How the two differ in kind
A battery bank holds a fixed quantity of energy and gives it back at whatever rate is asked, immediately and without being told. A generator holds almost no energy and manufactures it continuously, after a start-up delay, for as long as fuel arrives. Those two sentences contain most of the decision.
What they do not contain is that neither delivers its nameplate. A battery's USABLE energy is a fraction of its rated capacity, set by how deeply its chemistry tolerates being discharged — around half for lead-acid, eighty to ninety per cent for lithium iron phosphate — so two banks of the same nameplate can differ by a factor of nearly two in what they actually supply. And then the inverter takes its share on the way out.
The generator's version of the same trap points the other way. Its fuel consumption is not proportional to its output: a substantial share goes into simply keeping the engine turning, so at a light load the fuel per unit of electricity is far worse than at three-quarters load. An oversized generator running a fridge and a few lights does not sip fuel — it burns a disproportionate amount and, on a diesel, fails to reach its designed cylinder temperatures, so unburnt fuel and oil pass into the exhaust and foul it. The damage comes from under-use, which is not how most people expect machinery to fail.
The factors that actually differ
Show
| Standby generator | Battery bank and inverter | |
|---|---|---|
| Time to take over | Seconds to tens of seconds — start, stabilise, transfer. Anything sensitive sees the gap unless a small battery covers it. | Instant and seamless. This is the whole reason batteries sit in front of computers and medical equipment. |
| How long it lasts | As long as fuel keeps arriving. For a multi-day outage this is the only one of the two that answers the question. | Until the usable store is gone — hours for a household load, minutes for a heavy one. It does not extend. |
| What the nameplate overstates | Nothing about output, but the runtime: consumption is load-dependent and non-linear, so a runtime figure without a stated load is meaningless. | The energy. Usable depth of discharge is chemistry-dependent, and inverter and standby losses come off what is left. |
| Behaviour on a small load | Poor. Worse fuel per unit delivered, and on a diesel, wet stacking — fouling caused by running lightly loaded for long periods. | Excellent. A battery is indifferent to being lightly loaded; only the inverter's standby draw matters. |
| Where it can live | Outdoors, with clearance, away from openings. It produces carbon monoxide and noise, and both are why indoor operation kills people every winter. | Indoors, in a cupboard or a plant space, silent. Lithium installations have their own siting and fire-separation requirements, which are real but different. |
| Cold weather | Harder starting and thicker fuel, but its capacity does not fall. | Capacity falls in the cold, and lithium must not be CHARGED below freezing without heating — a real constraint in an unheated garage. |
| Maintenance | A machine: exercise runs, oil, filters, and fuel that ages. Petrol degrades in months; diesel grows microbial contamination in a part-full tank. | Almost none, and a calendar. Cells degrade with time and with cycles whether or not they are used, so the bank is nearer replacement each year regardless. |
| Shape of the cost | Moderate capital, then fuel only when it runs — so a rarely used generator is cheap to own and expensive per hour when needed. | High capital that is spent whether or not the power ever fails, and effectively no running cost. It is insurance paid up front. |
Which one, and when
Choose standby generator when…
- Outages are long — days rather than hours — which is the case a store cannot answer.
- The load is substantial: heating, a well pump, a workshop, a farm.
- There is somewhere safe outdoors for it and somewhere to keep fuel.
- The budget has to cover a large load, and battery capacity at that scale would be several times the price.
Choose battery bank and inverter when…
- Outages are short and frequent — minutes to a few hours — where a generator would spend more time starting than running.
- The transfer has to be seamless: computers, medical equipment, anything that resets.
- There is nowhere to site a generator, or noise and exhaust rule it out — a flat, a terrace, a dense urban site.
- There is already solar, so the battery earns its keep every day rather than only during an outage.
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 long will each actually run?
- For the generator: fuel divided by consumption at the load you are actually carrying, and that consumption is not proportional to load — a light load is disproportionately thirsty per unit delivered. Manufacturers publish consumption at a quarter, half, three-quarters and full load for this reason, and using the full-load figure to compute runtime on a small load gives an answer that is optimistic in the wrong direction. For the battery: usable capacity — nameplate times the depth of discharge the chemistry allows — divided by the load, less inverter conversion losses and the inverter's own standby draw, which matters when the load is small and the outage long. Both calculators state the assumption they used, because a runtime quoted without its load is not an answer.
- Why does my generator use so much fuel for so little?
- Because a large share of its fuel goes into turning the engine rather than into producing electricity, and that share is roughly fixed. At three-quarters load most machines are near their best fuel per kilowatt-hour; at a quarter load the same machine is materially worse. So an oversized generator running a modest load is not being gentle with itself — it is burning more fuel per unit delivered and, in a diesel, running too cool. Unburnt fuel and lubricating oil then pass into the exhaust, glaze the bores and foul the turbocharger, which reduces the machine's ability to take load at all. The remedy is a correctly sized machine, or a load bank, and the honest answer to a long required runtime is usually a bigger tank rather than a bigger generator.
- Can I run a generator in the garage with the door open?
- No. This kills people every winter and the reason is that carbon monoxide is odourless, heavier-than-air pockets form where you do not expect them, and an open door is not ventilation — it is an opening in one wall of a space that is filling faster than it clears. Portable generators belong outdoors, well away from doors, windows and vents, with the exhaust directed away from the building, and a carbon monoxide alarm inside is a cheap second line rather than a substitute. The same applies to a covered porch, a carport with three sides, or a doorway sheltered from rain — all of which are where the fatalities happen, because they look like outdoors and behave like indoors.
- Do I need a transfer switch?
- Yes, and it is not a formality. Connecting a generator to a building's wiring without one — back-feeding through a socket, typically — energises the supply conductors leaving the building, which can kill a line worker who has isolated the network and is working on it believing it dead. It also means the generator is connected when the utility supply returns, which destroys the generator. A transfer switch makes the two supplies mutually exclusive mechanically, so no sequence of operations can have both connected. It is a regulated item everywhere, it is installed by an electrician, and it is the part of a backup installation that is about somebody else's safety rather than your convenience.
- Does stored fuel go off?
- Yes, and faster than most people plan for. Petrol degrades in months — oxidising and, where it contains ethanol, absorbing water and separating — so a can kept since last winter is a poor bet in an emergency. Diesel lasts longer but is vulnerable to microbial growth at the fuel–water interface in a part-full tank, which produces sludge that blocks filters at exactly the wrong moment. Both are managed the same way: keep tanks full to reduce condensation, use a stabiliser, and rotate the stock through normal use rather than storing it indefinitely. A standby generator that is exercised monthly under load solves the rotation problem and proves the machine starts, which is the other thing you want to know before an outage rather than during one.
- Is the best answer actually both?
- Frequently, and it is not a compromise. A modest battery covers the first minutes without a gap — so nothing resets and nothing matters that the generator is still starting — and covers short outages entirely, which is most of them, without the machine running at all. The generator then handles the long event the battery cannot. That arrangement also fixes the generator's worst habit, because it starts under a real load rather than idling against a fridge. The cost is two systems and a control scheme that decides which runs, and for anything where an outage is genuinely consequential it is the arrangement that datacentres and hospitals use for exactly these reasons.