Battery Sizing Made Simple
Most homeowners ask the same question before buying: how many kWh do I need? The answer depends on what you want the battery to do. A small battery can cover evening lights and internet. A medium battery can cover normal night-time use. A large battery can support longer backup, bigger appliances, or high self-consumption from a large solar system.
Quick Answer
Small Homes
A 5–7 kWh battery can suit low-consumption homes that mainly want lights, Wi-Fi, fans, fridge, and basic evening use. It is not normally enough for heavy air-conditioning, electric heating, pumps, or whole-home backup.
Average Homes
A 10–13.5 kWh battery is the common middle ground for solar homes. It can often cover night-time consumption and essential backup loads, especially when paired with smart scheduling and realistic reserve settings.
Large Homes
A 15–20 kWh+ battery may be better for large families, all-electric homes, long backup windows, frequent load shedding, off-grid cabins, or homes trying to reduce grid imports as much as possible.
The Simple kWh Formula
Battery sizing starts with the energy you actually need to run after sunset or during an outage. Do not size only from your solar panel capacity. A 10 kW solar system does not automatically need a 10 kWh battery, and a 5 kW solar system does not automatically need a 5 kWh battery. Size the battery around your load, backup goal, usable capacity, and efficiency.
For example, if you need 8 kWh of usable energy at night, and your battery is designed around 90% usable depth of discharge and 92% round-trip efficiency, the nominal battery capacity is roughly 8 ÷ 0.90 ÷ 0.92 = 9.66 kWh. In practice, you would shortlist a 10 kWh or similar system, then check inverter power, surge requirements, and manufacturer limits.
This simple formula keeps you from under-sizing the system. It also prevents over-buying when a smaller battery would meet your real needs. The goal is not to buy the largest battery possible; the goal is to buy enough usable energy and enough power output for the loads you expect to run.
Typical Battery Size by Use Case
Use this table as a starting estimate. Your final size should be based on your bill, load profile, solar generation, tariff plan, inverter limits, and backup requirements.
| Battery size | Best for | Typical usable role | Watch out for |
|---|---|---|---|
| 3–5 kWh | Small backup, lights, router, fridge, fans, low evening load | Basic essential-load backup or partial self-consumption | Can run out quickly with AC, kettles, pumps, heating, or multiple appliances |
| 7–10 kWh | Small to average homes with moderate night-time use | Evening solar shifting, short outages, lighter peak shaving | Check usable capacity, backup reserve, and inverter output before assuming whole-home coverage |
| 10–13.5 kWh | Average solar homes and common retrofit systems | Good balance for night-time use, tariff optimisation, and essential backup | May still be too small for all-electric homes or long outages |
| 15–20 kWh | Large households, frequent outages, higher self-consumption targets | Longer backup, more overnight coverage, better use of surplus solar | Needs proper inverter power, current limits, cable sizing, and space planning |
| 20 kWh+ | Large homes, off-grid use, heavy loads, multi-day autonomy planning | Extended backup and deeper energy independence | Costs rise quickly; oversizing can reduce payback if the extra capacity sits unused |
kWh vs kW: Do Not Mix Them Up
kWh Means Energy
Kilowatt-hours tell you how much energy the battery can store. If a load uses 1 kW for 5 hours, it consumes 5 kWh. This is the number most people mean when they ask how big their battery should be.
kW Means Power
Kilowatts tell you how much power the battery inverter can deliver at one time. A battery may store 10 kWh but only output 3–5 kW continuously. That power limit decides which appliances can run together.
You Need Both
A good system has enough kWh for duration and enough kW for peak loads. A battery that has enough energy but too little power may shut down when a pump, AC, induction cooktop, or motor starts.
Size Your Battery in 5 Steps
Find Daily kWh Use
Check your electricity bill or smart meter. Note average daily consumption, but separate daytime use from night-time use because solar already covers some daytime load.
Choose the Backup Goal
Decide whether you want essential backup, overnight backup, or whole-home backup. Essential backup is much cheaper because you only support selected circuits.
List Critical Loads
Write down the wattage and run-time for fridge, lights, Wi-Fi, fans, medical equipment, security, pumps, and any appliances you must keep running during outages.
Add DoD and Efficiency
Adjust for depth of discharge and round-trip efficiency. A 10 kWh nominal battery may not deliver 10 kWh after reserve, BMS limits, conversion losses, and backup settings.
Check Inverter Output
Confirm continuous power, surge rating, battery current, backup port rating, grid rules, and BMS compatibility. Capacity alone does not guarantee performance.
Backup Runtime Examples
These examples are simplified. Real runtime depends on weather, battery reserve, inverter efficiency, appliance cycling, temperature, and whether solar is charging the battery during the day.
| Loads running | Approx. load | 10 kWh battery | 15 kWh battery | 20 kWh battery |
|---|---|---|---|---|
| Fridge, Wi-Fi, LED lights, phone charging | 300–500 W average | About 18–28 hours | About 27–42 hours | About 36–56 hours |
| Essentials plus fans, TV, laptop, small appliances | 700–1,000 W average | About 8–12 hours | About 12–18 hours | About 16–24 hours |
| Normal evening home use with some cooking or pumps | 1.5–2.5 kW average | About 3–5 hours | About 5–8 hours | About 6–10 hours |
| Air-conditioning or heavy electric heating | 2.5–5 kW average | Short duration | Moderate duration | Better, but still load-dependent |
Solar Self-Consumption Sizing
Size Around Night Load
For grid-connected solar, the best starting point is your evening and night-time consumption, not the total size of your solar array. If you only use 6 kWh after sunset, a 20 kWh battery may not improve savings enough to justify the cost.
Allow for Bad Weather
If your area has cloudy days, winter production drops, or frequent outages, consider extra capacity. But do not rely on battery size alone; make sure solar generation is large enough to recharge it.
Track Export Waste
If your monitoring app shows regular midday export, a battery can capture some of that surplus. If you rarely export, the battery may need grid charging or tariff arbitrage to make economic sense.
Common Sizing Mistakes
Using Solar kW as Battery kWh
A 6.6 kW solar system does not mean you need a 6.6 kWh battery. Solar capacity is power; battery capacity is stored energy. Match the battery to usage patterns.
Ignoring Backup Reserve
If you reserve 20% for outages, only 80% is available for daily cycling. A battery that looks large on paper may feel smaller in real use when reserve settings are active.
Forgetting Peak Loads
High-starting loads such as pumps, AC compressors, induction cooktops, ovens, and welders can exceed inverter output even when there is plenty of energy left.
Buying Too Much Battery
Oversizing can reduce payback because unused capacity does not save money. Extra kWh are useful for backup and autonomy, but only if you actually need them.
Ignoring Compatibility
The battery must communicate correctly with the inverter. Closed-loop CAN/RS485 communication helps the inverter read SoC, current limits, temperature and protection states.
Confusing Whole-Home Backup
Whole-home backup requires more than a big battery. You may need a backup gateway, transfer switch, supported inverter mode, load management, and approved wiring.
Simple Load Worksheet
Use a worksheet like this before you request quotes. It helps installers give more accurate advice and helps you compare options clearly.
| Load | Typical power | Hours per day | Daily energy |
|---|---|---|---|
| Fridge/freezer | 80–200 W average | 24 hours cycling | 1–2 kWh/day |
| Wi-Fi router and small electronics | 20–80 W | 8–24 hours | 0.2–1.5 kWh/day |
| LED lighting | 50–200 W total | 3–6 hours | 0.2–1.2 kWh/day |
| Fans or small cooling | 60–300 W | 4–10 hours | 0.3–3 kWh/day |
| Air-conditioner | 700–2,500 W average | 2–8 hours | 2–15 kWh/day |
Buying Checklist
Ask for Usable Capacity
Quotes should state both nominal and usable kWh. Usable capacity is what matters for runtime, while nominal capacity is the gross battery size before limits and reserves.
Check Warranty Terms
Look beyond years. Read cycle limits, throughput limits, retained capacity, operating temperature, installation rules, approved inverter list, and remote monitoring requirements.
Confirm BMS Integration
The battery management system should communicate with the inverter using an approved profile. Manual voltage-only settings are riskier and may reduce performance or support.
Plan Essential Circuits
List what you want to run during outages. Put essential circuits on the backup board and keep heavy loads off unless the inverter and battery are designed for them.
Final Rule of Thumb
Choose a battery that can cover your normal night-time kWh, your essential backup kWh, and your most important peak loads without exceeding inverter limits. For many homes, the practical sweet spot is around 10–13.5 kWh. Smaller homes may be fine with 5–7 kWh. Larger homes, all-electric homes, and outage-prone properties may need 15–20 kWh or more.
The most accurate answer comes from your own data. Review your smart meter, bill, solar monitoring app, and appliance list. Then compare at least two system sizes: a budget option that covers essentials and a larger option that covers comfort loads. This makes the buying decision much clearer than asking for a single “best” size.