How big a battery — and can it carry the load?

Energy and power are separate problems, so this checks them separately. No chemistry limit is ever applied without you choosing it.

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How big a battery?

Energy and power are separate problems. A bank can hold plenty of kilowatt-hours and still refuse to start your pump.

1 · What it has to supply

From the backup load calculator, or your own figure.
Used for the runtime estimate.

2 · What your battery can actually give back

Nothing is assumed here. Depth of discharge varies enormously between chemistries — running a lead-acid bank to a lithium bank's depth will ruin it in months. Pick a chemistry to load a labelled example, then replace it with the figure from your own battery's datasheet.
Insurance for an outage longer than planned.
100% means no derate. Cold cuts usable capacity sharply.

3 · The battery you are considering

4 · Putting it back

Good sun hours, not daylight hours.

The two checks, made separately

Working

Runtime at different loads

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Your data stays in this browser. Everything on this page is calculated on your own device. Your electricity use, roof dimensions, prices and equipment details are never uploaded, stored on a server, or included in analytics.
Planning estimate — not an engineering or installation plan. Results depend on the information and assumptions you enter. Output, savings and equipment behaviour vary with weather, shading, tariffs, temperature, ageing and installation. Verify equipment limits against current manufacturer documentation. Permits, structural capacity, electrical protection, conductor sizing, grounding, rapid shutdown, fire access, utility interconnection and code compliance must be reviewed by qualified professionals and the applicable authorities.
How this is calculated

requiredUsable = dailyEnergy × daysOfAutonomy

requiredNominal = requiredUsable ÷ (1 − reserve) ÷ (depthOfDischarge × dischargeEfficiency × temperatureDerate)

Module count rounds up — you cannot buy two thirds of a battery.

The power checks are separate and neither substitutes for the other. Continuous output must cover the running load; surge rating must cover the largest start. A bank that fails either one will not do the job however many kilowatt-hours it holds, and when power rather than energy sets the module count the tool says so explicitly.

runtime = usableEnergy × inverterEfficiency ÷ averageLoad, shown as a band because temperature, age, the real load pattern and the inverter's cutoff all move it — nearly always downwards.

Why reserve costs more than it looks

Holding 20% back is not 20% more battery. The reserve comes off the usable share, which was already reduced by depth of discharge and efficiency, so the effects multiply. Working through a typical example: 5 kWh a day, 90% depth of discharge, 95% discharge efficiency and a 20% reserve needs roughly 7.3 kWh of nominal capacity, not 5.5 kWh. That is the honest number, and it is why banks always end up larger than the first estimate.

The recharge has to balance

A battery that supplies 5 kWh a day needs 5 kWh a day put back into it, and the charging window is shorter than people assume — good generating sun is a few hours either side of noon, not the whole of daylight. If the arithmetic does not close, the bank runs down a little further each day until it is flat, and no amount of extra storage fixes it. That check is run above and flagged as an error rather than a note, because it is the failure mode that actually strands people.

Limitations and safety

This sizes capacity. It says nothing about wiring, fusing, disconnects, ventilation, mounting, temperature management or interconnection, all of which are governed by regulations and manufacturer requirements that vary by location and by product. Lithium batteries in particular have specific installation and clearance requirements. Have the design reviewed by a qualified installer before buying, and treat the battery's own documentation as the authority over anything here.