Solar energy Storage backup battery
Quick answer: Solar energy storage and backup batteries are LiFePO4 packs, typically 48V or 51.2V, specified from the loads they must carry and the hours of autonomy required. The hardware is the same whether the pack is cycled daily or held in reserve — what changes is the design emphasis: cycle life for daily storage, low self-discharge and reliable float behaviour for backup duty.
Storage duty and backup duty are not the same design
Both applications use the same cells and largely the same BMS, but the stress they impose is different, and buying for the wrong one is a common and expensive mistake. A daily-cycled storage bank is retired by cycle count. A backup bank that sits at full charge for months at a time is retired by calendar aging at high state of charge — and by the self-discharge that quietly reduces what it can deliver when it is finally called on.
| Daily storage duty | Standby backup duty | |
|---|---|---|
| Cycle pattern | One or more full cycles per day | Rare, deep discharges |
| Retired by | Cycle count | Calendar aging and self-discharge |
| Design emphasis | Cycle life, round-trip efficiency | Float behaviour, low self-discharge, standby reliability |
| Sizing basis | Daily energy shifted | Hours of autonomy required at full load |
| Charging | Daily solar absorption cycle | Maintained at full charge by charger or inverter |
Sizing from loads, not from a catalogue
| Load | Typical running watts | Hours needed | Energy |
|---|---|---|---|
| Refrigerator / freezer | 150–400W | 24h (duty-cycled) | ≈2.5–4kWh/day |
| LED lighting | 5–15W per room | 5h | ≈0.3–0.6kWh |
| Internet, router, comms | 20–50W | 24h | ≈0.5–1.2kWh |
| Well pump | 700–1,500W | 1–2h | ≈1–3kWh |
| Air conditioning | 1,000–3,500W | 4–8h | ≈4–20kWh |
| Medical equipment (CPAP etc.) | 30–60W | 8h | ≈0.3–0.5kWh |
Add the loads you actually want to keep running, multiply by the hours you need them, divide by the usable depth of discharge (about 90% for LiFePO4), then add roughly 20% headroom for inverter surge and for capacity fade over the warranty period. See our sizing calculator for the full worked method.
LiFePO4 versus lead-acid for solar storage
| LiFePO4 | Lead-acid (AGM/GEL) | |
|---|---|---|
| Usable depth of discharge | 90–95% | 50% recommended |
| Cycle life | 2,000–5,000 | 500–1,500 |
| Replacements over 15 years | 0 | 2–3 |
| Round-trip efficiency | ≈95% | 80–85% |
| Maintenance | None | Ventilation and periodic checks |
| Indoor installation | Yes — no gassing | Needs a vented enclosure |
| Weight per usable kWh | ≈1/3 of lead-acid | Heavy; floor loading matters |
The comparison changes completely once usable depth of discharge and replacements are counted. Lead-acid looks cheaper per nameplate kWh, but you buy twice the nameplate capacity for the same usable energy — see LiFePO4 vs lead-acid for the full cost argument.
What to specify
| Specification | Typical value | Why it matters |
|---|---|---|
| Chemistry | LiFePO4 (LFP) | Safe indoors, long calendar life, no gassing |
| Nominal voltage | 48V / 51.2V | Matches mainstream hybrid inverter battery inputs |
| Usable capacity | 90–95% of nameplate | More of the capacity you paid for is available |
| Cycle life | 6,000+ cycles @ 80% DoD | Quoted at a defined condition |
| Inverter link | CAN / RS485 | Compatibility with the inverter’s battery profile |
| Enclosure | IP20 indoor / IP65 outdoor | Matches the installation location |
| Certifications | UN38.3, IEC 62619, UL 9540A, CE | Inspection- and import-ready |
Adding storage to an existing array
The retrofit route depends on what inverter is already installed. A hybrid inverter usually has a battery input and the work is to match the bank voltage and the communication protocol. A grid-tied string inverter has no battery input, so the choice is an AC-coupled battery system on the house side or replacing the inverter — and for a large existing array the second option often costs less than the first over its lifetime.
Backup configuration: the part people miss
A battery alone does not provide backup power. During a grid outage a grid-tied inverter must shut down to avoid energising the network while line crews are working. Backup requires a designated backup or EPS output and an automatic transfer switch so the critical-loads panel is isolated from the grid and served from the battery. If backup is a requirement, it has to be designed into the inverter selection — it cannot be added afterwards.
Certifications and shipping
Every unit ships with its transport and safety documentation. UN38.3 covers transport; IEC 62619 covers cell and battery safety for stationary use; UL 1973 and UL 9540A are the North American system-level expectations; CE covers the EU declaration. We supply the test data an inspector or utility needs.
See certifications and compliance for the documents we hold, and battery certifications explained for what each one means.
Solar storage battery FAQ
What is the difference between an energy storage battery and a backup battery?
Functionally none in hardware terms — the difference is the duty. An energy storage battery is cycled daily to shift solar generation into the evening. A backup battery sits at full charge and is discharged only during an outage. The second case wants a different design emphasis: lower self-discharge, reliable float behaviour and confidence that the pack will deliver after months of inactivity.
Is LiFePO4 the right chemistry for solar storage?
For most stationary solar and backup applications, yes. LiFePO4 offers 2,000 to 5,000 cycles, tolerates deep discharge without the damage lead-acid suffers, needs no ventilation, and is the most thermally stable mainstream lithium chemistry. Its lower energy density relative to NMC does not matter in a stationary installation.
How do I size a solar backup battery?
From the loads you need to keep running and for how long, not from a round number. List the essential loads, multiply their watts by the hours of backup required, divide by the usable depth of discharge, and add 20% headroom for inverter surge and aging.
Can I add a battery to an existing solar array?
Yes. If the array already has a hybrid inverter, it usually has a battery input and you match the bank voltage and communication protocol. If it is a grid-tied string inverter, you either add an AC-coupled battery system or change the inverter — the second option usually being cheaper for a significant retrofit.
Will the battery work during a blackout?
Only if the system is configured for backup. A grid-tied inverter shuts down during an outage for safety unless it has a designated backup or EPS output with an automatic transfer switch. Tell us you need blackout protection and the transfer path is specified with the inverter.
How long will a solar storage battery last?
CMX Battery LiFePO4 units are rated 6,000+ cycles at 80% depth of discharge. At one cycle per day that is roughly sixteen years, quoted against a defined cycle and temperature condition rather than a bare number of years.
What certifications do I need for import?
UN38.3 for transport is mandatory everywhere. EU buyers expect IEC 62619 and CE; US projects look for UL 1973 and UL 9540A at system level. The full test file ships with every unit.
Can the system be configured off-grid?
Yes. Off-grid configurations size the battery for several days of autonomy without solar input rather than for a single overnight, so the bank is substantially larger and the inverter has to be rated for the full peak load with no grid to fall back on.
Explore related pages
- Residential energy storage
- Whole home battery backup
- Off-grid solar systems
- LiFePO4 vs lead-acid
- Battery sizing calculator
- Cost per kWh
Request a solar storage quote
Tell us your daily kWh, backup hours, inverter make and model and installation market — we return a matched battery and transfer configuration within three working days.
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