Solar panels only charge when the sun is out. So an off-grid system needs enough stored energy to ride through the times it isn't — nights, and especially long cloudy stretches. Days of autonomy is how you put a number on that safety margin: how many full days the battery bank can carry your loads with no solar input at all.
How the sizing works
Start with the energy you use in a day and multiply by the number of autonomy days you want — that's the usable energy the bank must deliver. But you can't drain a battery all the way, so you divide by the usable depth of discharge to get the total capacity you actually need to install. Finally, divide the total kWh by your system voltage (and multiply by 1000) to express it in amp-hours. For example, 3 kWh a day over 3 days is 9 kWh of usable energy; at 80% depth of discharge that's about 11.25 kWh of total battery, or roughly 234 Ah at 48 V.
How many days should you choose?
Most off-grid homes aim for 2–3 days. In sunny, dependable climates you might design for 1–2; in cloudy regions or for critical loads, 3–5 is safer. Each extra day multiplies the battery cost, so it's a trade-off between reliability and budget — the chart above shows how quickly the bank grows.
Related tools
Work out your daily use with the daily load calculator, convert between kWh and Ah with the battery Ah to kWh converter, size from daily load with the battery calculator, and design the whole system with the off-grid system wizard.
Frequently asked questions
What does days of autonomy mean in solar?
It's how many days your battery bank can power your loads with no solar input — your buffer for cloudy weather. Three days of autonomy means three days on stored energy alone.
How many days of autonomy do I need?
Usually 2–3 days for off-grid homes; 1–2 in sunny climates, 3–5 for cloudy regions or critical loads. More days means a bigger, pricier bank.
How do I calculate battery bank size for days of autonomy?
Daily use × autonomy days = usable energy; divide by depth of discharge for total capacity; divide total kWh by voltage × 1000 for amp-hours. This tool does it all.