A battery moves available energy to another time. It deserves assessment when regular solar surplus coincides with later demand that storage can serve. If almost all generation is already consumed during daylight, the additional financial value may be limited.
Three different objectives
| Objective | Required measurements |
|---|---|
| Higher self-consumption | Daytime surplus and demand after sunset |
| Backup | Critical-load power and outage duration |
| Peak management | Interval power data and applicable demand charges |
These objectives can coexist but require different control settings. Energy reserved for a possible outage is not always available for everyday savings. Make this trade-off explicit in the design brief.
Follow the energy balance
In a hypothetical day with 8 kWh of surplus, 90% round-trip efficiency leaves up to 7.2 kWh for later use before other constraints. If evening demand is only 4 kWh, a larger battery does not automatically create additional useful consumption. If winter surplus is 2 kWh, a large capacity may remain underused.
The 90% figure is an example input, not a specification for any particular product. Actual efficiency depends on measurement boundaries, temperature, operating power and auxiliary consumption. Model seasonal conditions, not just one sunny day.
Evaluate incremental benefit
Compare the extra investment with additional avoided electricity purchases, subtracting the export value forgone by storing surplus. Include losses, warranted energy throughput where specified, ageing and possible replacement. The payback of the complete PV installation is different from the incremental payback of its battery.
A useful comparison keeps panel capacity and demand identical in both scenarios. Otherwise changes attributed to storage may actually come from a different array or load assumption.
Storage is not automatically backup
A grid-connected battery does not guarantee power during an outage. Backup requires a compatible inverter, designed grid isolation, protection and often a separate critical-load board. Check transfer time as well: a backup output is not a substitute for every UPS application.
Published installations and next steps
The 10 kW plus 10 kWh Anissaras installation and 215 kWh industrial storage in Heraklion illustrate different application scales. No unpublished payback is attributed to either project.
For a battery assessment, provide measured generation and demand, the existing inverter model, required autonomy and load priorities. Compare with and without storage using the same underlying consumption profile.
Before selecting capacity
First measure how much solar energy is normally surplus on operating days and how much electricity is used after sunset. An oversized battery that seldom charges fully ties up capital without equivalent use. An undersized unit may frequently hit its power or energy limit. This comparison needs interval data, not annual kWh alone.
Ask separately whether the aim is evening energy shifting, outage backup or supplying critical loads for a specified duration. Each objective leads to different wiring, inverter, protection and evaluation. See also the battery-sizing guide.
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