Sized for the worst week, not the average
A grid-tied system can be undersized with no consequence: on a poor week you simply import more.
Off-grid has no such fallback. The array and the battery bank have to carry you through the darkest, coldest stretch your location produces — which in much of the country means a design built around a handful of days a year.
That is why off-grid arrays and battery banks are typically several times larger than a grid-tied system serving the same household.
The generator is part of the design
Sizing purely on solar for the worst week is possible and almost always uneconomic, because you buy an enormous battery bank that sits unused for eleven months.
The standard answer is a generator covering the small number of days the solar cannot. This is a deliberate trade: a modest amount of fuel a year against a much smaller and cheaper battery bank.
Anyone selling you an off-grid system with no backup strategy is either oversizing dramatically or has not thought it through.
Surplus has nowhere to go
In summer, an off-grid array sized for winter will make far more than you can use or store.
With no grid connection there is no export credit and no bank of value. The charge controller simply throttles the array back. That lost production is a real cost of being off-grid and it never appears in a payback calculation.
Loads change
Off-grid living usually involves looking at what draws the most power and deciding whether it earns its place.
Resistance heating, electric water heating and air conditioning are the big three. Many off-grid households move heating and cooking to propane or wood, use a heat pump water heater, and run heavy loads during the day when the array is producing rather than from the battery at night.
None of this is hardship, but it is a change, and it should be discussed before the system is designed rather than discovered afterwards.
How the work runs
- Load auditEvery appliance, its draw and when it runs. Off-grid design starts here and nothing downstream is valid without it.
- Worst-case solar assessmentProduction in your darkest month at your site, not the annual average.
- Autonomy decisionHow many days without sun the battery must carry, and where the generator takes over.
- Battery bank sizingFrom the load audit and the autonomy target. Usually the largest single cost.
- Array and charge controlSized to refill the bank in your worst month, with a controller matched to the bank.
- Generator and integrationAutomatic start and transfer, sized to charge the bank rather than to run the house directly.
- Permits and codeOff-grid is still electrical work and still inspected. Being unconnected does not exempt it.
What moves the price
We do not publish a national average and present it as your price. These are the variables that separate two quotes for the same job — run your own numbers with the cost guides and calculators, and check what incentives you actually qualify for.
| Factor | Why it moves the number |
|---|---|
| Battery bank capacity | The dominant cost, and it scales directly with how many sunless days you insist on covering. |
| Array size | Larger than a grid-tied equivalent because it must refill the bank in the worst month. |
| Generator and automation | Generator, fuel storage, and automatic start and transfer gear. |
| Load profile | Electric heat or air conditioning can multiply every number above. |
| Site remoteness | The reason you are off-grid is often the reason the install costs more. |
| Replacement cycle | The battery bank is a consumable with a finite life. Budget for its replacement from day one. |
What to ask before you commit
Take this list to the conversation. A professional who answers these directly is telling you a lot; one who deflects is telling you more.
- What load audit is this design based on, and can I see it?
- How many days of autonomy, and what happens on day four?
- What is the generator runtime assumption per year?
- What is the expected battery bank life, and what does replacement cost today?
- What would a grid connection cost here, and how does that compare?
Common questions
Is off-grid solar cheaper than staying on the grid?
Almost never, where a grid connection is available at reasonable cost. Off-grid systems are sized for the worst week of the year rather than the average, which makes the array and especially the battery bank much larger. It is usually a location or independence decision, not a savings one.
Do I need a generator with off-grid solar?
In practice, almost always. Covering the darkest days on solar and storage alone means buying a battery bank that sits idle most of the year. A generator for a handful of days is far cheaper.
Can I go off-grid with my existing grid-tied system?
Not directly. Grid-tied inverters are designed to shut down without a grid reference. Converting means new inverter equipment, a battery bank and usually a redesign.
What happens to extra power in summer?
It is lost. With no grid connection there is nothing to export to, so once the battery is full the charge controller throttles the array back.