This is an old revision of the document!
The "Big Battery" Maths: Why our 14.75 kWh is a Game-Changer for us
If you're still on the fence about battery storage, let's talk numbers. I'm running a 14.75 kWh setup, and for my household, it's not just a hobby—it's a massive money-saver. I'm doing my level best to keep the lights on without having to sell a kidney to pay the energy bills!
My System Setup
- Panels: 21 x 435W (9.135 kW total) — 10 East, 7 West, and 4 South.
- Inverter: GivEnergy 5kW.
- Storage: 14.75 kWh GivEnergy battery bank (5.2 kWh Gen 1 + 9.5 kWh Gen 2).
What are we running?
We've got a busy house. Along with all the usual appliances, we're running a tech-heavy setup, including a Home Assistant server, a Hyundai Kona EV, and an electric hot water heater. We cook with gas and heat the home with oil and a log burner, so the battery is working hard to cover everything else.
Estimated Annual Cost according to my back of a fag packet maths
| Scenario | Estimated Annual Cost |
|---|---|
| No Solar / No Battery (at 35c/kWh) | ~€2,623 |
| With Solar / No Battery (inc. 25c export) | ~€1,700 |
| With 14.75 kWh Battery (inc. 25c export) | ~€95 |
💡 The takeaway: Battery takes it from €1,700 down to ~€95 — that's the buffer against high prices in action.
How I make it work
The Nightly Top-Up: I charge my 14.75 kWh battery every single night at my Pinergy EV rate of 6c a kWh. It's the closest thing to legal robbery you'll find in this country.
The Grid Bridge & Clipping Prevention: By dumping the battery to the grid in the morning, I create enough “headroom” to ensure I never hit a clipping limit when the sun hits the panels full force at midday—because we all know the Irish sun likes to be dramatic when it actually decides to show up.
The Smart Charging Routine: I charge the car only during the cheap 02:00–05:00 EV window. Why? Because charging the car from the sun is a false economy when I can earn 25c/kWh for every unit I export to the grid. I'm not a charity, after all!
Excess Management: All excess solar either goes straight into the house battery to top it up or heads out to the grid to earn me that 25c credit. If the forecast shows enough solar, I run this cycle 365 days a year.
The Bottom Line
By using a large battery to bridge the gap between my solar generation and the grid, I've effectively insulated my household from the high flat rate (42c kWh), resulting in substantial annual savings compared to solar alone.
If you drive a lot in an EV or a heavy household load, the math isn't just “grand”—it's a no-brainer. Don't look at the battery as just a backup box; look at it as your personal buffer against high energy prices and the inevitable madness of the utility companies.
☕ Solar Sid's Tip: Want to run your own numbers? Plug your MPRN data into my Irish Solar Calculator and test different battery sizes — that's exactly what this strategy is based on.