How the two differ in kind
Both arrangements put solar generation and batteries on a building; they answer different questions. Grid-tied with battery backup uses the grid as the primary supply and the storage as a bridge — through an outage, or across the gap between generation in the day and consumption in the evening. Off-grid has no grid, so the local system has to meet every load at every hour of every day, on its own.
The number that separates them is AUTONOMY: how many days of poor generation the system must survive without help. For grid-tied backup, autonomy is measured in the hours of a typical outage, and beyond that the grid returns. For off-grid, autonomy is a design decision that has to cover the worst stretch of weather the site sees — and that is where the cost lives.
It lives there because off-grid sizing is not driven by average consumption. It is driven by the WORST case: a run of overcast days in midwinter, in a climate where winter daily yield is already a fraction of the summer figure, against a load that in winter is usually higher rather than lower. The battery bank and the array that get you through that week are large, and for the remaining months of the year most of that capacity sits unused. There is no way to average it away, because the deficit does not average — it accumulates day by day until either generation returns or the bank is empty.
The practical consequence is that almost every off-grid system carries a generator, and that is not a failure of design. It is the economically rational answer to a rare deficit: sizing the array and the bank for a once-a-year week of bad weather costs far more than running an engine for a handful of days. The honest version of off-grid is a hybrid, and any proposal that omits the generator should be asked how it covers the worst week.
The factors that actually differ
Show
| Off-grid | Grid-tied with battery backup | |
|---|---|---|
| What sets the size | The worst stretch of the year — days of poor generation, at the season of lowest yield and often highest load. | The expected outage duration and the loads you choose to back up. Everything else is the grid's problem. |
| Battery bank | Large. Days of autonomy multiplied by daily consumption, with depth-of-discharge and temperature derating on top. | Modest. Hours rather than days, and only for the circuits you selected. |
| Array size | Sized to recharge the bank in the worst month, not the average one — substantially oversized for summer. | Sized to offset consumption economically. Winter shortfall is bought from the grid. |
| Generator | Effectively required. It is the rational way to cover a rare deficit rather than paying to store for it. | Optional, and a separate decision about longer outages. |
| Load management | Central. Every large load has to be scheduled, deferred or designed out — this is a lifestyle constraint, not a setting. | Only during an outage, and only for the backed-up circuits. |
| Surplus generation | Wasted once the bank is full, unless there is a dump load to absorb it — a real inefficiency in summer. | Exported, or used to charge, depending on the tariff. Nothing is thrown away. |
| When the grid fails | Nothing changes. There is no grid. | The system islands and runs the backed-up circuits — which requires equipment capable of islanding, not merely an inverter. |
| Connection cost | Avoided entirely, which on a remote site can be the decisive number — grid extension over distance is priced per length. | Paid, plus standing charges for as long as the connection exists. |
| Consequence of getting it wrong | You run out. The lights go off and stay off until the sun returns or the generator starts. | The backup runs out and the grid carries on, unless the grid is the thing that failed. |
| Growth | Hard. Adding a load means revisiting the whole sizing chain. | Easy. The grid absorbs new loads without redesign. |
Which one, and when
Choose off-grid when…
- There is no grid connection and extending one is expensive — this is the case where off-grid wins outright.
- The site is remote enough that a connection would cost more than the entire generating system.
- Consumption is modest and controllable, and the occupants are willing to schedule around the weather.
- Independence from the utility is a goal in itself and the cost of achieving it is understood.
Choose grid-tied with battery backup when…
- There is already a grid connection, in which case abandoning it to solve an outage problem is rarely economic.
- The objective is riding through outages rather than leaving the network.
- Loads are large, variable or not easily scheduled — electric heating, workshop equipment, vehicle charging.
- A tariff makes stored energy worth something: time-of-use arbitrage, export payments, or grid services.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How many days of autonomy do I need off-grid?
- Enough to cover the worst realistic run of poor generation at your site, which is a local-climate question rather than a number anybody can hand you. The inputs are the longest stretch of heavily overcast days your location sees, the season it occurs in, and what the generation is during it — not zero, but a small fraction of a clear-day figure. The output feeds through several multipliers: usable capacity is less than nominal because batteries are not discharged fully, capacity falls in cold weather, and the bank ages. Then comes the economic judgement, which is where most designs land: rather than sizing for the very worst week, size for a typical bad spell and cover the rare extreme with a generator. Sizing for the extreme means paying for capacity that is idle every other week of the year.
- Why does almost every off-grid system have a generator?
- Because covering a rare deficit with stored energy is extraordinarily expensive, and covering it with an engine is not. The capacity needed to ride out a once-a-year week of overcast midwinter weather is far larger than the capacity needed for ordinary operation, and it sits unused for the rest of the year — so the last increment of autonomy is the most expensive one bought. A generator inverts that: a fixed modest capital cost plus fuel only on the days it runs. It also provides something storage cannot, which is recovery — after a deep discharge, a generator recharges the bank on demand instead of waiting for the sun. Batteries left at a low state of charge for extended periods degrade, so the generator is protecting the most expensive component as well as the lights.
- Will my solar keep working in a power cut?
- Only if the system was specified to, and a plain grid-tied array will not. Standard grid-tied inverters are required to shut down when the grid goes away — anti-islanding protection, which exists so that a system cannot energise lines that utility workers believe are dead. So a house with solar panels and no storage is dark in an outage, on a sunny day, which surprises a great many owners. Riding through requires equipment designed for it: a hybrid or battery inverter with a changeover arrangement that disconnects from the grid and forms its own island, supplying a defined set of backed-up circuits. That backed-up subpanel is a design decision made at installation — which circuits are on it, and whether its total is within what the inverter can supply.
- Can I go off-grid to escape an unreliable grid?
- You can, and in most cases it is the expensive answer to that particular problem. Leaving the network means sizing for every hour of the year including the worst week, which is a much larger system than one sized to bridge outages — and if the connection already exists, disconnecting discards an asset that works most of the time and costs a standing charge. Grid-tied storage addresses the actual complaint: it carries the chosen circuits through the outage and then goes back to using the grid, at a fraction of the capacity. The cases where full off-grid genuinely wins are where the connection does not exist and would be costly to create, where it is so unreliable that it cannot be counted on at all, or where independence is valued for its own sake and the premium is accepted knowingly.
- What happens to surplus generation off-grid?
- It is lost, unless something is arranged to use it, and in summer that is a large quantity. Once the battery bank is full the charge controller has to stop harvesting — there is nowhere for the energy to go — so an array sized for midwinter spends much of the summer curtailed. The usual responses are to put the surplus somewhere useful: a diversion or dump load heating water, space heating, a pool, or scheduling discretionary consumption into the middle of sunny days. This is the mirror image of the grid-tied case, where surplus is exported and either paid for or at least not wasted, and it is one of the underappreciated inefficiencies of off-grid: the system is oversized for eleven months in order to work in the twelfth.
- Does storage pay for itself on a grid-tied system?
- It depends almost entirely on the tariff, and the answer changes by market and by year rather than by technology. Where there is a wide spread between peak and off-peak prices, storage can arbitrage — charge cheaply, discharge at the expensive time — and that spread times the cycles per year is the return. Where export is paid poorly relative to import, storing your own generation for later use is worth the difference between those two rates. Where import and export are close and rates are flat, the financial case is weak and the value is resilience instead, which is real but is insurance rather than a return. Two things to check before any projection: the round-trip efficiency, since a proportion of what goes in does not come out, and the warranted cycle life, because the arithmetic only works over the cycles the battery is actually rated for.
- What does off-grid mean for how you use electricity?
- It becomes something you manage rather than something that is simply there, and that is a genuine lifestyle change rather than a technicality. Large loads get scheduled for sunny days — laundry, workshop tools, vehicle charging, water pumping — and deferred when the forecast is poor. Loads that draw continuously get scrutinised, because a small constant draw over a day is a substantial share of a modest bank. High-power resistance loads become the enemy: electric heating, immersion heaters, ovens and electric showers are each capable of consuming a meaningful fraction of a day's generation in an hour, which is why off-grid houses commonly heat and cook with something else. None of this is a hardship for people who chose it deliberately. It is a poor surprise for people who expected the system to behave like a grid connection.
- How does winter change the numbers?
- More than almost anyone expects, because two effects compound. Generation falls sharply — shorter days, lower sun angle, more cloud, and at higher latitudes a midwinter daily yield that is a small fraction of the midsummer figure — and consumption usually rises at the same time, with more lighting hours and more heating demand. Batteries make it worse rather than better: usable capacity falls at low temperature, so the bank that holds a given amount in a warm month holds less in a cold one, which is why bank location and insulation are design items rather than details. Snow adds an outage of its own by covering panels, and a steep tilt that sheds snow also optimises the low winter sun angle, which is why off-grid arrays are frequently tilted steeper than a grid-tied array optimised for annual yield.