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Off-grid battery: how many days of autonomy do I need?

“Days of autonomy” is how many days your off-grid system can run with little or no solar before the battery is flat. There is no single right number: it depends on your loads, your winter sun and whether you have a generator. We design for the worst week, not the best day.

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What autonomy means

Industry guidance defines autonomy as the number of days the system can operate without energy from a generator, before the battery exceeds its design depth of discharge. Government guidance says the battery should get you through most days of the year with only rare need for a back-up generator, and that systems should be designed for periods of low solar generation.

The sum

Battery energy you can use (kWh) divided by your daily use (kWh) gives days of autonomy. As an illustration only: 30 kWh usable and 10 kWh a day is about three days with no solar at all. Real days of cloud still bring some solar, so the true figure is usually better, but a long wet week in the hills is what we plan around.

What changes the answer

  • Your loads. Water pumps, fridges and freezers, workshops and air conditioning. Cutting or shifting big loads is the cheapest way to add autonomy.
  • Winter sun and shading. Panels are sized for winter, not summer, and tall trees and valley sides matter more off-grid than anywhere.
  • A generator. A back-up generator that starts automatically and tops up the battery covers the longest stretches, so the battery does not have to be sized for every possible week.
  • Battery type and condition. Modern lithium batteries replace ageing lead-acid ones in many older hinterland systems.

Sigenergy’s manual makes the same point for backup systems: how long the loads run depends on the supply capacity of the solar and storage. Read more about how we design and maintain these: off-grid solar and batteries in the Northern Rivers hinterland, and off-grid solar systems.

Why locals choose us

  • Our own in-house team designs every system and does every install. 25 years in solar and large commercial electrical work. No subcontractors.
  • A face-to-face site visit before every install: compliant battery position, switchboard and RCBO check, no surprises on the day.
  • SAA-accredited installer and Sigenergy-approved, with a 25-year workmanship warranty on our installation work.

Questions we get asked

How many days of battery do I need off-grid?

There is no fixed number. It depends on your loads, winter sun, and whether you have a generator. We design from your real use and for the worst week.

Do I need a generator?

Often yes for the longest wet spells. It lets us size the battery sensibly rather than for every possible week.

Can I reduce what I need by changing how I use power?

Yes. Shifting big loads to sunny hours and trimming standby loads reduces the battery you need.

Do off-grid batteries get the federal rebate?

Off-grid solar panels can earn STCs and off-grid batteries can be eligible for the federal battery rebate. We check eligibility for your system.

Sources

Last checked 7 October 2026. Only the manufacturer’s, the network’s or the government’s own documents are used; app menus and defaults change between models and versions, so if your screen looks different, call us rather than guess.

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