Off-Grid Solar Battery Bank Calculator: How Many Ah Needed?
A load that never switches off has to be replaced in full, every day, by an array that only works in daylight. Enter the continuous rack draw and this returns a first-pass PV array wattage, a 48 V LiFePO4 bank size in kWh and amp-hours, and the daily energy the system has to move.
What the calculator assumes
These are first-pass figures, not a bill of materials. Every constant below is a stated assumption you should replace with the real number from your own datasheets and site data before ordering anything.
- The load runs 24 hours a day. Daily energy is the entered wattage multiplied by 24. If the rack idles lower overnight, meter it and enter the true average rather than the peak.
- System round-trip efficiency of 90% is applied to the battery bank, covering charge and discharge losses and inverter conversion. A system with a long DC run or an older inverter will do worse.
- A composite derate of 0.83 is applied to array output, standing in for soiling, wiring loss, module mismatch and cell temperature. Hot roofs and dusty sites justify a lower figure. Array sizing applies the 90% round-trip figure as well, because the array has to replace what the load takes back out of the battery.
- A 48 V nominal bank at 51.2 V is assumed for the amp-hour conversion, which is the resting voltage of a 16-cell LiFePO4 string. A 12 V or 24 V bank needs four times or twice the amp-hours for the same energy.
- Peak sun hours are the worst-month figure, not the annual mean. The dropdown offers three broad bands spanning 3.5 to 5.5, which is already a 1.6x swing in array wattage for an unchanged load. Look up your actual coordinates in the Global Solar Atlas or PVGIS before committing, because a genuinely bad high-latitude December can fall to 2 sun hours and push the array past double what the 4.5 default returns.
- Depth of discharge is a longevity choice. 80% is the conservative default for LiFePO4 cycle life; 90% buys capacity at the cost of cycles.
What it deliberately does not size
Three components decide whether the system is safe and whether it survives winter, and none of them can be derived from these four inputs: the charge controller's rated charge current and maximum PV input voltage, the inverter's continuous and surge rating, and the DC overcurrent protection between the bank and everything else. Array open-circuit voltage on a freezing morning is the constraint that destroys controllers, and it depends on your string layout and your local record cold.
Guides that go with this
- Sizing an off-grid solar system for always-on homelab loads - the order of operations, from measured load to inverter.
- Solar panels for a homelab: how many do you need? - worst-month peak sun hours, derate factors and worked panel counts.
- MPPT vs PWM charge controllers for homelab solar - harvest, module compatibility and LiFePO4 charge profiles.
- Solar batteries not charging: how to diagnose it - the diagnostic order when the bank stops accepting charge.