Solar-charged batteries vs cheap night-rate batteries — and how big do you actually need?

 


Solar-charged batteries vs cheap night-rate batteries — and how big do you actually need?

Ever looked at your shiny home battery app and thought, “Brilliant… but is this powering my house, or just keeping the fridge emotionally supported?” Let’s put some numbers on it — especially if you’re heading towards an all-electric home (heat pump + EV).

The two battery “jobs”

A home battery usually does one (or both) of these:

  1. Solar shifting (day → evening)
    Store surplus solar at lunchtime, use it at tea time.

  2. Tariff shifting (cheap night → expensive day)
    Charge from the grid overnight (cheap rate), run the house in the morning/early evening (expensive rate).

Most households end up doing both, but the “right” battery size depends on which job matters more for you.

Step 1: Work out your daily electricity use (today)

Ofgem’s “typical” household electricity use is about 2,700 kWh/year. That’s roughly:

  • 2,700 ÷ 365 ≈ 7.4 kWh/day Ofgem

So, if you don’t have electric heating or an EV yet, your battery is mainly covering evening + overnight basics (lights, cooking, telly, fridge, broadband, etc.).

Rule of thumb (non-EV, non-heat-pump):
A 5–10 kWh usable battery often covers a big chunk of evening usage — especially with solar.

Step 2: Add the “all-electric” extras

A) Heat pump electricity

A common way to estimate is: average home needs around 12,000 kWh of heat/year; with a CoP ~3, that can be about 4,000 kWh/year of electricity. viessmann.co.uk+1

That’s:

  • 4,000 ÷ 365 ≈ 11 kWh/day (but very seasonal — much higher in winter)

So your new daily total becomes roughly:

  • 7.4 + 11 ≈ 18.4 kWh/day (average across the year)

B) Electric car charging

EV efficiency is often 3–4 miles per kWh in real-world use. allcarleasing.co.uk+1

If you drive, say, 8,000 miles/year:

  • At 3.5 mi/kWh → 8,000 ÷ 3.5 ≈ 2,285 kWh/year

  • Per day average: ~6.3 kWh/day

Now you’re at roughly:

  • 18.4 + 6.3 ≈ 24.7 kWh/day average

That’s the average; real life is lumpier (car charging days, cold snaps, guests, Christmas cooking marathons…).

Step 3: Decide what you want the battery to cover

Option 1 — “Evening & overnight cover” (common goal)

If your aim is to run the home from late afternoon to morning (say 12–16 hours), many households target roughly one day of non-EV household electricity in usable storage.

For an all-electric home, a sensible starting band is often:

  • 10–15 kWh usable: good for solar shifting + some peak shaving

  • 15–25 kWh usable: more realistic for heat-pump homes wanting strong evening/overnight coverage

  • 25–40 kWh usable: heavier loads, bigger houses, more EV miles, or “I really want to avoid peak rates” mode

(“Usable” matters — batteries are rarely 100% usable in practice.)

Option 2 — “Charge from grid at night, run most of the day”

If you’re doing tariff shifting, your battery size is basically determined by:

  • How many kWh you want to move from cheap hours to expensive hours

  • Your battery’s inverter power (kW) — can it run cooking + heat pump + kettle at once?

A widely used benchmark battery (example: Tesla Powerwall) has about 13.5 kWh usable. Heatable+1
One of those often feels “useful”; two starts to feel “transformational” in an all-electric home (depending on demand).

Option 3 — Backup power during outages

Backup sizing is different: you decide what you must run (fridge, lights, broadband, maybe heating circulation, maybe a kettle if you’re lucky), then size for hours of autonomy.

The big reality check: winter solar

If you size your battery purely to “store my excess solar,” you might oversize it — because in winter you often won’t have much excess solar to store. That’s when cheap overnight charging becomes more important than solar shifting.

So the best battery size usually matches your usage pattern + tariff, not just your panel size.

A simple battery-sizing recipe (do this in 10 minutes)

  1. Pull your smart meter or inverter app and note:

    • kWh used 4pm–midnight (evening peak window)

    • kWh used midnight–7am (overnight)

  2. Add them together → that’s the storage you’d need to cover evening + night.

  3. Add a buffer of 20–30% (real life + efficiency losses).

  4. Check your peak power (kW) needs: heat pumps + ovens can push power, and a battery must supply instantaneous power, not just energy.

Quick example (typical-ish all-electric household)

If evening+overnight is, say, 60% of that, you’re around 15 kWh to cover it — then add losses/buffer → ~18–20 kWh usable feels like a practical target.

So… solar-charged or night-rate charged?

Best answer: both, if your tariff allows it.

  • Solar shifting saves you exporting cheap and buying back expensive later.

  • Night-rate charging rescues winter and dull weeks, and can help power heat pumps/EVs more cheaply.

And if you’re already running solar + batteries (like your setup), the all-electric leap is mostly about honestly estimating winter demand, then deciding how much peak electricity you want to avoid buying

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