- Two stores, two failure modes
- Write the hours before any money
- The listing and the transfer are part of the machine
- Maintenance and aging are costs you have not seen on the sticker
- A blank comparison, with no winner filled in
- Frequently asked questions
- Is a battery always the cleaner backup?
- Can the generator recharge the battery during a long outage?
A diesel generator and a home battery can each keep loads alive when the utility is gone. They store the ability to do that in different places. The battery is a tank of kilowatt-hours that was filled earlier, from the array or from the grid, and it is empty when those kilowatt-hours have been used. The generator is an engine that makes power while it has fuel, air, cooling, and a load it is allowed to carry. Calling both of them backup does not make the costs comparable. Hours, fuel, maintenance, and the transfer equipment come first. This page does not pick a winner, and it does not invent a fuel price.
Two stores, two failure modes
A battery fails closed when it reaches the floor you set: output stops, or the inverter sheds load, and nothing you do at the tank will create energy that was not stored. Refilling waits on the array, the grid, or a generator. Quiet and instant start are the reasons people accept that ceiling. A generator fails open when fuel, oil, a start battery, or a clogged filter is neglected, and it can also fail by being too large or too small for the load. It can run for as long as fuel arrives. Exhaust, noise, and the need for someone to notice it is unhappy are the costs of that open-ended runtime.
Neither store covers a load the inverter or the generator cannot start. A well pump's surge is a battery problem and a generator problem. How to add up inverter loads and solar inverter overload are the battery-side version. The generator manual has its own motor-starting figure. Matching the two nameplates by their running watts and ignoring the surge is how both machines get blamed for a pump they were never asked, on paper, to start.
Write the hours before any money
A battery sized to shift two evening hours of a tariff is not a battery sized for a two-day storm. Those durations are separated in a battery for the bill versus a battery for an outage and counted from a circuit list in sizing backup for critical loads. Do that count before you ask what a generator costs. Ten hours of outage in a year and four hundred hours are different ledgers even when both machines are rated for the same running kilowatts. The battery's ledger is dominated by whether the usable energy covers the longest outage you refuse to sit through, and by how the pack ages. The generator's ledger is dominated by those hours multiplied by fuel and by the maintenance those hours require.
Fuel in a tank you can resupply is a different constraint from a battery that refills only when the sun or the grid returns. In a short outage with sun the next morning, the battery may never need the engine. In a week of storm the engine is the stretch the battery was not sized for, which is also why off-grid solar carries more equipment often keeps both. A slogan that solar is always cheaper than diesel skips the hours. So does a slogan that a generator is always cheaper because the sticker price is lower. Both slogans are unfinished until the blank sheet below has numbers from your own quotes.
The listing and the transfer are part of the machine
An inverter does not automatically accept a generator. If the manual has no start contact, no generator input, and no approved way to parallel or transfer, the generator is a separate scheme. How not to tie two sources onto one panel, and why the battery usually stays in an off-grid design, is in a generator with off-grid solar. That article does not include a wiring sequence, and this one will not add one. A cord from a running generator into a household outlet can energize the utility side of an open breaker. That is a backfeed. It is not a thrifty version of a transfer switch.
The battery path has its own listing test. A pack on a grid-tied inverter may have no legal way to feed the house when the utility is gone. Can solar panels run a house in an outage is that limit. Home battery backup for outages is the equipment that has to be present before the battery's kilowatt-hours become lights. Comparing a generator that includes a transfer switch with a battery that does not include an island mode compares a working backup with a tariff battery. Add the missing listing to the battery side, or stop calling it backup.
Maintenance and aging are costs you have not seen on the sticker
A battery ages on cycles and on the calendar. Cycle life and calendar life refuses to collapse those into one number of years, and a warranty is not a measured life. A generator ages on running hours, on the starts it was asked to make, and on the oil and filters those hours consume. A generator that sits for eleven months and then fails to start has an aging mode the battery, which was cycled all year for a tariff, did not have. A battery that was held full for a storm and never cycled has an aging mode the generator's hour meter does not show. The quote that compares sticker with sticker has skipped both.
Exhaust and carbon monoxide stay in the generator manual. Solar experience does not replace them. Placement, a carbon monoxide alarm where the manual and the local code require one, and a refusal to run the engine in a garage are part of the generator's cost of ownership even when they do not appear as a line item. This page does not convert those hazards into a siting procedure.
A blank comparison, with no winner filled in
| Question to answer from your quotes | Battery column | Diesel generator column |
|---|---|---|
| Longest outage you refuse to sit through | Usable kWh divided by the load, after the reserve | Runtime at that load, and the fuel that runtime requires |
| What refills it | Array, grid, or a generator | Fuel you can actually obtain |
| How it starts | Listed backup mode, already connected | Start battery, choke or controller, and a person or an auto-start that the manual allows |
| What ages | Cycles and calendar life on this sheet | Hours, starts, oil, filters |
| What must not be improvised | A grid-tied inverter that cannot form an island | A cord into a wall outlet |
Fill the cells from datasheets and from quotes. Leave a cell blank rather than borrowing a neighbor's fuel price or a brochure cycle life. When both columns can cover the hours you wrote down, the rest is the maintenance you will actually do and the nuisance you are willing to live with. When only one column can cover the hours, the other column is not cheaper. It is a different product, sized for a different outage.
Some houses end up with both, because the battery covers the short, frequent gaps and the generator covers the long one. That combination is coherent when the inverter is listed to accept the generator and the transfer keeps either source off the utility. It is not a default, and it is not free just because each machine looked incomplete alone. A hybrid versus an off-grid inverter is the fork if the utility is staying and you only need an island for selected circuits. Buy that listing, then decide whether the engine still has a job.
Frequently asked questions
Is a battery always the cleaner backup?
Only under the emissions condition in does a home battery cut emissions: the charge has to be cleaner than the generation the discharge replaces, after losses. A generator's emissions follow its hours and its fuel, which this page does not convert into a universal figure. Compare the two after the hours are known. Do not award the battery the cleaner title by category.
Can the generator recharge the battery during a long outage?
Only if this inverter is listed to accept that generator and the charge settings match the battery sheet. A generator running the loads through a separate transfer switch does not, by itself, refill a pack that is on the other side of an open inverter input. Read the generator section of the inverter manual before you count on that refill.
