☀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 after 30% federal credit—

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?

Yes. The 30% Residential Clean Energy Credit applies to off-grid solar installations on your primary or secondary residence, including the battery storage component (≥3 kWh). RVs typically don't qualify unless used as a primary residence.

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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