Solar Battery Storage: Sizing, Chemistry and When It Pays Off

Battery storage is the upsell customers ask about most — and the one installers most often get wrong, either over-selling it or sizing it badly. Here’s how to advise honestly.

Why a customer actually wants a battery

Usually one of two reasons: backup during outages, or storing daytime surplus to use at night. Be clear which goal you’re solving, because it changes the sizing completely.

Lead-acid vs lithium

Lead-acid batteries are cheaper upfront but heavier, shorter-lived and shallower in usable depth. Lithium (LiFePO4) costs more upfront but lasts far longer, allows deeper discharge and needs less maintenance — usually the better lifetime value where budget allows.

Sizing storage to the goal

For backup, size to the essential loads the customer must keep running, and for how long. For self-consumption, size to the evening usage you want to cover with daytime surplus. Oversized batteries sit idle; undersized ones disappoint — match capacity to the real need.

When storage actually pays

With strong net metering, exporting surplus to the grid can be more economical than storing it — so a battery is bought for resilience, not pure savings. Where outages are frequent or export credit is weak, storage makes more financial sense. Tell the customer which case they’re in.

• Backup is a resilience purchase — value peace of mind, not just payback.
• Self-consumption pays best where export credit is low.
• Lithium usually wins on lifetime cost; lead-acid on upfront price.

SolarStack lets you model systems with and without storage so the customer can see the cost and the trade-off clearly before they decide.