home battery storage system

When homeowners consider installing a battery energy storage system, one of their first questions is usually straightforward: Is a 5 kWh battery enough, or should I choose 10 kWh?

A common sales argument is that a larger battery offers greater flexibility and peace of mind. That can be true, but it does not automatically make the larger system the better investment.

An oversized battery may cost more without delivering proportionally greater savings. A smaller battery, meanwhile, may not provide enough energy for overnight backup or may leave valuable solar energy unused.

The right choice depends on three primary factors:

Your electricity consumption pattern: How much energy do you use, and at what times?

Your electricity tariff: Is there a meaningful difference between peak and off-peak prices?

Your solar PV system: Do you have solar panels, and how much surplus electricity is available to charge the battery?

Your backup requirements, inverter power, usable battery capacity, installation costs and warranty conditions also matter.

The key is to size the battery around the energy you can realistically use-not simply the largest capacity your budget allows.

Before comparing capacities, identify the primary purpose of the system. Residential battery storage generally serves three main use cases.

1.1 Time-of-Use Arbitrage

A battery can charge when electricity is relatively inexpensive and discharge when electricity prices are higher.

The potential benefit depends on the usable energy delivered during expensive periods, the difference between electricity prices, system efficiency and the number of economically useful cycles.

A larger battery can deliver more savings if the household has enough peak-period consumption to use its additional capacity. If demand is limited, however, some of that capacity may remain unused.

This use case is especially sensitive to local tariff structures. China's National Development and Reform Commission has explained that time-of-use pricing is designed to reflect differences in electricity-system costs across periods, but specific arrangements vary by region and customer category. Residential customers should verify their own applicable tariff rather than assume that a particular peak-valley spread is available everywhere. 

1.2 Backup Power During an Outage

For backup power, the important question is how much electricity essential appliances consume during the outage.

A battery may need to support a refrigerator, lighting, communications equipment, medical devices or air conditioning. These loads have different energy requirements, and some appliances draw substantially more power when starting.

Battery capacity determines how much energy can be supplied over time. Inverter power determines how much load can be supported simultaneously.

Both must be checked.

1.3 Solar Self-Consumption

When a home has rooftop solar, the battery can store surplus solar electricity for use later, such as after sunset.

The objective is not necessarily to store all the solar energy produced. Instead, the battery should capture an appropriate share of the surplus that would otherwise be exported or curtailed, then deliver that energy when the household needs it.

For this use case, the timing of solar generation and household demand is often more important than daily generation alone.

Let's use an illustrative calculation to compare 5 kWh and 10 kWh systems.

The following figures are example assumptions, not a quotation or a statement of current electricity prices in every Chinese city. Actual savings depend on the local tariff, usable capacity, system efficiency and installation cost.

Example assumptions

For simplicity, this first comparison uses nominal capacity as a proxy for daily energy shifted. A real quotation should use the system's usable capacity and clearly defined efficiency measurement boundary.

2.1 Illustrative Annual Savings

A simplified estimate is:

Annual gross arbitrage value = Energy shifted per cycle × Price difference × Efficiency factor × Useful cycles per year

For a 10 kWh system:

Daily gross value: 10 kWh × RMB 0.80/kWh × 90% = RMB 7.20

Annual gross value: RMB 7.20 × 330 days = RMB 2,376

For a 5 kWh system:

Daily gross value: 5 kWh × RMB 0.80/kWh × 90% = RMB 3.60

Annual gross value: RMB 3.60 × 330 days = RMB 1,188

<text>Important: This is a simplified screening calculation, not a bankable financial model. In a detailed analysis, the energy purchased to charge the battery, energy delivered after losses, usable capacity, auxiliary consumption, degradation and the actual tariff for each period must be modelled consistently. Efficiency should not be applied twice if the input and output energy values already account for it.

2.2 What Do the Payback Periods Look Like?

Using the illustrative installed prices above:

These figures do not include financing, maintenance, future tariff changes, battery degradation, replacements or the time value of money. The simple payback period therefore should not be interpreted as a guarantee of investment recovery.

At first glance, the 10 kWh system appears more attractive because its estimated payback is shorter. But the incremental investment deserves separate attention.

2.3 The More Useful Question: Is the Additional 5 kWh Worth the Extra Cost?

The larger system costs an additional RMB 12,000 in this example.

Its additional estimated annual gross value is:

RMB 2,376 − RMB 1,188 = RMB 1,188 per year

The simple payback on that additional investment is:

RMB 12,000 ÷ RMB 1,188 = approximately 10.1 years

This is the more revealing comparison. The extra capacity makes economic sense only if the household can use it often enough, and if the resulting additional savings justify the incremental cost over the expected service life.

The conclusion is not that 5 kWh is always better or that 10 kWh is always better. It is that the value of the additional capacity must be demonstrated using the household's actual energy profile.

2.4 Why Daily Peak Consumption Matters

Suppose a household typically needs only 4–5 kWh of battery-delivered energy during the expensive tariff period.

A 10 kWh battery may still be useful if it serves other purposes, such as backup or solar self-consumption. But if its only job is to shift electricity into that 4–5 kWh window, much of its capacity may not contribute to additional daily savings.

A useful first screening rule is:

Start with the energy you can realistically discharge during the target period, then assess whether the larger battery has another valuable use.

Do not confuse a battery's rated capacity with the energy that your household can actually use each day.

[5 kWh product technical details]