Home Outage & Backup
Interlock Kit vs Transfer Switch: Which Do You Need?
Both do the same safety job. One is cheaper and gives you more circuits. Here is how to pick, and the shortcut that kills people.
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What's in this article
What each one actually is
Transfer switch
A separate box mounted beside your panel. During installation an electrician moves a chosen set of circuits into it — the furnace, the fridge circuit, some lights, maybe the well.
You flip it from GRID to GENERATOR and those circuits draw from your battery or generator instead.
Upside: clearly labelled, obvious to use, hard to get wrong. Downside: you're locked to the circuits chosen at install, and it's a second box on the wall.
Interlock kit
A metal plate that mounts on your existing panel. It physically blocks the main breaker and the generator breaker from being on together — you have to turn one off to reach the other.
Upside: cheaper, and you get access to every circuit. Want to run the well this hour and the furnace the next? Just switch breakers. Downside: must be a listed kit for your panel. And it relies on you managing your own load — nothing stops you switching on more than your generator can carry.
How to pick
| Your situation | Pick |
|---|---|
| A listed kit exists for your panel | Interlock — cheaper, more flexible |
| No kit for your panel brand | Transfer switch |
| You want it foolproof for anyone in the house | Transfer switch |
| You want maximum circuits for minimum money | Interlock |
| Renting | Neither — you can't modify the panel |
⛔ The shortcut that kills people
Do you even need one?
Probably not, and this is worth checking before you spend anything.
Everything you need has a plug? Fridge, lamps, router, phone chargers, a fan. Then extension cords cover you completely. No panel work, no electrician, no cost.
You need hardwired things? Furnace blower, well pump, sump pump, electric water heater. Those have no plug and nothing but panel work reaches them.
Swipe the table sideways to see every column.
| Price | |||||||
|---|---|---|---|---|---|---|---|
| EcoFlowEcoFlow DELTA Pro 3LiFePO4 · expandable | 4,096 Wh | 4,000W | 6,000W | 113 lb | — | 5 yr | Check price for EcoFlow DELTA Pro 3 (opens in a new tab, affiliate link) |
| AnkerAnker SOLIX F3800LiFePO4 · expandable | 3,840 Wh | 6,000W | 9,000W | 132.3 lb | — | 5 yr | Check price for Anker SOLIX F3800 (opens in a new tab, affiliate link) |
| EcoFlowEcoFlow DELTA 2LiFePO4 · expandable | 1,024 Wh | 1,800W | 2,700W | 27 lb | 1h 20m | 5 yr | Check price for EcoFlow DELTA 2 (opens in a new tab, affiliate link) |
Specs come from data/products.json. Capacity is the figure on the box — real usable output is roughly 85% of it once you account for inverter losses.
The middle path most people miss
You don't have to back up the whole panel.
One circuit for the gas furnace blower, with an interlock, is often the entire answer in cold country. The blower is a modest load — around 400 watts — and running it heats your whole house rather than one room.
That's a much smaller job than backing up everything, and it delivers most of the benefit for a fraction of the cost.
On cost
Get local quotes. Any number in an article is guesswork — labour varies enormously by region and by how awkward your panel is.
But price the whole thing before committing: battery plus the electrical work. At that total, get a standby generator quote too. They start themselves, run on gas or propane, and don't run out on day three.
They're also loud, need servicing, and can't run indoors. It's a real trade — just make it with both numbers in front of you.
The picks, in detail
Specs come from data/products.json, so they're the same everywhere on this site.
EcoFlow
EcoFlow DELTA Pro 3
Specs unverifiedThe first portable unit that can honestly claim to back up parts of a house, including 240V circuits.
- Capacity
- 4,096 Wh
- AC output
- 4,000W
- Weight
- 113 lb
What's good
- 240V output covers well pumps and some dryers, which almost nothing else portable does
- Pairs with a transfer switch so you stop running extension cords through a window
- Chains with extra batteries into genuinely serious capacity
What's annoying
- Costs about the same as a permanently installed standby generator once you add a transfer switch
- 113 lb means it lives where you put it
- The install to make it useful is an electrician job, not a weekend project
Who this is wrong for
Anyone who wants to move it more than once. It has wheels for a reason, and stairs will beat you.
Anker
Anker SOLIX F3800
A home backup system wearing a portable power station's clothes. Treat it as an install, not a purchase.
- Capacity
- 3,840 Wh
- AC output
- 6,000W
- Weight
- 132.3 lb
What's good
- 6000W output covers nearly anything in a normal house, one at a time
- Real 240V, so the well pump and the dryer are on the table
- Stacks with expansion batteries into a very large bank
What's annoying
- 132 lb, and it needs a permanent home
- The transfer switch and electrician push the real cost well past the sticker
- At this price a standby generator deserves a serious look
Who this is wrong for
Renters. You cannot get the value out of it without wiring work you don't own.
EcoFlow
EcoFlow DELTA 2
The one most people should buy. It refills fast enough that a short sunny break between storm bands actually matters.
- Capacity
- 1,024 Wh
- AC output
- 1,800W
- Weight
- 27 lb
What's good
- Wall recharge is genuinely quick — useful when power flickers back for an hour
- LiFePO4 cells, so it tolerates being left plugged in for years
- Six AC outlets means no daisy-chained power strips on the kitchen counter
- Add an extra battery later instead of buying a whole second unit
What's annoying
- The fan is audible in a quiet bedroom at night
- 1800W ceiling rules out most space heaters and hair dryers
- Expansion batteries cost nearly as much as the unit itself
Who this is wrong for
Anyone who needs to power a well pump, an electric water heater or a central AC. It will not start them, and it is not a whole-home unit.
Questions people actually ask
What is the difference between an interlock kit and a transfer switch?
Is an interlock kit legal?
Which is cheaper?
Do I need either one for a portable power station?
Can I install an interlock kit myself?
Who wrote this
Dhruvil Domadiya
Founder and editor, GreenCommuteGuide
I started GreenCommuteGuide because backup power is sold with numbers that sound impressive and mean very little — watt-hours on the box, surge ratings that turn out to be a boost mode, charge times quoted to 80% and printed as though they were full. I set the rules this site runs on: every spec is checked against the manufacturer's own documentation, anything we have not verified is marked as unverified on the page rather than quietly published, prices are never shown because they go stale, and every review names at least one product not to buy and who it is wrong for. Where I have not tested something myself, the page says so.
- · Sets and enforces the editorial standards on this site — unverified claims are flagged on the page, and the build fails if they are not
- · Specs are taken from manufacturers' own published documentation, never from retailer listings, and the source and date are recorded for each product
- · No payment is accepted for a rating, a ranking, or a positive verdict
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