Stop guessing. Stop wasting money. This is how you size batteries for Australian off-grid life. No theory. Just what works.
Stop guessing. Stop wasting money. This is how you size batteries for Australian off-grid life. No theory. Just what works.
Four-step flow for sizing an off-grid battery bank from daily loads to final kWh result.
Depth of Discharge (DoD): The Dealbreaker
DoD is how much you can actually use before killing your battery. Don't ignore it. Ever.
Lead-acid (AGM/Gel): Max 50% DoD. Use 50% of capacity, then recharge. 100Ah battery = 50Ah usable.
Lithium (LiFePO4): 80-90% DoD. 100Ah battery = 80-90Ah usable. That's why they cost more.
Example: You need 200Ah usable. With lead-acid: 200Ah / 0.5 = 400Ah battery. With lithium: 200Ah / 0.8 = 250Ah battery. Lithium wins on cost-per-use.
Temperature Correction: Your Battery Dies in Winter
Australia's cold snaps wreck battery performance. Calculator must adjust for this.
Rule of thumb: At 0°C, lead-acid loses 30% capacity. At -10°C, it's 50% loss. Lithium handles cold better but still needs correction.
Practical tip: Always add 20% buffer for winter. If your calculator doesn't do temp correction, it's useless. Use Victron's free calculator (they've got it right).
How to Size Your Battery Bank (Step-by-Step)
Do this. No exceptions.
Calculate daily energy use (Wh): Add up all your appliances. 100W fridge x 24h = 2400Wh. 50W lights x 4h = 200Wh. Total: 2600Wh.
Factor in days of autonomy: How many cloudy days can you survive? 2 days = 2600Wh x 2 = 5200Wh.
Enerdrive (Lead-acid): 200Ah AGM. $850-$950. Cheap but you'll replace it every 3-4 years. Only use if you're on a tight budget and in a warm climate.
Battery World (All types): Price match on lithium. 100Ah LiFePO4: $1,100-$1,300. Check your local store. They'll rip you off if you don't know the price.
Final Tip: Don't Be a Dumbass
Most people oversize by 20% to be safe. Do it. Your battery bank is the heart of your off-grid system. If it's wrong, you're living like a caveman with a solar panel.
Use the Victron battery calculator (free, Australian-friendly). It handles DoD and temp correction. Stop using random online tools. They're garbage.
⚠️ SAFETY WARNING: Working with electrical systems, structural modifications, or gas installations carries inherent risks. If you are not confident in your abilities, always engage a licensed professional.
It walks through the daily Ah calculations and battery bank voltage choices, directly applying the same sizing logic my calculator uses. He also points out why under‑sizing the bank leads to frequent cycling, which I see a lot in central QLD installs.— Dave Miller
When to Call a Professional
While many off-grid projects are achievable as DIY, certain situations require licensed professionals:
Electrical work beyond basic 12V DC additions — requires a licensed electrician
Structural modifications to buildings or load-bearing elements
Gas line installation or modification
Solar array installations above safe voltage thresholds
Any work that affects the structural integrity of your property
Always check local regulations and obtain necessary permits before commencing work.
