Energy Flow Hub · Tools
Home Battery Sizing Calculator
Estimate a sensible home battery size based on the electricity you actually want to shift — not simply your total daily consumption.
“A home battery should be sized around the electricity you want to move, not simply the electricity the home uses.”
This tool provides an educational estimate. Final battery sizing should consider the actual inverter, battery system, tariff, property loads and installer design.
Start here
Scenario mode
Preset scenarios
Section A
Household electricity
Used for context only — a battery is not sized from total daily use.
This is usually more important than total daily use because this is the energy a battery may need to carry from one period to another.
Solar
Solar PV surplus
Use surplus after household daytime loads, not total solar generation.
Useful for understanding seasonality — it should not decide the whole battery size.
Tariff
Cheap overnight charging
Reserve & losses
Backup reserve and efficiency
This portion of the battery is held back rather than used for normal daily energy shifting.
Not all energy put into the battery comes back out again.
Core input
Electricity you want the battery to shift each day
Auto uses your evening/overnight demand alongside available solar surplus and cheap tariff charging. Current auto target: 7 kWh/day.
Power
Battery power check inputs
Optional
Future loads
Select any that apply for a short future sizing note.
Live result
What is happening?
- Your evening and overnight demand is around 7 kWh, while typical usable solar surplus is around 8 kWh. A battery of about 10–12 kWh nominal capacity would shift roughly 7 kWh a day while keeping a 20% reserve.
- Typical good-day solar surplus could fill most of the battery.
- Holding back 20% for backup adds roughly 1.8 kWh to the nominal capacity required.
Diagnosis
What is limiting the size?
Your evening and overnight demand is the main limit.
Energy flow
Where the energy moves
Solar surplus
8 kWh
Battery
10–12 kWh
Evening / night load
7 kWh
Breakdown
Sizing breakdown
Evening / overnight demand
7 kWh
Target daily shift
7 kWh
Charging energy needed
7.8 kWh
Target ÷ 90% efficiency
Required usable capacity
7 kWh
Required nominal capacity
8.8 kWh
Usable ÷ (1 − 20% reserve)
Sensible range
10–12 kWh
Rounded to common capacity bands
Power
Battery power check
Peak load to support
4 kW
Suggested discharge power
Around 5 kW
Your largest short-duration load (7 kW) is well above your continuous target, so short surge capability matters as well as continuous power.
kWh tells you how much energy is available. kW tells you how quickly the battery can help.
Solar
Solar fill check
Typical solar surplus
8 kWh
Battery usable space
8 kWh
Typical good-day solar surplus could fill most of the battery.
A battery can only store solar that actually exists.
Seasonal comparison
A battery sized around summer solar may behave very differently in winter.
Backup
Backup reserve check
Reserved energy
2 kWh
Reserve share
20%
| Steady load | Approx. runtime |
|---|---|
| 500 W | 4 h |
| 1 kW | 2 h |
| 2 kW | 1 h |
Simplified energy-duration maths only. Backup duration is about energy. What can run at the same time is limited by inverter power and backup architecture.
Comparison
What if I choose…
8 kWh
Smaller
Covers most evening demand but may run empty earlier.
Usable after reserve: 6.4 kWh
10 kWh
Balanced
Good match to your intended daily shift.
Usable after reserve: 8 kWh
12 kWh
Larger
Good match to your intended daily shift.
Usable after reserve: 9.6 kWh
Educational comparison only — not a product recommendation.
Education
What this calculator demonstrates
- Total daily electricity use is not the same as battery sizing requirement.
- Battery sizing depends on how many kWh need moving between time periods.
- Solar surplus can limit how much a battery can regularly fill.
- Evening demand can limit how much stored energy is actually useful.
- Reserve reduces usable battery capacity.
- Charging power can limit whether a battery fills inside a cheap tariff window.
- Discharge power determines whether the battery can support larger loads.
- A larger battery is not automatically a better battery.
A battery can become twice as large in kWh without becoming twice as powerful in kW.
Choose the job first. Then choose the battery.