Free solar installed through the U.K.'s ECO4 Scheme cut one homeowner's costs almost right away, but their next decision — whether to choose a 5.2-kWh or 9.4-kWh battery — prompted a Reddit conversation focused less on present-day usage than on what the household could look like in a few years.

Here's what to know

The poster said they live alone and keep electricity use low. Even so, exported power from May through October had already brought in about £250 ($335), which they said would cover electricity bills from January through October 2026.

Before solar, the homeowner calculated that 12 months of electricity had cost about £480 ($643) including standing charges, and they said around £180 ($241) of that was the standing charge itself. 

They explained on r/SolarUK that free east-west panels were fitted in March, the MCS paperwork was completed in May, and although local approval for batteries arrived only after the work was finished, the installation included a battery-ready Hanchu inverter for adding storage later.

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Much of the response leaned toward adding more capacity. One commenter wrote, "The bigger the battery, the more you can benefit from time of use tariffs, moving your usage to either be covered entirely by solar, or the cheap overnight rates," before adding: "I'd go on the bigger side for the battery, go for the 9.4, then you've got headroom for the future."

More background

Future plans were a major part of the dilemma. The homeowner said they want to buy an EV "in a couple of years," may rely on some electric heating during winter, and could later look at an air-source heat pump, making the decision about more than their current modest demand.

Others said the answer depends on what the battery is meant to do. One commenter broke it into two cases: "For most people a 5kWh battery is enough for the first scenario, and a 10kWh battery is required for the second," meaning either carrying summer solar into the evening or using cheap overnight charging in winter. 

Another warned that "beyond a certain amount you have [diminishing] returns," saying storage should mainly cover "your core usage only" because extra capacity can lengthen payback.

For someone with relatively low consumption, that economics question was important. The original poster said their own math pointed to a payback period of roughly eight to 10 years if energy prices stay around current levels.

What can be done?

One recurring suggestion was to size the system using actual consumption patterns, especially in the colder months. 

One commenter asked, "How much power did you use on a typical December day?" while another recommended checking smart meter or inverter readings because winter loads and any future heating changes are more useful than annual spending figures.

The thread also clarified that storage does not automatically provide blackout protection. After one commenter asked, "But will a battery work during a power outage?" another replied, "No unless he gets a gateway of some type as well; then it will." In other words, backup power has to be built into the setup from the outset.

For homeowners looking for a smaller-scale option, Pila offers strong battery backup systems. Pila's plug-and-play batteries cost a fraction of a whole-home backup. 

In the Reddit thread, the most consistent advice was to work backward from the end goal: EV charging, a future heat pump, and whether cheap overnight tariffs will be part of the plan.

One commenter said, "The battery doesn't have to cover the most extreme days, as that would mean buying capacity that you aren't using most of the time."

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