Off-Grid System Designer

How This Tool Works

The Off-Grid System Designer sizes a complete stand-alone power system: solar array, battery bank, inverter, and backup generator. Unlike grid-tied solar where the utility acts as a virtual battery, off-grid systems must produce 100% of your energy on the worst day of the worst month. That makes the sizing math fundamentally different — you're not averaging across a year, you're surviving the week where it rained for four days straight in December.

This calculator uses conservative design rules: solar sized for 120% of daily usage at your worst-month sun hours, battery sized for the number of autonomy days you specify (typically 2–3), inverter sized to start your largest motor (well pumps and AC compressors need 3–6× surge), and a generator backup if you want winter reliability without doubling the battery bank.

The cost estimate is rough — within ±30% of a real installed quote. Off-grid systems cost 2–4× more per kWh than grid-tied because of the battery oversizing required. A 25 kWh/day off-grid system typically costs $40,000–$70,000. The same daily usage on grid-tied solar with a small backup battery would be $25,000–$35,000. The premium buys true independence — no utility bill, no outage risk, no net metering policy exposure.

If you're going off-grid by choice (rural property, cabin, homestead), this tool gives you the system size. If you're considering off-grid because of frequent outages, consider whether a battery-only backup on a grid-tied system might serve you better at half the cost.

1. Daily kWh = your real daily consumption. Off-grid homes typically run 10–30 kWh/day because they're more efficient by necessity.
2. Peak sun hours = the WORST month average, not annual. Off-grid must work in December, not July. NREL maps give monthly data.
3. Autonomy days = how many cloudy days you need to survive on battery alone. 2 is typical, 3 is conservative for cloudy climates, 1 is acceptable if you have a generator.
4. Depth of discharge = how much of the battery you actually use. LFP: 80%. Lead-acid: 50% (don't push it). The calculator sizes nominal capacity based on this.
5. Largest motor = the biggest surge load. Well pumps (1–2 HP), AC compressors (3–5 ton), table saws all draw 3–6× running watts for 1–3 seconds at startup.

The 4000W inverter minimum is non-negotiable for whole-home — anything less won't start a fridge compressor or well pump. If your motor wattage is high, the inverter must be 2× that.

When to Use This Calculator

Why off-grid costs 2–4× grid-tied

Grid-tied solar can be sized for annual production — excess in summer is sold to the grid, deficit in winter is bought back. The grid is a free, infinite, 100%-efficient battery. Off-grid has no such luxury. You must size the battery to bridge multi-day cloudy stretches, which means 3–5× the daily usage in nominal capacity. A 25 kWh/day off-grid system needs 50–75 kWh of battery. At $600/kWh installed, that's $30,000–$45,000 in battery alone — more than the entire grid-tied system.

The autonomy days tradeoff

Autonomy is how many days you can go without sun. Each additional day adds about 25 kWh of battery capacity (at 25 kWh/day usage) — $15,000 more. Most off-grid designs settle on 2–3 days because beyond that, a generator backup is cheaper. A $4,000 generator + 20 gallons of propane covers the rare 5-day storm better than $30,000 of extra battery.

Motor starting surges

Induction motors (well pumps, AC compressors, table saws, air compressors) draw 3–6× their running wattage for the first 1–3 seconds as they spin up. A 1 HP well pump rated at 750W running can pull 4,500W at startup. If your inverter can't deliver that surge, the motor stalls or the inverter trips off. Off-grid inverters must be sized for the largest motor's surge, not the average load. Soft-start kits on AC compressors cut surge by 60% and are worth installing.

Winter is the design constraint

Off-grid systems fail in winter, not summer. December in the US Northeast gives 2.5 peak sun hours vs 6.5 in July. December loads are also higher (more lighting, more heating). The system must be sized for December or it will fail every year. Most off-grid homes supplement with a generator in winter — solar handles 80% of annual energy, generator covers the December shortfall.

LFP vs lead-acid for off-grid

LFP (lithium iron phosphate) batteries cost 2× upfront but last 4× longer (6,000 cycles vs 1,500) and allow 80% DoD vs 50%. Over a 15-year horizon LFP is roughly half the cost per kWh delivered. Lead-acid only makes sense for cabins used seasonally or budget-constrained setups where you accept replacing the bank every 5 years.

Frequently Asked Questions

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Further Reading

Deep-dive articles and guides related to this calculator.