From watt-hours to amp-hours for battery banks
How daily load, depth of discharge, efficiency and voltage combine to set battery bank size, and why the amp-hour number changes with voltage.
Start with an honest daily load
Every sizing calculation rests on the daily load, the total energy your equipment consumes in a day measured in watt-hours. Add up each device as its power in watts multiplied by the hours it runs, so a 60 watt fridge running the equivalent of 8 hours contributes 480 Wh. Be generous rather than optimistic, because an underestimated load is the most common reason off-grid banks fall short in winter. This single number drives everything downstream.
Why the raw bank is bigger than the load
You never build a bank equal to the daily load. Two derations push the size up. Depth of discharge reflects how deeply you can cycle the chemistry without shortening its life, so a lead-acid bank held to 0.5 must be twice the usable energy you need. Round-trip efficiency accounts for energy lost while charging and while the inverter converts DC to AC. The formula divides the load by both factors, which is why 4000 Wh at 0.5 depth and 0.85 efficiency becomes 9411.76 Wh of raw storage.
Turning watt-hours into amp-hours
Battery capacity is usually sold in amp-hours at a nominal voltage, so the tool divides the required watt-hours by your system voltage. The same 9411.76 Wh is 784.31 Ah on a 12 volt bank but only 196.08 Ah on a 48 volt bank, because higher voltage carries the same energy at lower current. Higher voltage also allows thinner, cheaper cable and smaller losses, which is why larger systems tend to run at 48 volts. Pick the voltage first, then read the amp-hour target it implies.
Adding days of autonomy
Days of autonomy set how long the bank alone must carry your loads with no charging, covering cloudy stretches or a quiet generator. Each day multiplies the requirement, so planning for two days doubles the bank while three days triples it. More autonomy buys resilience but adds cost and weight quickly, so most homes settle on one to three days and lean on a backup charging source for longer outages. Balance the comfort of extra days against the price of the extra cells.