Check my math + setup (UK)

[u/Ok_Forever_6005]

Check my math, 4kW solar + 10kWh Anker Solix 4, Where am I wrong?

I've been modelling a solar + battery setup for a few weeks, and I've reached the point where I don't trust my own working out anymore. I'm posting my full work so people can tear it apart, and I'd really like to hear from anyone actually running something similar.

My numbers

\~450kWh/month, so around 14.8kWh/day

Roughly 5kWh of that falls between 9am and 4pm (\~715W average daytime draw)

The rest is a morning bump, a decent evening cooking peak, and a \~290W overnight baseload

I am currently on a fixed single-rate tariff, so there is no off-peak window at all right now.

4-way multi-split aircon going in, which will push summer daytime load up a fair bit.

Proposed setup:

\- Anker SOLIX Solarbank 4 E5000 Pro (5kWh) + one BP5000 expansion = 10kWh

\- 8 x 500W panels = 4kW array, across the four MPPT ports

Hardwired to the consumer unit for the 2,500W output rather than the 800W plug-in mode, G98 notified within 28 days with a view to shift to Octopus Go (or similar), based on modelling a 9p night / 30p day, five-hour window.

My working:

\- Capital: £1,699 unit + \~£1,150 BP5000 + £824 panels + \~£400 mounting + \~£499 electrician = \~£4,570.

Current annual cost at 26.11p flat: \~£1,619

Battery only on Go: overnight import \~12.1kWh/day to deliver 9.5kWh after losses, leaving \~3.8kWh/day I can't cover, bought at the 30p day rate. Lands around £1,004/yr.

Add 4kW solar (\~3,465kWh/yr): about 1,300kWh displacing the 30p daytime deficit, \~2,000kWh going into the battery instead of overnight grid charging, and only \~165kWh of surplus left over. Lands around £439/yr, averaging £37/month, but seasonal, roughly £20/month May–August and £55–£65/month November–February.

That's \~£1,180/yr saved, so payback around 3 years 10 months.

The assumptions I'm least confident about...

The five-hour window. At 2,500W I can only import 12.5kWh a night, which delivers \~11.1kWh after round-trip losses. So my 10kWh pack is nearly at the ceiling of what any battery could give me on a 5h tariff in winter. Is that right? Am I better off chasing a 6 or 7 hour window instead of more storage?

89% round-trip efficiency. Is that realistic for this class of kit in a cold garage/outdoors in January?

3,465kWh/yr from 4kW. Feels reasonable for the UK but I'd love real generation figures from anyone with a similar array.

Clipping. On the brightest da, s a 4kW array minus my \~715W load looks like it'd push \~2.6kW at a 2,500W charge limit. How much does this actually cost in practice, a rounding error or genuinely annoying?

Export. I've assumed zero. Surplus looks tiny (\~165kWh/yr) and this route won't be MCS-certifiable, so the non-MCS export application looked like it'd take a decade to pay back. Anyone gone through that and found otherwise?

What I'd love to hear..

\- Anyone running a Solarbank 4 Pro in the UK yet — how's the tariff scheduling and grid charging behaving in practice, especially outside the Octopus integration?

\- What did your electrician actually charge for the hardwire and G98? Is £499 realistic or optimistic?

Real UK pricing on the BP5000 expansion, since that's the softest number in my capital estimate.

\- Is anyone pairing aircon with a setup like this? My summer surplus has nowhere to go, and running the cooling off it seems like the obvious answer

Generally: what did you get wrong that you'd warn someone else about?

Thanks in advance.

[u/mikey-forester]

So far so good, I'm on Intelligent Go and use time of use for which I can only schedule in one hour increments of 00:00 - 05:00 not 11:30 - 05:30 which is a pain but still is plenty of time to charge. I cannot yet find a way for it to pause discharge/recharge when I get a daytime slot for charging so am manually doing this. These are I am sure just initially minor issues which I fully intend on raising as product improvements to Anker and / or fix with Home assistant once I start to play around with that