Off-Grid Solar Calculator 2026 — Panels & Battery Size

Understanding your results

Short answer: A cabin using 3 kWh a day with 5 peak sun hours needs about 0.9 kW of panels and a 10 kWh LFP battery bank for 3 days of autonomy — roughly $10,700 at this calculator’s cost assumptions. A home using 10 kWh a day needs about 3.1 kW of panels and 33 kWh of storage, roughly $36,000. Homeowner-owned off-grid systems installed in 2026 get no federal tax credit.

Off-grid sizing logic: The solar array is sized as daily kWh ÷ (peak sun hours × 0.80 system efficiency) × 1.25, a 25% buffer for cloudy days and seasonal variation. The battery bank is daily kWh × autonomy days ÷ usable depth of discharge. The inverter is sized conservatively at 2 kW per daily kWh to allow for motor start-up surges; many homes can use a smaller inverter matched to their actual peak load. The charge controller is sized for a 48 V system with a 25% margin.

Cost assumptions: $2,500 per kW of panels (including mounting and wiring), $600 per kWh of battery, and $400 per kW of inverter. These are planning figures, not quotes; remote sites, ground mounts and generator integration add cost.

Why battery chemistry matters so much off-grid: LFP batteries tolerate roughly 90% depth of discharge versus 50% for AGM lead-acid, so an LFP bank needs about 40–45% less nominal capacity for the same usable storage. LFP also lasts 3,000–6,000 cycles (8–15 years of daily cycling) versus a few hundred to about 1,000 cycles for AGM. Despite a higher upfront price, LFP is usually cheaper over the life of a permanent system.

Generator sizing: A backup generator lets you avoid oversizing panels and batteries for the worst week of winter. Example: recharging a 30 kWh bank from 20% to 80% means replacing about 18 kWh; a 4,000 W generator running a charger at about 3.5 kW does that in roughly 5 hours.

Tax credits and incentives in 2026: Until Dec 31, 2025, homeowners could claim 30% of an off-grid system’s cost under §25D; the One Big Beautiful Bill Act (Public Law 119-21, signed July 4, 2025) ended that credit for systems installed after that date, so owned off-grid homes and cabins installed in 2026 get no federal credit (IRS). Business, farm or rental-property systems may still qualify for the §48E investment credit — typically 30% for qualifying projects — if construction begins by Jul 4, 2026 or the system is placed in service by Dec 31, 2027, subject to foreign-entity-of-concern (FEOC) rules; businesses can also use 5-year MACRS depreciation. Tax-exempt organizations can use elective (direct) pay for §48E within the same deadlines. Farms and rural small businesses can apply for USDA REAP grants when funding rounds are open. Enter any grant or rebate you expect in the calculator.