Sizing a home battery is not about buying the biggest box you can afford. It is about matching two independent numbers: how much energy you need (kWh) and how much power you need at any instant (kW). Get either wrong and the system disappoints.
The two numbers that matter
- Capacity (kWh) — how long the battery can run your loads. This is the "fuel tank".
- Power rating (kW) — how many loads it can run at the same time. This is the "engine".
A 10kWh battery with a 3kW inverter cannot run a 5kW air conditioner, no matter how much capacity is left. A 5kW inverter with a 5kWh battery will run the air conditioner beautifully — for about an hour.
Step 1: Read your electricity bill
Find your average daily consumption in kWh. Divide the monthly total by 30. Typical figures:
| Home profile | Daily consumption | Typical battery size |
|---|---|---|
| Apartment / small home | 8–12 kWh/day | 5 kWh |
| Standard family home | 15–25 kWh/day | 10 kWh |
| Large home with A/C | 30–50 kWh/day | 15–20 kWh |
| Home with EV charging | 40–70 kWh/day | 20 kWh+ or dedicated charger |
Step 2: Decide what you are solving for
Three different goals produce three very different sizes:
- Solar self-consumption — store daytime surplus for evening. Size to roughly 50–70% of daily consumption.
- Backup during outages — size from your critical load list and the outage duration you want to survive.
- Time-of-use arbitrage — charge off-peak, discharge at peak. Size from your evening peak-window consumption.
Step 3: Build a critical load list
List what must stay on during an outage, with wattage and hours. Wattage is usually on the appliance label.
| Load | Typical power | 8 hours overnight |
|---|---|---|
| Lighting (LED, whole home) | 100–200 W | 0.8–1.6 kWh |
| Refrigerator / freezer | 150–400 W (cycling) | 1.2–2.5 kWh |
| Router, modem, IT | 30–80 W | 0.3–0.6 kWh |
| TV + media | 100–250 W | 0.8–2.0 kWh |
| Well pump | 700–1,500 W | 1.0–2.0 kWh (intermittent) |
| Air conditioner (12,000 BTU) | 1,000–1,500 W | 8–12 kWh |
| Electric stove / oven | 2,000–3,500 W | Only if cooking during outage |
Step 4: Convert to usable capacity
Battery capacity is quoted as nominal, but you should not plan to use all of it. With LiFePO4 at a recommended depth of discharge below 80%:
Usable kWh = nominal kWh × 0.8 × inverter efficiency (typically 0.92–0.95)
So a 10kWh LiFePO4 bank delivers roughly 7.4–7.6 kWh of usable AC energy. Working backwards: if your overnight critical loads total 7kWh, you need about a 10kWh nominal bank.
Worked example
A family home uses 20 kWh/day and wants backup for lighting, refrigeration, internet, a few sockets and one bedroom A/C through an 8-hour night.
- Lighting 0.4 kWh + fridge 1.6 kWh + IT 0.5 kWh + sockets 1.0 kWh + A/C 8.0 kWh = 11.5 kWh
- Divide by 0.8 depth of discharge → 14.4 kWh nominal
- Divide by 0.93 inverter efficiency → ~15.5 kWh nominal battery
- Simultaneous peak (fridge + A/C + lighting + sockets) ≈ 2.5 kW, with A/C start surge ≈ 4 kW → 5 kW hybrid inverter
That points to a ~15kWh stackable LiFePO4 system with a 5kW hybrid inverter — comfortably inside the configurable range of our home energy storage system.
Match the solar array as well
A battery that never fully charges is wasted money. As a rough guide, you need about 1–1.5 kW of solar per 5 kWh of battery in a sunny climate, and 2–2.5 kW per 5 kWh in a cloudy or high-latitude one. In winter, output can drop to a third of summer, so size for the worst month you actually care about.
Five sizing mistakes we see repeatedly
- Sizing on nominal kWh and forgetting depth of discharge — the system feels 25% smaller than expected.
- Ignoring surge current — the inverter trips when the pump or A/C starts.
- Oversizing for solar self-consumption — the top of the battery never cycles, so that capacity never earns anything.
- No critical-loads sub-panel — the battery tries to run the whole house and empties in two hours.
- Forgetting future loads — an EV or a heat pump arrives in year two and the system is suddenly undersized. Choose stackable capacity where you can.
Not sure which size fits your project? Send us your last three electricity bills and your critical load list via the inquiry form and we will size it for you.
FAQ
Most family homes land between 10kWh and 15kWh. Homes that only want evening solar self-consumption often sit at 5–10kWh; homes backing up air conditioning usually need 15kWh or more.
With about 7.5kWh of usable energy, a 10kWh LiFePO4 system runs essential loads (lighting, refrigeration, internet, sockets) for roughly 8–12 hours. Adding air conditioning typically cuts that to 4–6 hours.
Yes, if you choose stackable or modular LiFePO4 modules from the start. Plan the inverter power rating for your final capacity, since that is the part that is expensive to change later.