How much battery storage does a house need for autonomy?

The correct answer to the question "how much battery does a house need" does not come from its square footage or a general rule. It comes from when you consume energy, which appliances you want to power during an outage, and how much autonomy you actually expect from your photovoltaic system. A small battery will drain early. An excessively large one will tie up capital without corresponding financial benefit.

For a modern house with photovoltaics, storage is not chosen to "contain" all daily consumption. It is chosen to transfer solar production to the hours you need it most: in the afternoon, evening, early morning, or during a grid outage. This difference is what determines whether you need 5, 10, 15, or more kWh.

How much battery does a house actually need?

In most homes, a LiFePO4 battery with a nominal capacity of 5 to 10 kWh is a reasonable starting point for increasing self-consumption. It covers part of evening use, such as lighting, refrigerator, TV, router, small appliances, and selected kitchen loads.

For a family with increased evening consumption, an electric water heater, heat pump, air conditioners, or electric vehicle charging, we usually consider solutions of 10 to 20 kWh. In fully autonomous homes, holiday homes with an unstable grid, or installations that must operate for days without grid support, the required capacity can exceed 20 or 30 kWh.

These numbers are not an offer or a final study. They are indicative sizes. Two houses with the same monthly consumption may need completely different batteries: one consumes mainly at noon, when the photovoltaics are producing, while the other consumes after sunset.

Capacity in kWh is not power in kW

The most common confusion concerns the difference between kWh and kW. Battery capacity, in kWh, indicates how much energy it can store. Power, in kW, indicates how many appliances the system can power simultaneously.

A 10 kWh battery may be energetically sufficient for an evening but may not serve a house if the inverter does not have the necessary power. For example, oven, hobs, water heater, and air conditioners can create high instantaneous peaks. In an installation with backup, the inverter and battery must be sized together, not as two independent purchases.

The technical study considers both daily energy and the simultaneous power of critical loads. This is particularly important when the goal is the operation of the house during a power outage and not just the reduction of energy purchased from the grid.

The calculation starts with consumption after sunset

The electricity bill is useful, but not enough on its own. It shows how much energy was consumed overall, not at what hours. For proper battery selection, we need to know how many kWh the house consumes from the moment photovoltaic production decreases until the next morning.

If, for example, evening consumption is 6 kWh, it is not enough to install a battery of exactly 6 kWh. There are conversion losses in the inverter, discharge limits, and a percentage of capacity retained for battery protection. With a LiFePO4 battery and a suitable hybrid inverter, a nominal capacity of approximately 8 kWh may be more realistic to cover these 6 kWh.

A simple initial formula is as follows:

Required nominal capacity = daily energy you want to cover × days of autonomy / depth of discharge / system efficiency

If you want to cover 10 kWh, with a useful discharge of 90% and a total efficiency of 92%, the theoretical minimum capacity is approximately 12.1 kWh. In practice, the final choice may move to the next available battery size, to allow for operational margin and future expansion possibilities.

Days of autonomy: the point that changes the whole study

In a hybrid system connected to the grid, it is often sufficient for the battery to cover the evening hours or critical loads for a few hours. The grid acts as a backup, and photovoltaic production recharges the battery the next day.

In an autonomous house, however, the logic changes. You need storage for periods of low sunlight, especially in winter. A daily consumption of 12 kWh with a target of two days of autonomy theoretically requires over 30 kWh of nominal storage, depending on discharge limits and losses. At the same time, sufficient photovoltaic power is required to properly recharge the battery during available daylight hours.

That is why full energy autonomy is not simply "installing a larger battery." It is a combination of consumption, photovoltaic power, load profiles, inverter, and energy management strategy.

Which loads do you want to operate during a power outage?

The need for backup is different from the need for savings. Before choosing capacity, decide whether you want to support the entire house or only a panel of critical loads. In the latter case, the investment often becomes more efficient and autonomy greater.

Critical loads usually include refrigerator, freezer, lighting, internet, alarm, work outlets, heating circulator, and selected air conditioners. In contrast, water heaters, electric ovens, hobs, dryers, and large resistors can quickly deplete available energy.

This does not mean that these loads are always excluded. In a larger system with a powerful inverter, they can operate, but they significantly increase the required budget and overall complexity. The right choice is based on the house's priorities, not on the promise that everything will operate simultaneously without limits.

When a larger battery is really worth it

Greater capacity is valuable when there is available surplus solar production that would otherwise not be utilized, when consumption shifts to evening hours, or when uninterrupted operation is essentially important. A house with a heat pump, electric car, and high evening needs can utilize a 15 kWh battery much better than a small apartment that consumes very little after 10:00 PM.

Conversely, oversizing can delay payback. A battery that remains half-full for a large part of the year does not offer the same financial result as a properly selected unit that charges and discharges systematically. Scalability is often a smarter choice: you start with the capacity you need today and anticipate the possibility of adding modules later.

LiFePO4: the technology suitable for residential applications

LiFePO4 batteries have become established in modern residential installations due to their long lifespan, high safety, and ability to utilize their capacity more deeply. However, not all batteries are the same. Compatibility with the inverter, BMS communication, warranty, parallel expansion capability, and technical support are factors that directly affect the reliability of the project.

Choosing premium equipment is not just about the manufacturer's name. It's about consistent operation over time, proper charge and discharge management, and the availability of technical support when needed. In an investment that will operate daily for years, compatibility and careful study are worth more than a superficial comparison based only on the price per kWh.

The right battery is part of an integrated system

Hellenic Energy designs each solution based on real consumption data, the available photovoltaic field, the required autonomy, and the loads that need to be protected. This avoids both insufficient capacity and costly oversizing.

The most useful first step is to record evening consumption and identify which appliances are essential in the event of an outage. From there, a free technical study can turn the estimate into a system that substantially reduces electricity costs and provides the autonomy your home truly needs.