Solar Battery Calculator – Size & Runtime for Off-Grid Systems

Depth of Discharge represents how much of your battery's capacity you use before recharging. Using higher DoD than recommended will significantly reduce battery lifespan. Lithium batteries can safely discharge to 80–90%, while lead-acid batteries should be limited to 50% to prevent sulfation and permanent damage.

Our calculations provide estimates based on ideal conditions. Real-world performance can vary by ±15–25% due to temperature, battery age, actual inverter efficiency, and load variations. Cold temperatures and older batteries will reduce actual runtime.

Lithium batteries cost 2–3× more upfront but last 10–15 years with 3000–5000+ cycles versus 3–5 years and 500–1000 cycles for lead-acid. When you factor in replacement costs, lithium batteries typically have 20–40% lower total cost of ownership over 10 years.

Temperature significantly impacts battery performance. At 0°C, lead-acid batteries lose about 20% capacity, while lithium loses 10–15%. At −20°C, capacity can drop by 50% or more. Always size your battery bank considering your coldest operating temperatures.

Series connections increase voltage (12V + 12V = 24V) while maintaining the same amp-hour capacity. Parallel connections maintain voltage but add capacity (100Ah + 100Ah = 200Ah). For most off-grid systems, parallel 12V batteries or series-parallel combinations work best.

Add more batteries if you need extended runtime during cloudy periods or overnight. Add more solar panels if your batteries regularly discharge too deeply or don't fully recharge during the day. A balanced system should fully recharge batteries on average sunny days with 1–2 days of battery backup.

Inverters consume power just by being switched on — typically 10–50W — even with no load attached. This idle draw is called tare loss and is a common cause of unexpected overnight battery drain. Always factor it into your runtime calculations.