How to Size a Home Battery: kWh, Loads, and Backup Days Explained

The short answer

To size a home battery, take your daily electricity consumption in kWh, multiply it by the number of days you want the battery to cover without solar input, then divide by the battery's usable depth of discharge — typically 0.8 for LiFePO4. For a home using 15 kWh per day that wants one full day of backup: 15 × 1 ÷ 0.8 = 18.75 kWh. You would install two 10.24 kWh units in parallel, or a single larger system if available.

That formula works, but the real answer is more useful. Sizing a battery isn't a single calculation — it depends on what you are trying to do: self-consumption, backup, or both. The right number changes with your solar array size, your load profile, and how flexible you are willing to be during an outage.

What are you sizing it for?

Most residential battery installations serve one or more of these goals. Each pulls the sizing calculation in a different direction.

Goal 1: Solar self-consumption

You have rooftop PV. During the day, the panels produce more than the house consumes. Without a battery, the surplus goes to the grid — often at a feed-in tariff far below the retail electricity price. A battery stores the excess and discharges it in the evening.

Sizing rule of thumb: match the battery capacity to your daily excess solar generation. If your 5 kWp array exports 12–16 kWh on a good summer day and you import 8–10 kWh overnight, an 8–10 kWh battery captures most of the usable surplus without leaving capacity idle.

Goal 2: Backup power

You want lights, refrigeration, internet, and perhaps a heat pump circulating pump to stay on during grid outages. This is about covering a defined set of loads for a defined duration.

Sizing method: list the essential circuits, estimate their daily consumption, multiply by the number of backup days you need.

A household protecting only critical loads — fridge, lights, internet, boiler — might need 3–5 kWh for a 24-hour outage. Adding a heat pump changes the numbers dramatically: a modern air-source unit can draw 2–4 kW in cold weather, consuming 15–25 kWh over a winter day. That is the difference between one battery module and a full stack.

Goal 3: Both (the most common case)

Most homeowners want daily self-consumption with some backup reserve. A common approach: size for self-consumption first, then add 20–30% headroom for backup reserve — or accept that during an outage, non-essential circuits get turned off.

The formula, with real numbers

Here is the calculation step by step, using a typical European household as the example.

Step 1: Find your daily consumption. Check your electricity bill or smart-meter portal. A 3–4 person household in Germany typically uses 10–18 kWh per day (3,500–6,500 kWh/year). A smaller 1–2 person household uses 5–10 kWh/day.

Step 2: Decide backup duration. One full day is common; off-grid or weak-grid installations may need 2–3 days.

Step 3: Account for depth of discharge. LiFePO4 batteries are typically rated to 80% DoD for their cycle-life claims. A 10 kWh battery delivers 8 kWh of usable energy per cycle. Do not size on the nameplate number — size on usable kWh.

Step 4: Apply the formula.

Required battery capacity (kWh) = daily consumption (kWh) × backup days ÷ 0.8

These are starting points. Every house is different. The installer who walks through the house and measures actual circuit loads will always produce a more accurate number than a formula.

Why 80% DoD matters more than the headline capacity

A battery's nameplate capacity and its usable capacity are not the same thing. Most LiFePO4 home batteries specify cycle life at 80% depth of discharge — meaning the manufacturer guarantees the cycle count (e.g., 6,000 cycles) when you cycle between 100% and 20% state of charge, not 100% to 0%.

This is not a weakness. It is how the chemistry works. Discharging a LiFePO4 cell below its minimum voltage shortens its life, so the battery management system (BMS) will cut off before that happens regardless. The practical takeaway: if you need 10 kWh of usable energy, buy 12.5 kWh of nameplate capacity. Round up.

How capacity maps to the Storage Wall series

Senneon's Storage Wall series offers four capacity tiers in a wall-mounted form factor. Each unit is a self-contained 48 V or 51.2 V LiFePO4 module with its own BMS, and multiple units can be connected in parallel for larger systems.

All four use the same LiFePO4 chemistry, the same CAN/RS485 communication protocol for inverter compatibility, and the same wall-mount chassis design. The SEN-W48050 adds an IP65-rated enclosure for sheltered outdoor installation.

Parallel operation supports stacking multiple units behind a single inverter — up to 16 units depending on the model — so a system can start small and grow as household consumption or budget allows.

Four things that change the sizing calculation

1. Your inverter's battery input limit

If your hybrid inverter has a maximum battery charge/discharge rate of 5 kW, stacking 30 kWh of battery behind it won't let you pull 30 kW during an outage. The inverter remains the bottleneck. Size battery and inverter together, not separately.

2. Winter solar production

A 5 kWp array that produces 25 kWh on a July day in central Europe might produce 5–8 kWh on a grey December day. If you size the battery for summer self-consumption, you will still draw from the grid in winter — the battery simply cannot store what the panels do not generate. This is expected and not a sizing failure.

3. Load management during an outage

During a grid outage, most households reduce consumption: they turn off the dishwasher, delay laundry, and avoid electric heating where possible. A battery sized for "normal day" consumption suddenly becomes a "two-day" battery when loads are managed. Build this assumption into the sizing conversation with the homeowner.

4. Future load growth

An electric vehicle charger, a heat pump retrofit, or a home office that wasn't there before can shift daily consumption by 5–15 kWh. A modular battery system that accepts parallel expansion lets the homeowner add capacity later rather than over-buying upfront.

What not to do

• Don't size on average daily consumption alone. Peaks matter. A family that averages 12 kWh/day might spike to 20 kWh on a laundry-and-cooking Sunday. The battery should cover the peak day, not the average.
• Don't forget inverter idle consumption. A typical hybrid inverter draws 30–60 W continuously. Over 24 hours, that is 0.7–1.4 kWh of overhead — roughly 10% of a 10 kWh battery. Subtract it from usable capacity.
• Don't mix nameplate and usable kWh. Always work in usable kWh after DoD. Confusing the two is the most common sizing mistake we see.

When to bring in an installer

A formula gets you to a reasonable estimate. A site visit gets you to an accurate number. An installer will measure actual circuit loads, check the existing electrical panel for space and rating, confirm the inverter's battery input limits, and assess the wall space and environmental conditions for mounting. If you are an installer reading this, the four-capacity Storage Wall series gives you a single platform to cover everything from a 2.5 kWh apartment retrofit to a 40+ kWh multi-unit stack — with one set of mounting hardware, one communication protocol, and one supplier to deal with.

View the Storage Wall series or contact us for a quote and datasheet.