Solar Battery Size Calculator
Calculate required battery capacity (kWh) to back up a home for a chosen number of days at a given daily energy usage and battery depth-of-discharge limit. Use it when sizing storage for solar self-consumption, grid-outage backup, or off-grid systems.
Last updated: May 2026
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About this calculator
The calculator sizes a battery to cover a number of days of consumption while staying within the depth-of-discharge (DOD) limit set by the battery chemistry. The formula is: Required Capacity (kWh) = (Daily Usage × Backup Days) / (DOD / 100). Variables: Daily Usage is average daily electricity consumption (US average ~28 kWh/day for whole-home, but backup-essential loads are typically 5-10 kWh/day — refrigerator, lights, internet, occasional cooking); Backup Days is how many days the battery must support consumption (1 for outage ride-through, 2-3 for severe weather events, more for off-grid); Depth of Discharge is the percentage of nameplate capacity you can use without harming battery life (modern lithium-iron-phosphate / LFP batteries: 90-100%; modern lithium-ion: 80-90%; lead-acid: 50% for long life, 80% for occasional use). Edge cases: undersized batteries deep-cycle quickly and need replacement; oversized batteries are mostly empty and waste capital. Modern residential batteries (Tesla Powerwall 13.5 kWh usable, Enphase IQ Battery 10 kWh modular) target single-day backup of essential loads; whole-home extended backup typically requires multiple batteries or generator backup. For solar self-consumption (not backup), battery sizing is different — you want enough to capture daily solar excess, typically 8-15 kWh for a 6-8 kW system. The economics also differ: backup justifies high cost-per-kWh because the alternative is darkness; self-consumption competes against net-metering credits and is harder to justify financially in markets with good 1:1 NEM.
How to use
Example 1 — Whole-home backup, 2-day weather event. Daily usage 25 kWh, 2 days backup, DOD 90% (modern lithium battery). Step 1: required = (25 × 2) / 0.90 = 50 / 0.90 ≈ 55.6 kWh. Verify ✓. This is about 4 Tesla Powerwalls — a $40-50k battery installation. Most homeowners narrow to "essential loads only" backup (fridge, lights, networking, medical devices, maybe partial HVAC) at 5-8 kWh/day, dramatically shrinking battery requirements. Example 2 — Essentials-only backup, 1-day outage. Daily essential usage 8 kWh, 1 day backup, DOD 95% (LFP battery). Step 1: required = (8 × 1) / 0.95 ≈ 8.4 kWh. Verify ✓. A single Enphase IQ Battery 10T (10.5 kWh usable) covers this with margin to spare. Pricing 2025: about $10,000-15,000 installed including the inverter electronics — adds 30-50% to a baseline solar system cost. The economic case depends on outage frequency, grid resilience priorities, and net metering structure in your area.
Frequently asked questions
What's a realistic daily energy usage to assume?
Whole-home consumption varies dramatically: US average is ~30 kWh/day; smaller homes/apartments 10-20; larger homes 40-60; EV-owning homes 50-100; all-electric homes (heat pump heating + EVs + induction) can exceed 100 kWh/day in winter. Essential loads only — what you'd actually want to run during a power outage — typically run 5-10 kWh/day: refrigerator (1-2 kWh/day), lights (1 kWh), networking/devices (1 kWh), water heater (heat-pump 5-10 kWh, gas 0), critical medical devices, occasional cooking, plus partial HVAC (window AC for one room ~2-5 kWh, gas furnace blower ~0.5 kWh). Whole-home AC or electric resistance heating during outages requires much larger battery banks (40-80 kWh) or whole-house generator backup. Always check your actual usage from utility bills; the calculator should reflect your specific situation, not a generic assumption. For backup sizing, decide upfront: essential loads only (single battery affordable) or whole-home (4+ batteries, $40k+).
How do battery chemistry and depth-of-discharge interact?
Different chemistries support different cycling depths without damage. Lead-acid (flooded or sealed AGM): nominally 50% DOD for long life (3-5 years), 80% for occasional use; cycle life drops fast above 50%. Lithium-ion (NMC, the original Tesla Powerwall chemistry): 80-90% DOD with manageable degradation. Lithium iron phosphate (LFP, newer Tesla, Enphase IQ Battery, many DIY systems): 90-100% DOD with minimal degradation, 5000-10000 cycles to 80% capacity. Manufacturer ratings reflect 'usable' capacity that has DOD already factored in: a Tesla Powerwall 3 is rated 13.5 kWh usable, meaning the chemistry actually has more nominal capacity but is limited by software to give consistent life. When sizing, always use the usable rating not nominal. DOD also affects cycle life: a battery cycled to 100% daily lasts shorter than one cycled to 50% daily, but modern LFP batteries are robust enough that for residential backup (not heavy daily cycling) the difference rarely matters in practice.
What are the most common mistakes when sizing batteries?
The biggest is sizing for whole-home backup when essential-loads backup is actually all most people need — this 3-5x oversize wastes $20-30k. Most outages last under 8 hours; even severe weather events average 24-48 hours. The second is forgetting that batteries themselves consume some energy (round-trip efficiency typically 88-95% for modern lithium); a 10 kWh battery delivers ~9 kWh to your loads. The third is not accounting for inverter inefficiency separate from battery efficiency; the inverter system consumes 50-200W continuously even when idle, draining battery slowly during long backup periods. The fourth is comparing battery costs to grid electricity rates on a simple savings basis — backup value is mostly about resilience (cost of being without power for hours/days) not pure financial savings. The fifth is undersizing the inverter relative to the battery — a 13.5 kWh battery feeding a 5 kW inverter cannot run high-load appliances like an EV charger or central AC even if the battery is full.
When should I NOT use this calculator?
Skip it for off-grid systems which require detailed seasonal modeling — winter consumption with little solar production can require 7-10 days of battery autonomy (or backup generator), much more than this simple multiplier captures. Avoid it for solar self-consumption sizing (not backup) where the relevant question is daily excess solar production vs evening consumption, not number of days of full coverage; for self-consumption, target battery size = daily excess production at peak season (usually 8-15 kWh). Do not use it for utility-scale or commercial battery storage where dispatch economics, demand-charge management, and ancillary services all matter beyond simple kWh sizing. Skip it for short-duration backup applications (small UPS for computers/networking) where the relevant unit is amp-hours and runtime in minutes, not days. And do not rely on it without separate inverter sizing — the battery may have enough energy but the inverter must support the load's peak power draw.
How do battery economics compare to grid electricity?
Pure financial battery payback is harder than solar payback. A $12,000 battery storing $0.30/kWh peak shifted from $0.10/kWh off-peak saves roughly $0.20 × 8 kWh × 365 days = $584/year — about a 20-year payback on the battery alone, longer than its 10-15 year warranty. Battery economics improve with: (1) larger peak-vs-off-peak rate spread (California, Hawaii excel); (2) net metering disruption (post-NEM-3 California makes self-consumption far more valuable than exports); (3) frequent grid outages where the resilience value of having backup is substantial; (4) wildfire shutoff areas where multi-day outages are common; (5) critical loads (medical equipment, home offices, work-from-home) where outage cost is high. For pure financial returns, batteries rarely beat grid electricity alone. The case for adding batteries is mostly: (a) financial benefit in post-NEM markets, plus (b) resilience and outage protection, plus (c) clean-energy alignment for owners who value 24/7 renewable backing. The federal ITC (30%) helps significantly — without it most residential batteries would not be economic; with it they're marginal for most US homes.