Three Common Battery-Sizing Mistakes
Mistake 1: Assuming a Larger Battery Automatically Provides Better Backup
A 10 kWh battery generally stores more energy than a 5 kWh battery of otherwise comparable design. But it does not necessarily provide more instantaneous power.
Consider a home with a 10 kWh battery and a 3 kW inverter. If the inverter's permitted backup output is 3 kW, the system cannot continuously support a 4 kW load simply because the battery has enough stored energy.
Buyers should check three separate specifications:
Energy capacity (kWh): How much energy is stored or available for use?
Continuous power (kW): How much load can the system supply at one time?
Peak or surge power: Can the system handle the starting current of motors, pumps, compressors or other appliances?
Also verify whether the system supports backup operation during a grid outage, whether essential circuits need to be wired to a separate backup output, and whether the battery and inverter can supply the required current.
Mistake 2: Assuming Battery Storage Will Eliminate Electricity Bills
Battery storage can reduce grid purchases, but it does not automatically eliminate them.
Savings depend on how much electricity can be shifted, the applicable tariff and the energy lost during charging and discharging. Fixed charges and electricity consumed outside the battery's operating window may remain.
Tariff structures can also change. China's national time-of-use policy guidance allows arrangements to reflect local system conditions, and the NDRC has described the need for mechanisms to be adjusted as electricity demand and supply characteristics change. [1]
A payback calculation based on today's tariff should therefore be treated as a scenario, not a prediction that prices will remain unchanged for the entire battery life.
For a more realistic assessment, model at least three scenarios:
Conservative: A narrower peak-valley price difference or fewer useful cycles
Base case: Current verified tariff and realistic household use
Upside: A larger price difference or greater use of otherwise surplus solar energy
If the investment only appears attractive under the most optimistic scenario, buyers should understand that risk before placing an order.
Mistake 3: Assuming a Solar Installation Always Needs a Large Battery
Consider a home with a 6 kW solar PV system that generates approximately 24–28 kWh on a favourable day. This is an illustrative daily output, not a year-round forecast; actual generation varies with location, season, shading, orientation and weather.
Suppose the household consumes 10 kWh per day.
The difference between generation and consumption might appear to suggest that a large battery is necessary. However, that daily balance does not reveal when the electricity is generated or consumed.
If much of the household's consumption occurs during daylight, less surplus may be available for charging. If the occupants are away during the day and use more electricity in the evening, a larger share of solar generation may be available for storage.
The correct approach is to compare the solar generation curve with the household's consumption curve, preferably using hourly or sub-hourly data.
A 6 kW solar system paired with a 5–8 kWh battery may be a reasonable starting point for some homes, but it is not a universal sizing rule. The optimal configuration depends on the actual surplus, evening load, tariff, backup requirements and battery operating limits.
How to Choose a Battery for Different Household Scenarios
The following framework can help buyers narrow their options before requesting a formal system design.
Scenario A: Time-of-Use Arbitrage Without Solar
Begin by checking whether your electricity meter and tariff support time-of-use pricing.
Then establish:
The actual off-peak charging price
The price during the period when the battery will discharge
The amount of electricity consumed during that period
The energy the battery can deliver after accounting for usable capacity and losses
The installed cost of each proposed system
A price difference below RMB 0.50/kWh may make arbitrage less attractive, but this is not a universal pass-or-fail threshold. The result depends on system cost, efficiency, cycling frequency and the applicable tariff.
A larger price spread can improve the economics, but only if the household has enough demand to use the battery's additional capacity.
Buyer tip: Request a comparison based on your actual tariff and at least two battery sizes. Ask the supplier to show the annual energy shifted and calculation assumptions, rather than quoting only a theoretical saving per cycle.
Scenario B: Backup Power Is the Main Priority
For emergency backup, list the appliances that must continue running and estimate both their power demand and duration of use.
Here is a simplified example:
| Appliance | Illustrative average power | Runtime | Energy required |
|---|---|---|---|
| Refrigerator | 100 W | 8 hours | 0.80 kWh |
| Router | 20 W | 8 hours | 0.16 kWh |
| Phone charging | 20 W | 2 hours | 0.04 kWh |
| One light | 10 W | 8 hours | 0.08 kWh |
| Total | - | - | 1.08 kWh |
This calculation uses average power assumptions for illustration. Refrigerators cycle on and off, and actual consumption varies by model, ambient temperature and usage.
The table also excludes conversion losses and any reserve that the system may retain. If the same household needs 1.2 kWh of delivered energy, it should not automatically assume that a nominal 1.2 kWh battery will provide it under real operating conditions.
Now consider air conditioning. An air conditioner drawing an average of 1.0–1.5 kW over eight hours would consume approximately 8–12 kWh during that period, before accounting for other loads and system losses. Actual energy use depends on the unit's efficiency, outdoor temperature, room conditions and compressor duty cycle.
This can make the difference between a small backup system and a much larger one.
However, even a 10 kWh battery does not guarantee a full night of air conditioning. Buyers need to verify usable battery energy, inverter continuous and surge power, backup circuit limits, and whether the air conditioner can be supported by the system.
Buyer tip: Design backup around essential loads first. If air conditioning, well pumps or other high-power equipment must operate during outages, ask for a load assessment rather than choosing capacity from a headline kWh figure.
Scenario C: Solar Self-Consumption
For a home with rooftop solar, review at least 12 months of electricity bills if available. Combine this with solar generation records and, ideally, interval-meter data.
Look for:
The amount of solar energy exported during daylight
Electricity imported after sunset
The times when household demand is highest
Seasonal differences in solar production and consumption
The amount of surplus energy that a battery can realistically absorb
Whether the battery can discharge at the required rate
The objective is to capture surplus energy that would otherwise have limited value and use it later when the household needs it.
A battery that is too small may fill early and leave additional solar surplus unused. A battery that is too large may rarely fill during periods of weak solar generation or may not discharge fully before the next charging window.
The right size balances these competing effects.
Don't Ignore Battery Degradation Over Time
A battery's usable energy changes over its service life. Capacity retention depends on both cycle-related ageing and calendar ageing.
The U.S. National Renewable Energy Laboratory's System Advisor Model (SAM) distinguishes between cycle degradation and calendar degradation. Its modelling guidance explains that capacity loss can depend on depth of discharge, cycle count, temperature, state of charge and elapsed time. This is why one simple rule cannot accurately predict every battery's condition after ten years.
For example, suppose a 10 kWh battery has a contractual or modelled capacity-retention assumption of 80% at a defined point in its service life.
The corresponding retained nominal capacity would be:
10 kWh × 80% = 8 kWh
That calculation is straightforward, but buyers must check what the 80% figure actually means. It may refer to nominal capacity under specified test conditions, and it does not automatically guarantee that 8 kWh will be delivered to household appliances after system losses and operating limits.
What Buyers Should Check
Before purchasing, review the manufacturer's documentation for:
- The warranted capacity-retention threshold and the year it applies to
- Any throughput or cycle limits within the warranty
- The conditions required to maintain warranty coverage
- Permitted operating temperature and charging conditions
- The definition of end of life
- Whether the guarantee applies to battery cells, the battery pack or the complete system
A headline cycle-life figure is not enough. A product described as having thousands of cycles should also state the conditions under which that figure was established.
When Is 5 kWh Enough, and When Does 10 kWh Make Sense?
Neither capacity is inherently the right answer for every home. The following table provides an initial screening framework.
| Household profile | 5 kWh may be worth considering when… | 10 kWh may be worth considering when… |
|---|---|---|
| Time-of-use arbitrage | Peak-period demand is relatively low and additional daily energy shifting is limited | There is enough peak-period consumption to use the extra energy regularly |
| Backup power | The priority is essential low-power loads for a limited period | Longer backup duration or more essential loads are required, subject to inverter limits |
| Solar self-consumption | The typical surplus available for storage is modest | More surplus solar energy is regularly available and can be used later |
| Budget-sensitive purchase | Lower upfront cost is important and smaller capacity meets the objective | The additional savings or backup value justifies the incremental investment |
| Future load growth | Household demand is expected to remain relatively stable | New loads, such as an electric vehicle or additional electric appliances, are expected-but should be modelled first |
This is a screening guide, not a substitute for system design. A household may need a different capacity depending on inverter compatibility, the minimum and maximum operating state of charge, reserve settings and the battery's usable energy.
It is also worth asking whether the system can be expanded later. Modular expansion may offer flexibility, but only if the manufacturer supports the proposed configuration, parallel operation, firmware compatibility and warranty conditions.
A Better Way to Size Your Home Battery
A useful sizing process can be completed in five steps.
Step 1: Define the Main Objective
Decide whether the priority is electricity bill savings, backup power, solar self-consumption or a combination of these goals.
If several objectives matter, rank them. A system optimised for arbitrage may not be the same size as one designed to run air conditioning through a long outage.
Step 2: Collect Real Household Data
Use electricity bills, smart-meter data and solar monitoring records where available.
For arbitrage, focus on consumption during the expensive tariff periods. For backup, list the essential appliances and their expected runtime. For solar self-consumption, compare surplus generation with later demand.
Step 3: Calculate Usable Energy and Required Power
Do not compare systems on nominal capacity alone.
Ask for the usable energy under the proposed settings, the permitted depth of discharge, the system's conversion efficiency, continuous output power and surge capability.
For backup applications, confirm the actual backup output and which circuits will be powered during an outage.
Step 4: Compare Total Costs and Realistic Savings
Request a written comparison of 5 kWh and 10 kWh configurations, including equipment, installation, inverter or PCS requirements, electrical work, commissioning and any additional accessories.
Calculate the additional annual value of the larger system. Then consider degradation, warranty conditions and the possibility that tariffs may change.
Step 5: Verify Compatibility and After-Sales Support
Check whether the battery is compatible with the selected inverter, whether the system supports the required operating modes and whether local installation and maintenance support is available.
The purchase decision should be based on the full system, not the battery pack in isolation.
How Tenz Energy Can Support a More Informed Storage Purchase
Tenz Energy is a Chinese manufacturer focused on new-energy battery research, development and manufacturing, including residential energy storage solutions.
For buyers evaluating home battery systems, the most productive starting point is to define the household's energy needs and then match those needs to the proposed system configuration.
Before selecting a capacity, buyers should confirm the usable energy, inverter compatibility, supported charging and discharging power, intended operating modes, installation requirements and warranty terms for the exact model under consideration.
For solar-connected homes, the design should reflect the site's generation and consumption profile. For backup applications, the essential loads and expected outage duration should be clearly specified. For time-of-use arbitrage, the financial calculation should use the customer's actual electricity tariff and realistic energy throughput.
Tenz Energy recommends a project-specific evaluation rather than assuming that every household needs the same battery size.
The most reliable decision process is to:
Identify the main purpose of the battery.
Analyse real electricity use and, where applicable, solar generation.
Calculate usable energy and check output power.
Compare the additional cost with the additional value.
Review degradation assumptions, warranty conditions and system compatibility.
The best battery is not necessarily the biggest one. It is the one whose usable capacity, power output and lifetime value match the household's actual needs.
Mail：Leeana.long@tenzenergy.com
Common Problem
Is a 5 kWh home battery enough for an average household?
It depends on the household's consumption, backup priorities, solar generation and tariff. A 5 kWh system may be suitable for shifting a modest amount of electricity or supporting essential low-power loads, but it may be insufficient for extended backup or high-power appliances.
Is a 10 kWh battery always more economical than a 5 kWh battery?
No. A 10 kWh system can deliver more value only when its additional capacity is used effectively. Buyers should compare the incremental installed cost with the additional annual savings or backup value, rather than comparing capacity alone.
Can a 10 kWh battery run an air conditioner overnight?
Possibly, but not in every installation. The answer depends on the air conditioner's average consumption, runtime, battery usable energy, inverter continuous and surge power, system losses and other simultaneous loads. A load assessment is recommended before purchase.
What matters more: cycle life or a capacity-retention warranty?
Both matter, but they describe different aspects of battery life. Cycle life should be assessed under its stated test conditions, while a capacity-retention warranty explains the manufacturer's commitment under specified terms. Buyers should review both alongside the warranty duration, throughput limits and exclusions.