Home Battery Size Calculator: Get the Right kWh for You

Start With the Question: What Do You Want the Battery to Do?

Home batteries serve two main purposes, and the right size depends entirely on which you prioritise. Understanding this distinction is the most important step.

Purpose 1 — Solar self-consumption: Store surplus solar generation during the day and use it in the evening rather than buying from the grid. This typically requires 5–15kWh depending on your solar system size and your evening electricity use. For most households with rooftop solar, this is the primary use case and it determines the minimum useful battery size.

Purpose 2 — Outage backup: Keep critical appliances running during a power outage. This could mean a few hours of essentials or multiple days of significant loads. Backup requirements vary enormously — from 5kWh (run a fridge and lights overnight) to 30kWh+ (whole-home backup for extended periods). This purpose can require a much larger battery than solar self-consumption alone.

Most home batteries serve both purposes, but which one drives your sizing decision matters significantly.

Sizing for Solar Self-Consumption

The goal here is to store enough surplus solar to cover your evening electricity use (from sunset to midnight, typically the highest-consumption period for most households).

Step 1: Find your average evening electricity use. Check your smart meter data or electricity bill — evening use (5pm–11pm) is typically 30–50% of total daily consumption for most households. If your total daily use is 25kWh, evening use is approximately 7–12kWh.

Step 2: Find your typical solar surplus. This is total daily solar generation minus daytime consumption. If your 8kW system generates 32kWh on an average day and your daytime use is 15kWh, surplus is approximately 17kWh. Without a battery, this surplus is exported to the grid.

Step 3: Your ideal battery for self-consumption is the smaller of: your evening use OR your typical solar surplus. There's no point storing more solar than you can use in the evening, and no point buying a larger battery than your surplus can fill on an average day.

Example: Evening use of 9kWh, daily solar surplus of 14kWh → ideal battery size for self-consumption: approximately 9–10kWh. A 13.5kWh Tesla Powerwall would be partially wasted on typical days; a 10kWh battery would usually fill completely and discharge fully each day — maximising the battery's utilisation and therefore its financial return.

Sizing for Backup Power

Calculate how much energy your critical loads use over your target backup duration.

Identify your critical loads: List the appliances you must keep running during an outage. Common examples: refrigerator (150W average, runs ~8 hours out of 24 = 1.2kWh/day); freezer (200W average, ~8 hours = 1.6kWh/day); LED lighting for key rooms (100W total, 6 hours = 0.6kWh/day); phone/laptop charging (100W, 4 hours = 0.4kWh/day); internet router (20W, 24 hours = 0.48kWh/day); CPAP machine (30W, 8 hours = 0.24kWh/day).

Total critical load example: 1.2 + 1.6 + 0.6 + 0.4 + 0.48 + 0.24 = approximately 4.5kWh per day of critical load.

Choose your target backup duration: Most grid outages last 2–8 hours. Planning for 24 hours covers 95%+ of outage scenarios. For bushfire-prone or severe weather areas, 48–72 hours of backup gives genuine resilience.

Battery size for backup = Critical load (kWh/day) × Target duration (days)

Example: 4.5kWh/day × 1 day = 4.5kWh minimum. A 10kWh battery covers 2+ days of critical loads without any solar recharging.

Add air conditioning if needed: A window AC unit (1,000W) running 8 hours adds 8kWh/day. Central air conditioning adds significantly more. If keeping cool during an outage in a hot climate is a priority, add 8–16kWh to your sizing calculation.

The Battery That Does Both

Most homeowners want a battery that covers both self-consumption and backup. The sizing calculation: take the larger of your self-consumption size and your backup size, and use that as your target.

Example: Self-consumption analysis suggests 9–10kWh. Backup analysis suggests 4.5kWh for one day of critical loads. The self-consumption requirement is larger → size for 10kWh. The same battery that covers your evening solar use will also cover a standard outage.

If you want 3+ days of backup capability, the backup requirement would drive a larger battery than self-consumption alone — in that case, 13–15kWh becomes the right target.

Leading Home Battery Options by Size

5–10kWh range: BYD Battery-Box Premium HVS (5–12.8kWh scalable), Sonnen eco (5–15kWh), Alpha ESS Smile (various). Good for self-consumption focused buyers with moderate backup needs.

10–15kWh range: Tesla Powerwall 3 (13.5kWh) — the benchmark choice for this capacity. Seamless solar integration, best app, strong installer network. Enphase IQ Battery 10T (10.08kWh) — best choice for Enphase microinverter systems.

15kWh+ range: Multiple Tesla Powerwalls; BYD Battery-Box stacked; Sonnen eco 20. For households with high consumption, EVs, or extended backup requirements.

The Financial Sense Check

A home battery is typically not the fastest-payback solar investment — solar panels on their own usually have shorter payback periods. A battery's financial case depends on: the difference between your retail electricity rate and your export/net metering rate (larger gap = better battery economics); your battery cycling frequency (daily cycling = better return than weekly); any battery incentive programs in your area; and whether you participate in a Virtual Power Plant (VPP) program that pays you for grid services.

Before buying the largest battery your budget allows, run the numbers on a correctly-sized battery first. A well-matched battery that cycles daily outperforms an oversized battery that half-cycles.