 8.6 hours;

• 2 kWh nominal: approximately 1.71 kWh delivered, equivalent to 17.1 hours;

• 4 kWh nominal: approximately 3.42 kWh delivered, equivalent to 34.2 hours.

Theoretical runtime at an average load of 300 W

• 1 kWh nominal: approximately 2.9 hours;

• 2 kWh nominal: approximately 5.7 hours;

• 4 kWh nominal: approximately 11.4 hours.

The formula is delivered energy in kWh ÷ average power in kW. It assumes an initially charged battery, sufficient output power and constant efficiency. In practice, changing loads and the electronics’ own consumption may reduce runtime, particularly at low output power.

Charging and discharging power: limits to check

Even an appropriately sized battery may be poorly utilised if its power is insufficient.

When surplus arrives too quickly

A battery that accepts 300 W cannot fully absorb a 700 W surplus at the same moment. It can recover more energy later only if generation remains available.

Storing 1.5 kWh at a constant input power of 300 W and 95% charging efficiency would take approximately 5.3 hours. Variable solar generation means the actual time will differ.

When evening demand is concentrated

A home may use a lot of energy in one hour and little during the rest of the evening. A battery with limited output power may not deliver all its stored charge during that period, even if total demand appears sufficient.

Continuous power, peak power and the duration of the peak are different specifications. Peak power should not be used to estimate sustained output.

When it is better to wait before buying a battery

It may be useful to postpone the decision if generation and consumption are unknown, or if measured surplus is very small.

The choice between solar with or without storage also depends on whether generation can be used directly. Before adding a battery, consider scheduling suitable appliances during solar hours where household routines and equipment allow it. Direct use avoids the complete charging and discharging pathway.

Good exposure does not guarantee cost-effectiveness either. If the home already uses almost all generation directly, storage adds little extra self-consumption. In this case, the first option to compare with 1, 2 and 4 kWh is no battery.

How to assess the financial benefit of a larger battery

To compare two sizes, estimate the additional energy actually delivered and consumed in the home.

Suppose moving from 2 to 4 kWh allows an extra 80 kWh to be recovered per year. The benefit is not calculated from the additional 2 kWh of nominal capacity, but from those 80 kWh, using the variable cost of electricity purchases avoided and accounting for any value given up by not exporting that energy.

Use your own electricity contract and the times when purchases are reduced. With time-of-use pricing, not every stored kWh avoids the same cost. If the installation is eligible for export payments, include the revenue forgone when solar energy is stored instead. Export arrangements and eligibility differ by country, supplier and installation; they should not be assumed for a balcony kit.

Fixed bill charges do not disappear when grid consumption falls. Degradation, warranty, internal consumption and the additional investment also matter. Comparing solar battery costs and savings means assessing how much the battery is used, as well as its purchase price.

A complete financial assessment is separate from sizing: a battery can be technically suitable and still have an unattractive payback period.

Modularity and compatibility: what to check

An expandable system may allow you to start with less capacity and add more after reviewing operating data. Expansion must, however, be supported by the manufacturer.

Before buying, check:

• maximum capacity and permitted number of modules;

• compatibility between generations and firmware versions;

• conditions for adding modules of different ages;

• permitted voltages, currents and power ratings;

• compatibility with the controller and microinverter;

• temperature limits and installation conditions;

• warranty terms in years, cycles or energy throughput.

Compatibility cannot be established from connector shape alone. The protection rating must also be read alongside instructions covering rain, direct sunlight, ventilation and charging temperatures.

Battery capacity does not guarantee backup power

A grid-connected microinverter normally stops operating when the public electricity supply fails. A charged battery does not remove this limitation.

A battery system with backup functionality requires specific features separate from its capacity in kWh. A dedicated emergency outlet may power compatible appliances if the product provides that function. Supplying household circuits during an outage instead requires a specific architecture, with safe isolation from the grid and load management designed into the system.

The runtime calculations above therefore do not establish that the kit can keep the flat supplied during a grid outage.

Frequently asked questions about balcony solar batteries

How many kWh do you need for balcony solar panels?

Capacity depends on available surplus, later demand and system limits. Around 1 kWh may be enough for modest surpluses; 2 or 4 kWh requires more recoverable energy. There is no single battery size suitable for every balcony.

Is a 2 kWh battery always the best choice?

No. It may be too large if generation is already used directly, or too small if frequent surpluses remain that the home could use. Compare it with generation and consumption profiles.

Can an 800 W solar system charge a 4 kWh battery?

It depends on what the 800 W rating represents, generation conditions, self-consumption and losses. Filling it in one day is not guaranteed. Estimate the energy available at the storage input, rather than relying only on the kit’s advertised power.

How long does a 2 kWh battery last on one charge?

In the example using 90% usable capacity and 95% discharging efficiency, it delivers approximately 1.71 kWh. At a constant average load of 300 W, theoretical runtime is around 5.7 hours. Actual results depend on specifications and operating conditions.

Can a battery with a 600 W output contribute to an oven’s consumption?

Yes. In a grid-connected system configured to do so, it can contribute up to its power limit. The grid supplies the remainder. This does not mean it can power the oven independently during an outage.

Does the battery need to charge fully every day?

No. Partial cycles are normal. When assessing sizing, what matters is how much energy is recovered and used over time, particularly compared with a smaller battery.

Can I add a battery to an existing kit?

Only if there is a compatible, supported configuration. Components, electrical limits, control arrangements and applicable connection requirements must be checked. Matching connectors are not enough.

When is a modular system worth considering?

When expansion is supported and you want to base the final capacity on data collected over time. Before buying, check the limits, conditions for adding modules and their future compatibility.

How to choose between 1, 2 and 4 kWh

Start with recoverable energy, rather than the largest available size.

Consider around 1 kWh when surplus is modest and the home can use a similar amount after generation falls. Consider around 2 kWh when a smaller battery would frequently leave energy available for storage. Move to around 4 kWh when the additional capacity recovers more energy regularly enough, or meets a verified requirement.

Before deciding, check usable capacity, efficiencies, power, compatibility and seasonal patterns. If these data are missing, measuring over a representative period is more useful than choosing solely by the number of panels.

A well-sized battery stores energy the home can actually use, with an investment proportionate to the benefit it can deliver.