☀SolarSavingsMath

Off-Grid Solar System Calculator

Sizing for full off-grid living: cabin, tiny home, RV, or homestead. Accounts for worst-case winter production + battery autonomy + climate derating — the math grid-tied calculators get wrong.

For worst-case winter sun hours (typically 60-70% of annual average).
Off-grid use is dramatically lower than typical grid homes (~30 kWh/day US avg).
How many cloudy/no-sun days you can run on battery alone.
Most off-grid setups include a small generator as last-resort backup.
Typically December. AZ winter ~5 hr, MN winter ~2 hr.
Off-grid systems average 0.65 due to MPPT + battery losses (vs 0.80 grid-tied).
Off-grid installs run $0.30-$0.50/W premium over grid-tied (custom design).
2026 LFP avg $850-$1,100/kWh.
LFP usable 90-100%, lead-acid 50%.
Cold reduces effective capacity 20-30%.
Pick your state + usage to size your off-grid system.
Daily energy need—
Solar system size (sized for winter)—
Panel count (400W each)—
Battery capacity needed—
Solar cost—
Battery cost—
Total system cost (gross)—
Net cost (no federal credit for 2026+ installs)—

Why off-grid sizing is fundamentally different

Grid-tied solar systems can be sized for annual average production because the grid covers any shortfall. Off-grid systems have no fallback — if you under-size for winter, you sit in the dark in December. Three things change the math:

1. Size for the worst month, not the average

December typically produces 50-70% of June's output in most US locations. Anchorage drops to 25%. Phoenix only to 80%. Off-grid systems must be sized for the December minimum, not the annual average — which is why off-grid systems are 30-60% larger than equivalent grid-tied systems.

2. Battery autonomy days

You need enough battery to run through multi-day cloudy stretches without sunlight to recharge. The standard is 3 days of autonomy (handles most weather), with 5+ days for true wilderness or extreme climates. Multi-day cloudy weather is more common in winter, when production is already lowest.

3. System efficiency drops to ~0.65

Off-grid systems use MPPT charge controllers (lose ~5%), batteries (charge/discharge loses ~10-15%), and inverters (lose ~5-10%). The combined effective efficiency is about 0.65, vs 0.80 for grid-tied. You need more panels to deliver the same kWh to your home.

The off-grid sizing formula

Example: 5 kWh/day cabin in Montana

FAQ

How big does an off-grid solar system need to be?

Off-grid systems are sized 30-60% larger than grid-tied for the same usage because they must produce enough during the worst month AND charge a battery bank with 2-5 days of autonomy. A typical off-grid cabin (5 kWh/day) needs 1.5-3 kW of solar + 12-25 kWh of battery.

How many batteries do I need for off-grid solar?

Battery sizing = daily kWh usage × autonomy days × climate derating ÷ depth of discharge. For 5 kWh/day with 3-day autonomy and LFP batteries: ~17-22 kWh.

Do off-grid solar systems qualify for the federal tax credit?

Not anymore for new systems. The 30% Residential Clean Energy Credit covered off-grid solar and batteries on a residence, but it ended for installations completed after December 31, 2025. Systems completed by then can still claim it on a 2025 return. Details →

Should I use lead-acid or lithium (LFP) batteries?

LFP (lithium iron phosphate) is the modern standard — 90-100% usable depth of discharge, 6,000+ cycle life, no maintenance, no off-gassing. Lead-acid is cheaper upfront but only 50% usable, 1,000-2,000 cycle life, requires venting. LFP costs more per kWh but less per kWh of usable life.

Do I still need a backup generator?

Most off-grid setups include a small propane or gas generator (~5 kW) for emergencies — multi-week cloudy weather, equipment failures, or unexpected high usage. Generator cost: $1,000-$3,000 installed. It runs maybe 20-40 hours per year in a well-sized system.

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