SOLAR PV · BATTERY STORAGE · EMERGENCY BACKUP

How Much Backup Power Does Your Home Really Need?

Understanding battery capacity, inverter output and essential household circuits before investing in emergency backup power.

Imagine a power cut on a cold winter evening. The lights go out, the broadband stops working, the fridge and freezer lose power, and your heating system may stop operating.

You've already invested in solar panels and battery storage, but how much electricity would you actually need to keep your home running?

At Speedy Fit, we believe emergency backup systems should be designed around the homeowner's actual requirements, not simply the largest inverter or battery available.

1. Battery capacity and inverter power are two different things

Battery capacity is measured in kilowatt-hours (kWh). It determines approximately how much energy is available.

Inverter output is measured in kilowatts (kW). It determines how much electrical power can be supplied at any given moment.

For example, a 5kW backup inverter may supply up to 5kW of compatible loads simultaneously, subject to its EPS rating and operating conditions.

A battery containing 10kWh of usable energy could theoretically supply a constant 1kW load for approximately 10 hours.

The same battery supplying 2kW would theoretically last around five hours.

In practice, conversion losses, battery limits and operating conditions will affect these figures.

A large battery doesn't necessarily mean you can operate an electric shower, oven and heat pump simultaneously.

2. How much electricity do essential household appliances use?

The following figures are illustrative. Actual consumption varies by appliance, operating conditions and usage.

Some appliances, particularly motors and compressors, require higher starting currents than their normal running consumption.

The important question is which appliances are likely to operate at the same time.

3. A realistic essential-loads example

Consider a household wanting to maintain basic services during a power cut.

Relatively modest running loads can maintain a reasonable level of comfort. However, compressor starting currents, heating demand and temporary increases in electrical demand must also be considered.

4. How long could a 10kWh battery last?

Assuming 10kWh of usable stored energy and an illustrative overall delivery efficiency of 90%:

These are examples, not guaranteed operating times. Inverter standby consumption, battery reserve settings, state of charge and equipment efficiency can all change the results.

A battery that's only 30% charged when the power fails cannot provide the same duration as a fully charged battery.

5. Should you back up the whole house?

Whole-house backup sounds attractive, but it isn't always the best solution.

Electric showers, cooking equipment, immersion heaters, EV chargers and heat pumps can create substantial electrical demand.

Connecting every circuit without considering maximum demand can lead to overloads and inverter shutdowns.

For many properties, a separate essential-loads consumer unit offers a more practical solution.

This could supply selected lighting, refrigeration, broadband equipment, heating controls and designated socket circuits.

6. Automatic changeover or manual switching?

Some compatible hybrid inverters automatically supply designated circuits when the public electricity network fails.

Others require additional manufacturer-approved switching equipment. A manual changeover arrangement may also be appropriate.

Whichever arrangement is selected, the installation must safely isolate the backup supply from the public network and provide suitable earthing and electrical protection.

7. What about heat pumps?

Heat pumps deserve particular attention because their electrical demand varies with operating conditions.

Defrost cycles, compressor operation, circulation pumps and supplementary electric heaters must all be considered.

Before including a heat pump on an emergency backup supply, its electrical characteristics and the inverter's backup capabilities need to be assessed.

8. Can solar panels recharge the battery during a power cut?

Potentially, yes. Some hybrid inverters support solar generation and battery charging while operating independently of the grid.

However, this depends on the inverter, battery system, installation configuration and operating conditions.

Not every system can restart solar generation from a depleted battery during a grid outage.

Winter generation may also be insufficient to replace the energy consumed overnight.

9. The importance of battery reserve settings

Some battery systems allow homeowners to maintain a minimum state of charge for emergency use.

This can provide an additional safeguard if a power cut occurs overnight.

However, reserving energy means less stored electricity is available for everyday use.

10. How do you determine the right system for your home?

Before choosing an inverter or battery capacity, consider these five questions:

1. Which circuits must remain operational during a power cut?
2. What is their realistic simultaneous electrical demand?
3. Are there appliances with high starting currents?
4. How many hours of backup are required?
5. Can the existing inverter and battery system support EPS operation?

The answers help determine the inverter output, battery capacity, switching arrangement and electrical installation work required.

Don't pay for backup capacity you don't need

A properly designed emergency backup system isn't necessarily the largest or most expensive system available.

For some households, keeping a few essential circuits operational for 12–24 hours may be more valuable than attempting to run the entire property for a short period.

Speedy Fit undertakes solar PV, battery storage and electrical installation work throughout Worthing and West Sussex.

We can assess existing systems and discuss whether an EPS upgrade is practical, which circuits should be supported and what battery capacity may be appropriate.

No fake smiles, no fake promises — just professional, honest work.

Related reading

Solar Panels and Batteries: Will Your Home Still Have Power During a Power Cut?

← Back to Solar Advice & Guides