How this is calculated
Every number above comes from one chain of division, run in order. Start with your daily energy use in kWh — from a load audit or a utility bill, not a nameplate-wattage guess — and multiply by the days of autonomy you want to survive without meaningful solar input. That gives the energy your loads need to receive; it isn’t yet the energy the battery has to hold, because getting power from a battery to a load always costs something along the way.
Dividing by round-trip efficiency accounts for that loss — a charge controller and inverter pairing typically runs 88–95% efficient, so the battery has to put out more than the loads actually receive. Dividing again by usable depth of discharge does the same job for the battery itself: a LiFePO4 pack rated for 80% usable DoD only lets you draw 80% of its nameplate capacity before the BMS cuts it off to protect cycle life, so the nameplate (nominal) kWh you buy always has to be bigger than the usable kWh you actually need.
Amp-hours are just kilowatt-hours converted at your system voltage — the same nameplate capacity needs roughly four times the amp-hours at 12V that it needs at 48V, which is why higher-voltage banks run thinner, cheaper cabling for the same power. The suggested string rounds up to whole 100Ah modules, the most common off-grid LiFePO4 building block at any of the three standard voltages; real products range from about 50Ah to 280Ah per unit, so treat the module count as a starting point for a quote, not a parts list.
- The cost line uses BloombergNEF’s December 2025 pack-price survey (global average lithium pack price) — a raw cell/pack benchmark, not a retail or installed quote: ≈$108/kWh in the US edition, ≈€100/kWh in the Europe edition (approx. FX-adjusted). Assembled drop-in units with a BMS, enclosure and monitoring typically run 2–3x more per kWh.
- Depth of discharge and round-trip efficiency vary by manufacturer and by charge-controller/inverter pairing — check the datasheet for the specific products you’re pricing.
- This is a planning number, not a load calculation or an electrical design — confirm wiring, fusing and disconnects against your local code before buying.
For the chemistry-by-chemistry breakdown — LiFePO4 vs lead-acid vs Powerwall-class systems, cycle life, and a quarterly-updated $/kWh table — read the off-grid battery bank sizing guide.