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Days of autonomy is the number of days your battery bank can run your loads with no solar charging at all. Most systems use 1 day if there is a backup generator or grid, 2 days for typical off-grid homes in sunny climates, and 3 or more days in cloudy climates or for critical loads without a generator.
Solar battery bank calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
Why it matters
Your solar panels refill the battery every sunny day. On a run of heavily overcast days, production can fall to 10 to 25% of normal. Days of autonomy decides whether your lights stay on through that stretch.
It also directly multiplies battery size and cost:
Battery Ah = Daily Wh × Days of autonomy ÷ (Voltage × Depth of discharge)
How battery size grows
For a home using 5 kWh per day on a 48 V LiFePO4 bank (85% DoD):
| Days of autonomy | Battery capacity | Approximate energy |
|---|---|---|
| 1 | 123 Ah | 5.9 kWh |
| 2 | 245 Ah | 11.8 kWh |
| 3 | 368 Ah | 17.6 kWh |
| 5 | 613 Ah | 29.4 kWh |
Every extra day adds another full day of storage. Beyond three days, a backup generator is almost always cheaper than more batteries.
Choosing your number
| Situation | Recommended days |
|---|---|
| Grid-tied home with backup battery | 0.5 – 1 (essential loads only) |
| RV or van with alternator charging | 1 – 2 |
| Off-grid cabin with generator | 1 – 2 |
| Full-time off-grid, sunny climate (desert, Mediterranean) | 2 |
| Full-time off-grid, cloudy climate (UK, Pacific Northwest, Great Lakes) | 3 + generator |
| Critical loads (medical, telecom, remote monitoring) | 3 – 5 |
Panels also need to catch up
After a cloudy spell, your array must cover today’s use and recharge the battery. If panels only just cover daily use, an empty battery never fully refills. Designers often add 10 to 25% extra array capacity in off-grid systems so the battery recovers within a few sunny days.
Lead-acid needs extra caution
Lead-acid batteries suffer if they stay partially charged for days, and their 50% DoD limit means you need double the nameplate capacity. LiFePO4 tolerates partial state of charge well. See LiFePO4 vs AGM and depth of discharge.
Reduce autonomy needs instead of buying batteries
- Cut non-essential loads: on cloudy days, run only essentials (fridge, lights, internet, pump) and skip the washing machine.
- Generator: a small inverter generator for the few worst days of the year. See generator size for off-grid.
- Better winter harvest: steeper tilt, snow clearing and slightly more panels. See winter off-grid solar.
- Efficient appliances: every kWh you save per day saves autonomy days × that kWh of battery.
Our solar calculator has a “days of autonomy” selector (1, 2 or 3 days) that resizes the battery bank instantly.
Worked example: choosing between 2 and 3 days
A full-time off-grid home uses 6 kWh per day on a 48 V LiFePO4 bank.
| Option | Battery capacity | Usable energy | What it covers |
|---|---|---|---|
| 2 days | 6,000 × 2 ÷ (48 × 0.85) = 294 Ah | 12 kWh | Two fully dark days |
| 3 days | 441 Ah | 18 kWh | Three fully dark days |
| 2 days + generator | 294 Ah + 3–5 kW generator | 12 kWh + fuel | Unlimited, with a few generator hours on long storms |
Real cloudy days still produce 10 to 25% of normal solar, so a 2-day bank often lasts 3 calendar days in practice. If the home has a generator, the 2-day option with generator backup is usually the best value.
Days of autonomy by battery chemistry
| Daily use 5 kWh, 48 V bank | LiFePO4 (85% DoD) | AGM (50% DoD) |
|---|---|---|
| 1 day | 123 Ah (5.9 kWh) | 208 Ah (10 kWh) |
| 2 days | 245 Ah (11.8 kWh) | 417 Ah (20 kWh) |
| 3 days | 368 Ah (17.6 kWh) | 625 Ah (30 kWh) |
Three days of autonomy with AGM means about 30 kWh of nameplate batteries, which weighs well over a tonne. That is a big reason modern off-grid systems use lithium plus a generator rather than huge lead-acid banks.
How weather data can guide your choice
- Look up how many consecutive overcast days your area typically sees in the worst month (local weather history or solar resource tools help).
- Note how much solar you still get on those days (often 10 to 25% of normal).
- Size autonomy to cover a typical bad spell, not the worst one in 20 years.
- Plan the generator to cover anything beyond that.
Recovery: how quickly can the battery refill?
After two cloudy days, your array must cover today’s use and refill two days of battery. With 5 kWh per day and a 2.5 kW array at 4.5 sun hours (9 kWh per day):
- Surplus per sunny day: 9 − 5 = 4 kWh
- To refill 10 kWh: about 2.5 sunny days
If you want faster recovery, add panels, which are usually cheaper than more batteries.
Common mistakes
- Forgetting that every extra day costs another day of batteries.
- Using lead-acid DoD as 100%, halving real autonomy.
- No generator in a cloudy climate with only 1 to 2 days of storage.
- Never testing: run a “no-solar” day deliberately and check what your monitor shows.
Days of autonomy for RVs and vans
Mobile systems usually need less autonomy, because you can move to sun, drive to charge with the alternator, or plug in at a campground. One to two days of storage is typical, and a DC-DC charger is often cheaper than a bigger battery.
FAQ
Is one day of autonomy enough?
For RVs, grid-tied backup and off-grid cabins with a generator, yes. For full-time off-grid without a generator, it is too little.
Do days of autonomy count only night-time use?
No. It assumes a full day with no solar, so it covers your entire daily consumption, day and night.
Does autonomy change the inverter size?
No. Inverter size depends on peak power, not stored energy. See what size inverter do I need.
How many days of battery backup do I need for off-grid?
Two days is a common choice in sunny climates; three days plus a generator in cloudy ones.
Can a bigger solar array replace days of autonomy?
Partly. More panels help on lightly cloudy days and speed recovery, but on very dark days even a large array produces little.
Related guides
- How Many Batteries Do I Need for Solar? (Sizing Formula + Examples)
- Solar Inverter vs UPS Battery Backup: Which Is Better for Power Outages?
- What Size Solar Panel Do I Need for Camping?
- Solar Generator vs DIY Solar System: Which Should You Choose?
Sources and further reading
- Global Solar Atlas (World Bank): sun data and solar potential for any location.
- European Commission JRC: PVGIS: free solar output and optimum-tilt tool for Europe and most of the world.
- Battery University: BU-205 Types of Lithium-ion: how lithium chemistries, including LiFePO4, compare.
- U.S. Department of Energy: Solar PV System Design Basics: how panels, inverters, controllers and batteries fit together.