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BatteriesWhole home backup or partial backup? The runtime arithmetic
Whole home backup sounds like the obvious upgrade until you work out how long a battery lasts against a whole house. Here is the arithmetic that decides which tier you actually want.
Updated 4 August 2026 · written by the Clearline guidance team
The three tiers, in order of cost
Backup is not one product. It is three quite different specifications that get described with the same word on quotes, and the price gap between the cheapest and the dearest is large enough to fund a decent chunk of the battery itself.
| Tier | What runs during an outage | What it needs |
|---|---|---|
| EPS socket | Whatever you physically plug into one 13A socket. A lamp, a phone, the broadband router. | Wiring from the inverter's EPS terminals |
| Partial backup | A chosen set of circuits. Typically fridge and freezer, a lighting circuit, broadband, maybe the boiler. | A backup sub board with the selected circuits moved onto it |
| Whole home backup | Every circuit in the property, exactly as normal. | A gateway or whole house transfer switch between the meter and the consumer unit |
Whole home backup is the most expensive of the three because of the switchgear and the work at the incoming supply, not because the battery is different.
The middle tier is the one most people should be quoted and the one fewest people are offered. A sub board carrying the fridge, freezer, lighting and broadband covers almost everything that causes real loss or disruption in an outage, at a fraction of the cost of switching the whole property.
- Backup
- Requires a Backup Gateway 2
- On-grid output, continuous
- 3.68 kW to 11.04 kW, set to your DNO connection
Runtime is capacity divided by load
This is the calculation that settles the argument, and it is simple enough to do on the back of an envelope. Take the usable capacity of the battery in kWh, divide by the average draw in kW of whatever you are keeping alive, and you get hours.
The table below runs that for a 13.5 kWh usable battery, the capacity of a single Tesla Powerwall 3. The load figures are illustrative assumptions so you can see the shape of the curve, not measured averages for your house. Substitute your own.
| Average draw | Roughly what that looks like | Runtime from 13.5 kWh |
|---|---|---|
| 0.3 kW | Fridge, freezer, router, a couple of LED lights | 45 hours |
| 0.5 kW | The above plus a boiler and a television | 27 hours |
| 1.0 kW | A quiet but fully lit house going about its evening | 13.5 hours |
| 2.0 kW | Cooking, laundry running, everything on | 6.75 hours |
| 3.0 kW | Whole house at heavy draw, electric heating active | 4.5 hours |
Arithmetic on a 13.5 kWh usable capacity. Draw figures are illustrative assumptions for comparison, not measured household data.
The curve is the whole argument
The same battery gives you nearly two days of essentials or half an evening of everything. Whole home backup does not last longer than partial backup, it simply lets you spend the stored energy faster. If the outages you are worried about are long ones, partial backup is not the compromise, it is the correct answer.
Power output is the second ceiling
Capacity decides how long. Power decides what can run at the same moment. A battery rated for 3.84 kW continuous output, like the Enphase IQ Battery 5P, cannot deliver more than that no matter how full it is, so a single heavy appliance can consume the entire system output on its own.
At the other end, a Tesla Powerwall 3 delivers up to 11.04 kW continuous on the UK single phase grid, which is why it handles whole home backup on most domestic properties without anyone having to think about load shedding. Sigenergy quotes 200% peak output when running off grid, which covers the surge when a motor or a pump starts.
Whole home backup has a hidden requirement
It is not enough for the battery to hold the energy. It has to be able to deliver the peak the house draws when the shower, the oven and the kettle coincide. Where the output rating cannot cover that, the honest specification is partial backup with the heavy circuits deliberately left out.
Solar keeps generating, but only with the right hardware
A gateway creates a safe electrical island the array can generate into, so during a daytime outage the panels carry on producing and the battery recharges rather than draining steadily to nothing. That transforms multi day outage performance, because you are no longer working from a fixed 13.5 kWh but from 13.5 kWh plus whatever the roof makes each day.
On a basic emergency power supply socket that generally does not apply, so what you start with is what you have. Our guide to what a battery gateway does covers the hardware in detail.
Who each tier suits
EPS socket
Urban and suburban homes on a reliable network where outages are rare and short. Keeps the phone charged and the router alive, costs almost nothing extra.
Partial backup
The default sensible choice. Protects the freezer contents, keeps the house lit and connected, and stretches a given battery across a long outage.
Whole home backup
Rural properties on overhead networks, homes with medical equipment, households where a working day offline has a real cost, and properties with pumps that stop when power does.
No backup at all
A legitimate answer. If your supply is stable, the money buys a lot of electricity and the battery still earns every day on tariff arbitrage.
A free remote design call will tell you which tier your property actually needs, including when the honest answer is the cheapest one.
Get your free quote →What it does to the quote
Backup hardware is a visible line item. Tesla lists the Powerwall 3 from £5,000 without a gateway and from £5,975 with one before installation, and a single unit with Backup Gateway is commonly quoted around £7,995 fully fitted. Beyond hardware, whole home backup means work at the consumer unit and the incoming supply, which is a different job from hanging a battery on a wall.
Adding it afterwards costs considerably more than specifying it at the start, for the same reason a second visit always does. If backup matters to you at all, raise it before the first design rather than after the first quote. Our 2026 costs guide puts the rest of the pricing in context.
Sizing a battery for outages, not bills
If backup is the main reason you are buying, the sizing logic changes. Bill driven sizing matches the battery to your daily consumption. Outage driven sizing matches it to the longest interruption you want to survive multiplied by the load you want to keep running, which often points at a larger battery than the bills alone would justify. Our guide to battery sizing covers the bill led method.
Rural readers should weigh this properly, because the value of backup is genuinely local. Every county page in our coverage area carries a calculator, and the battery storage guide covers the product landscape. Generation sits in the solar panels guide.
Runtime figures above are arithmetic on stated assumptions, not measurements. Clearline gives independent guidance and a free remote design call. Installations are carried out by an MCS-certified installation team for your area.
Backup questions, answered
Can a battery power my whole house?
It depends on two numbers. The battery has to hold enough energy for the duration you want, and it has to be able to deliver the peak power the house draws when several heavy appliances coincide. A 13.5 kWh battery rated at 11.04 kW covers most domestic properties. A 5 kWh battery rated at under 4 kW does not, and should be specified as partial backup instead.
How long does whole home backup last?
Divide usable capacity by average draw. At 1 kW average, a 13.5 kWh battery runs about 13.5 hours. At 3 kW it runs about 4.5 hours. The same battery on partial backup covering only essentials can stretch to well over a day, because the load is a fraction of the size.
Is partial backup a compromise?
Not in any meaningful sense. It delivers longer runtime from the same battery and costs less, and the circuits it protects are the ones that matter during an outage. Whole home backup is the right answer when convenience across a short interruption matters more than endurance across a long one.
Does backup hardware affect my DNO application?
The application is driven by the cumulative inverter rating on site rather than by the backup specification. Up to 3.68 kW on a single phase supply falls under G98, which is installed then notified within 28 days. Above that requires a G99 application approved in advance. Ofgem's connection guaranteed standards give the network operator 45 working days to issue a quotation for a low voltage generation connection, and type tested systems under 50 kW are usually decided faster than that in practice.
Will the battery still save me money if I skip backup?
Yes, and this is the point most quotes bury. The daily economics come from storing cheap or free energy and using it when electricity is expensive, which happens whether or not you can ride out a power cut. Backup is a separate purchase decision with a separate justification.
Can I keep my heat pump running on backup?
That is a power rating question rather than a capacity question, because heating draws heavily and continuously. Most partial backup specifications deliberately exclude it. If it matters to you, it needs to be raised at design stage so the output rating and the battery size are chosen with that load included.
See the numbers next: Tesla Powerwall 3, installed, or what a battery costs.
Figures and specifications in this guide are sourced below and were checked on the date shown. Rates and product specifications change; we confirm the current picture on your free design call.
Sources
- Three tiers of backup: EPS socket, partial backup sub board, whole home via gateway or transfer switch viablepower.co.uk
- Whole home backup versus essential circuits solarlove.org
- Backup duration depends on stored kWh versus house load greenhatrenewables.co.uk
- When whole home backup matters: rural networks, medical equipment, home working sunsave.energy
- Tesla Powerwall 3: 13.5 kWh usable, up to 11.04 kW continuous on UK single phase tlgec.co.uk
- Enphase IQ Battery 5P: 5 kWh usable, around 3.84 kW continuous ohmsnap.com
- Sigenergy SigenStor 200% peak output off grid and claimed 0 ms switchover heatable.co.uk
- Gateway function: islanding the property so solar can keep generating during an outage energylibrary.tesla.com
- Tesla Powerwall 3 UK list pricing with and without Backup Gateway solar4good.co.uk
- Typical fully fitted price for a Powerwall 3 with Backup Gateway solaradvice.co.uk
- G98 and G99 thresholds and cumulative inverter assessment capture.energy
- Ofgem connection guaranteed standards: 45 working day quotation timescale for a low voltage generation connection, £80 failure payment connections.nationalgrid.co.uk