Solar + Storage Sizing Calculator
Sizing solar and storage together starts with two different questions: how much energy do you need to generate each day, and how much do you need to store for backup? This calculator takes your average daily electricity usage, desired backup duration, critical load percentage, peak sun hours, and system derate factor, then reports the critical backup load, the required battery usable capacity, and the required solar array size in kW DC. To estimate the sunlight resource that drives the solar array, see the Solar Irradiance Calculator, and for the generation figure the array sizing builds on, see the Solar Panel Output Calculator. For a deeper dive on the battery side alone, see the Home Battery Sizing Calculator.
A typical U.S. household uses roughly 900-1,000 kWh/month, or about 30 kWh/day.
How many hours you want the battery to cover during a grid outage.
Most residential backup systems target essential circuits (refrigerator, some outlets, well pump) rather than the entire home -- backing up 100% of load roughly doubles the required battery size.
See the Solar Irradiance Calculator to estimate this for your specific location.
Accounts for inverter losses, wiring, soiling, and temperature effects, consistent with the site's Solar Panel Output Calculator default.
average daily electricity usage (kWh) × (critical load percentage (%) ÷ 100)
(critical backup load (kWh) ÷ 24) × desired backup duration (hours)
(average daily electricity usage (kWh) ÷ peak sun hours) ÷ (system derate factor (%) ÷ 100)
Results update live as you type. For planning and field-check estimates — always verify against applicable standards and equipment ratings.
How we calculate this →Sizing solar and storage together starts with two different questions: how much energy do you need to generate each day, and how much do you need to store for backup? A household using 30 kWh/day needs roughly a 7.3 kW solar array to cover full daily consumption -- but only a 7.5 kWh battery to back up half the home's critical loads for 12 hours. Backing up the whole home instead of just critical circuits would roughly double the required battery size, which is why most residential backup systems target essential loads rather than whole-home backup.
How solar + storage sizing is calculated
This calculator estimates the critical backup load, the required battery usable capacity, and the required solar array size, tying five inputs together: the average daily electricity usage, the desired backup duration, the critical load percentage, the peak sun hours, and the system derate factor. Three quantities tie the calculation together.
Critical Backup Load (kWh) = Average Daily Electricity Usage (kWh) × (Critical Load Percentage (%) ÷ 100). The critical backup load is the portion of daily household consumption that essential circuits draw -- the load the battery must be sized to sustain during an outage. Applying the critical load percentage to total daily usage isolates that essential share. At the defaults (30 kWh/day, 50% critical load), that is 30 × (50 ÷ 100) = 30 × 0.50 = 15 kWh.
Required Battery Usable Capacity (kWh) = (Critical Backup Load (kWh) ÷ 24) × Desired Backup Duration (hours). Converting the daily critical load to an hourly draw (dividing by 24 hours) and multiplying by the desired backup duration gives the usable energy the battery must deliver to cover critical loads through the outage. At the defaults (15 kWh critical load, 12-hour backup), that is (15 ÷ 24) × 12 = 0.625 × 12 = 7.5 kWh.
Required Solar Array Size (kW DC) = (Average Daily Electricity Usage (kWh) ÷ Peak Sun Hours) ÷ (System Derate Factor (%) ÷ 100). Dividing daily usage by peak sun hours gives the raw DC array size needed to generate that energy over a sun-equivalent day, then dividing by the derate factor (expressed as a decimal) inflates the array to account for real-world losses from inverter efficiency, wiring, soiling, and temperature. At the defaults (30 kWh/day, 5.0 peak sun hours, 82% derate), that is (30 ÷ 5.0) ÷ (82 ÷ 100) = 6 ÷ 0.82 = 7.32 kW DC.
Two notes on the model. First, solar array size and battery size answer different questions and do not need to be proportionally matched -- the array is driven by daily energy consumption goals, while the battery is driven by backup duration and critical load needs, though many installers consider both together for overall system design. Second, this is a planning-level approximation: real battery sizing also accounts for depth-of-discharge limits, round-trip efficiency, and surge/peak loads on critical circuits, and real solar sizing accounts for seasonal sun-hour variation, shading, and orientation -- all typically refined with specialized modeling software for actual project design. Data sources: Solar array sizing methodology from NREL PV system design standards and solar engineering handbooks; battery usable capacity and backup duration calculation from residential battery storage design standards; critical load percentage and whole-home vs. critical-load backup comparison from residential solar + storage system design practices; peak sun hours and system derate factor from solar irradiance measurement standards and PVsyst documentation; solar + storage system design and sizing from residential solar + storage case studies and industry best practices. Verification: with defaults (30 kWh/day, 12-hour backup, 50% critical load, 5.0 peak sun hours, 82% derate), Critical Backup Load = 15 kWh, Required Battery Usable Capacity = 7.5 kWh, Required Solar Array Size = 7.32 kW DC.