Off-grid solar power systems: what one actually takes

Last reviewed 20 August 2026 · sizing math uses the same model as the calculator

The one difference that drives everything

Grid-tied solar plays a game where every kilowatt-hour you overproduce in July is banked by the utility and returned in January — the grid is a free, infinite, 100%-efficient battery. Off-grid solar has no such account. Every watt you use at night must have been generated that day and stored in batteries you paid for, and every cloudy week must be survivable on stored power, a generator, or conservation. That single difference — storage is yours — is why off-grid costs land where they do.

Sizing an off-grid array honestly

Start from daily energy, not from roof area: a typical US home uses somewhere between 6,000 and 15,000 kWh a year, so 16–41 kWh a day. Array size then follows your location's peak sun hours at the same performance ratio (0.8) the calculator uses:

kW needed = daily kWh ÷ (sun hours × 0.8)

The table shows what that means at the portfolio's sun extremes — Arizona at 6.5 peak sun hours versus Washington at 3.5 — with battery banks sized for two dark days at 80% usable capacity:

Two honest notes on that table. Winter sun is materially worse than the annual average — a system that must survive December without a generator wants another 25–50% of array. And the array cost column is only the panels: batteries at current prices add roughly as much again for the bank sizes shown, plus the inverter, charge controller, and installation.

The four components, and what each one is really for

• Panels. Same hardware as grid-tied; only the sizing discipline changes. More panel is often cheaper than more battery — running the generator less beats storing more.
• Battery bank. Almost always lithium iron phosphate (LFP) today: thousands of cycles, 80–90% usable capacity, no venting. Bank size = daily kWh × dark-days ÷ 0.8. This is the line item that decides whether the project is a cabin system or a house system.
• Off-grid inverter/charger. Not the same part as a grid-tied inverter: it must make the whole house's AC from nothing, start motors, and ideally charge the bank from a generator. Sizing to the household's simultaneous peak load — not daily kWh — is what people get wrong.
• Charge controller and backup generator. The controller sits between panels and bank (MPPT, sized to the array). The generator is not an admission of failure; nearly every serious off-grid home has one for the February that never clears.

When off-grid wins, and when it does not

Wins: remote builds where the utility's quote to extend line power lands in five figures; seasonal cabins where even a modest bank covers every visit; wells, barns, and workshops a few hundred metres from the meter; and places where the grid arrives unreliably enough that independence has value no payback formula captures.

Does not: anywhere the grid already stands. A grid-tied array at $3.00/W — with the utility absorbing every surplus kilowatt-hour — beats off-grid on cost by a wide margin; the payback calculator shows the grid-tied arithmetic in seconds, and the installation-cost guide breaks the quote down line by line. Going off-grid where grid power exists is a lifestyle choice, not an investment.

Costs, stated plainly

There is no honest single number, so here is the structure: panels and racking price like any install ($3.00/W); batteries scale with autonomy and dominate the budget; the off-grid inverter/charger and controller add a mid-four-figure block; installation of off-grid systems is more bespoke than rooftop work. Net effect: expect roughly two to three times the cost of a same-size grid-tied array before any generator. And the federal credit that used to soften these projects — it covered off-grid systems too — ended for systems completed after December 31, 2025, so plan on state and utility incentives only; the details are in the federal credit explainer.

A worked example, start to finish

A cabin using 8 kWh a day in a 4.5-sun-hour state: array = 8 ÷ (4.5 × 0.8) ≈ 2.2 kW (call it 3 kW for winter); bank = 8 × 2 ÷ 0.8 = 20 kWh; a 4 kW inverter/charger covers the water pump and tools; a small propane generator covers the bad fortnight. Panels ≈ $9,000, batteries roughly the same, electronics and installation add the rest — a realistic $25,000–35,000 project that replaces either a five-figure line-extension quote or years of generator fuel. That is off-grid solar at its best: not cheaper than the grid, but cheaper than getting the grid.

Sizing math: performance ratio 0.8 and $3.00/W from the same constants as the methodology page; sun hours from the state data table; battery usable depth of discharge 0.8 (LFP typical). Costs are planning ranges, not quotes — last reviewed 20 August 2026 by SolarDime editorial.