Calculate the optimal battery capacity for a solar-plus-storage system. Size your battery bank based on nighttime usage, backup needs, and depth of discharge.
Adding battery storage to a solar system enables energy independence, backup power during outages, and maximized self-consumption of solar energy. But sizing the battery correctly is critical — too small and you still rely heavily on the grid, too large and you overspend on capacity you'll never use.
Battery sizing depends on your nighttime energy consumption (when solar isn't producing), desired backup hours during an outage, and the battery's depth of discharge (DoD). Modern lithium batteries can discharge to 80–90% of their rated capacity, while older lead-acid batteries should only be discharged to 50%.
This calculator determines the total battery capacity you need based on your nightly energy consumption, desired backup duration, and the DoD of your chosen battery technology. The result tells you the minimum rated capacity to purchase.
Understanding this metric in precise terms allows energy managers to evaluate investment options, forecast savings, and build compelling business cases for efficiency upgrades and retrofits.
Battery storage is the most expensive component of a solar-plus-storage system. Proper sizing ensures you get the reliability you need without overspending on excess capacity. Regular monitoring of this value helps energy teams detect usage anomalies early and address equipment malfunctions or operational issues before they drive utility costs higher.
Nightly Storage = Nightly kWh / DoD Backup Storage = Backup Hours × Hourly Load / DoD Recommended Capacity = max(Nightly Storage, Backup Storage)
Result: 18.75 kWh total battery capacity
Nightly storage: 12 / 0.80 = 15.0 kWh rated. Backup storage: 10 × 1.5 / 0.80 = 18.75 kWh rated. The backup scenario requires more capacity, so 18.75 kWh is the recommended minimum — roughly equivalent to 1.4 Tesla Powerwalls.
Grid-tied batteries typically cover nighttime consumption and outage backup. Off-grid batteries must handle all non-solar hours plus multiple days of autonomy. Grid-tied systems need 10–30 kWh; off-grid may need 40–100+ kWh.
In TOU rate areas, batteries charge from solar during cheap midday hours and discharge during expensive evening peaks. This arbitrage can save $30–60/month beyond what net metering provides alone.
Lithium iron phosphate (LFP) batteries offer the best cycle life and safety for stationary storage. Nickel-manganese-cobalt (NMC) batteries are lighter and more energy-dense but have shorter cycle lives. Lead-acid is cheapest upfront but has the worst lifetime economics.
Divide your required capacity by 13.5 kWh (Powerwall usable capacity). A home needing 18-20 kWh of storage needs two Powerwalls. For whole-home backup during outages, you may need 2–3 depending on your consumption patterns.
Depth of discharge is the percentage of total battery capacity that can be used. A 20 kWh battery with 80% DoD provides 16 kWh of usable energy. Discharging beyond the rated DoD shortens battery life significantly.
Not necessarily. Grid-tied solar without batteries uses net metering for credit. Batteries add value if you have time-of-use rates, want outage backup, or your utility has reduced net metering credits. Batteries are not required for basic solar savings.
Modern lithium solar batteries are warrantied for 10–15 years or a set number of cycles (typically 4,000–6,000). Most retain 60–80% of original capacity at warranty end. Actual lifespan depends on cycling depth, temperature, and usage patterns.
It depends on battery capacity and your consumption. A single Powerwall can power essential loads (lights, fridge, router, charging) for 12–24 hours. Powering everything, including HVAC and cooking, requires 2–4 batteries or load management.
Lead-acid batteries cost less per kWh upfront but have 50% DoD limits, shorter lifespans (5–7 years), and lower round-trip efficiency (80–85% vs 95% for lithium). For daily solar cycling, lithium is more cost-effective over the system lifetime.