How big a battery do you really need?
An oversized battery is the most common mistake we see in systems installed by others. It costs a lot and is used little.
In short
- The battery is sized on the evening consumption, not the daily figure.
- Target: a full charge-discharge cycle almost every day.
- A battery that stays half full most of the year is capital that is not working.
- Backup during outages is a separate requirement and can justify a larger capacity.
We start from evening consumption
In a grid-connected system the battery has one clear role: to move energy from the hours you produce to the hours you consume. So the right question is not how much energy do I use in a day, but how much do I use from sunset to sunrise.
In a typical home, evening consumption is usually 40-55% of the daily total: lighting, cooking, television, washing machines, the fridge, air conditioning or a heat pump. That figure, increased for conversion losses and for the usable depth of discharge, gives the target capacity.
The rule we apply
Usable capacity is roughly the evening consumption of an average day
And no larger, unless there is a backup requirement or a genuine lack of grid. At the same time, the solar system must be able to fill that capacity after covering daily consumption – otherwise the battery never reaches 100%.
In practice, a home battery that charges and discharges a full cycle almost every day of the year is correctly sized. If the monitoring app shows the state of charge almost never dropping below 60%, you paid for capacity you are not using.
When a larger capacity is justified
- Backup: if you want the house to work during a power cut, the sizing changes – we calculate the critical loads and the hours of autonomy you ask for.
- Grid instability: in areas with frequent outages or voltage dips, the battery also acts as equipment protection.
- Electric car: changes the consumption profile dramatically – and it is usually better to charge directly from solar, without going through the battery.
- Zero Feed-In: in zero-export systems storage is practically essential, because otherwise the inverter curtails production.
Technology: why LiFePO4
We use lithium iron phosphate cells (LiFePO4) almost exclusively. They offer a significantly longer cycle life than other lithium chemistries, better thermal behaviour – critical in a Greek summer – and allow deep discharge without losing capacity prematurely. The cost per stored kWh over the service life is the number that matters, not the purchase price per kWh of capacity.
Equally important is the BMS and its compatibility with the inverter. A system where the battery and inverter communicate properly manages charging far better and protects the cells. See the service solar with batteries for the equipment we install.
To determine the right capacity in your case we need 12 months of bills and a short conversation about your habits. The an energy study does the rest.
Let us work out the right capacity for you
No oversizing and no surprises on the bill.
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