How to choose · Home batteries
How to size a home battery
Last updated 25 October 2026 · 2 sources listed · Reading time about 3 minutes
Short answer
Size on two separate numbers: energy capacity, which sets how long the battery can supply your evening and overnight load, and power output, which sets how many appliances it can run at once. Study your own interval data, your solar surplus, and whether you want backup during outages. Oversizing leaves capacity idle; undersizing leaves expensive grid use at peak times.
Who this is for: A household with or planning solar, or on a time-varying tariff, deciding how much storage capacity and output power to buy.
What to evaluate
Evening and overnight load
Capacity should relate to what you actually draw after solar fades, not to the largest model on offer.
How to check: Use smart meter or monitoring data to total consumption between sunset and morning on typical days.
Solar surplus available to store
A battery only fills as far as your panels produce beyond daytime demand.
How to check: Estimate average daily export in each season from your data or an installer's model.
Continuous and peak power rating
A battery with plenty of capacity but low power cannot start a heat pump or run an oven alongside other loads.
How to check: List the largest simultaneous loads and compare with the battery's continuous output.
Usable versus nominal capacity
Depth-of-discharge limits mean not all stated capacity is available.
How to check: Read the datasheet for usable energy and the conditions it applies under.
Backup requirement
Whole-home backup needs different wiring and sizing from simple bill shifting.
How to check: Decide which circuits must stay live in an outage and confirm the system can island safely.
Future changes in demand
A vehicle charger, heat pump or extra panels can change the right size.
How to check: Choose a modular system that can expand if you expect these additions.
Comparison
| Approach | Good when | Watch out for |
|---|---|---|
| Cover evening load only | Solar is present and tariffs are flat | Little help in poor-sun winter weeks |
| Cover overnight and peak shifting | You are on a time-based tariff | Cost grows with each increment of capacity |
| Essential-loads backup | Outages occur and some circuits matter most | Requires a separate backup panel |
| Modular start small | Budget or demand is uncertain | Mixing batteries of different ages can limit performance |
Decision worksheet
Rate up to three options against this guide’s own criteria. It runs in your browser and saves only on this device. It is a thinking aid, not a recommendation.
1. Have you checked each criterion?
0 of 6 criteria checked.
2. Compare options (0 = fails or unknown, 3 = meets it well)
| Criterion | |||
|---|---|---|---|
| Evening and overnight load | |||
| Solar surplus available to store | |||
| Continuous and peak power rating | |||
| Usable versus nominal capacity | |||
| Backup requirement | |||
| Future changes in demand | |||
| Total (max 18) | 0 | 0 | 0 |
Questions to ask before you commit
- What is the usable capacity as well as the nominal figure?
- What are the continuous and peak power ratings?
- Can the battery start the largest motor loads I own?
- Is it AC-coupled or DC-coupled, and what does that mean for my existing inverter?
- How does it behave in a grid outage, and is that automatic?
- Can I add modules later and are they compatible?
- What are the warranty terms in terms of cycles, years and retained capacity?
- How is software control handled, and is it dependent on a cloud service?
Typical trade-offs
Larger capacity
- Upside
- More resilience and overnight cover
- Downside
- Higher cost and parts of capacity may rarely cycle
Smaller capacity
- Upside
- Lower outlay and quicker cycling
- Downside
- Grid imports still happen in dark winter weeks
Higher power rating
- Upside
- Runs more appliances at once in backup
- Downside
- Costlier hardware you may not need
Common mistakes
- Sizing from a household average rather than from your own interval data.
- Confusing kilowatts with kilowatt-hours and buying power when capacity was needed.
- Planning whole-home backup without checking that the battery can handle starting currents.
- Assuming stored energy will last through a multi-day outage.
- Buying before adding solar or an electric vehicle that would change the right size.
Related questions
What is the difference between capacity and power?
Capacity, measured in kilowatt-hours, is the energy the battery holds, like the size of a fuel tank. Power, measured in kilowatts, is how fast it can deliver or absorb energy, like the width of the pipe. A large tank with a narrow pipe cannot run many appliances at once. Check both in datasheets, along with the continuous rating, not just the brief peak figure.
Do I need a battery if I have no solar?
A battery can still shift grid energy from cheap periods to expensive ones on a time-varying tariff, and can provide backup. Without solar, the value depends entirely on the tariff gap and the rules in your country. Ask suppliers for modelling using your real data, and check the regulator's guidance on grid-charging and export.
How long will a battery power my home in an outage?
It depends on stored energy and which loads are connected. A fridge, lights and router draw little and can run a long time. Electric heating, cooking and air conditioning drain a battery fast. Decide which circuits are essential and ask the installer to estimate runtime for that set, with and without solar recharging.
Can I add capacity later?
Some systems are modular, but mixing old and new units may be restricted because of differences in age, chemistry or firmware. Ask the manufacturer about expansion policy and whether additions share warranty terms. If you expect demand to grow, choose an expandable design from the start.
Sources and further reading
These pages were reachable when we last checked. They informed definitions and rules referred to above; they do not endorse this guide. Where rules differ by country, check the authority for your location.
- Homeowner's guide to going solar — U.S. Department of Energy, retrieved 27 October 2026
- Energy Explained — U.S. Energy Information Administration, retrieved 27 October 2026
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