The real cost of your energy: what it costs to produce one kWh at home with a LiFePO4 battery

When you open the fridge, you don't think about how many cents each minute of the compressor costs. You pay a monthly bill and that's it. But two prices of electricity coexist in your home, and if you want to make a good decision about self-consumption you need both.

The first is the price you pay the utility: what appears on your bill, made up of generation cost + grid fees + taxes + VAT. In Spain, with the 2.0TD tariff, it ranges between €0.19 and €0.32 per kWh depending on the time of day. That's the visible price.

The second is the price it costs you to produce it yourself: the price of the kWh that comes out of your panels or your battery. Nobody invoices it, so almost nobody calculates it. It has a name — LCOE, levelised cost of energy — and it is the only number that lets you compare producing against buying.

This article works it out end to end for a real installation, and shows the five places where the figure usually gets quietly improved.

What LCOE means: cost per kWh like the cost per kilometre of your car

The clearest analogy is the cost per kilometre of your car. When you work out what each kilometre really costs, you don't just add up petrol. You include the purchase price spread over the years you'll own it, insurance, tyres, servicing. LCOE is the same idea applied to energy:

LCOE = (all lifetime costs) ÷ (all useful kWh over that lifetime)

Two rules make the difference between a real number and a brochure number. Everything that has to be paid goes in the numerator, including replacements. And only energy you actually consume goes in the denominator — not everything the panels generate.

The Spanish 2.0TD tariff: why the grid has three prices a day

Since 2021 every home with up to 15 kW of contracted power is on the 2.0TD tariff, which splits the day into three periods with three different prices.

The reference prices in this article are a real, checkable contract: Som Energia's 2.0TD, in force from 1 October 2026. We use Som Energia because it is a co-operative with a fixed quarterly price and a public contract, sitting in the mid-to-upper band of the Spanish market. Comparing our numbers against it makes them conservative, not flattering.

For a typical home consuming 5,000-7,000 kWh/year, the weighted average grid price works out at about €0.23/kWh all included. That's the figure to hold in your head for the rest of this article.

One 2026 development worth knowing: with massive solar generation at midday, the middle of the day often comes out cheaper than the flat period, sometimes cheaper than the small hours — even though it is administratively "peak". Where the price really bites is the evening peak, 18:00-22:00, when the sun has gone. That is exactly the window a battery covers, and it is why the value of storage keeps concentrating in the evening.

Solar production: catalogue figures vs reality on the roof

A 650 Wp panel gives 650 W under laboratory conditions (1,000 W/m², cell at 25 °C, AM1.5 spectrum) that essentially never occur on a roof. The European standard tool for estimating real output is PVGIS, which applies a Performance Ratio typically between 75% and 85% for a well-designed southern-European installation.

For our case — 8 × Longi LR7-72HVH-650M (5.2 kWp), south-facing, 30° tilt, Catalan coast:

That 19% gap is not pessimism. It is heat (the biggest single loss: −0.30%/°C, and modules run at 65-70 °C in summer), inverter conversion, wiring, soiling, mismatch and degradation. Every number below uses the post-PR figure.

The journey of a kWh from the sun to the lightbulb

Not everything the panels generate reaches your consumption, and what matters is how many conversions each kWh goes through:

• Flow A — Direct consumption: panels (DC) → inverter (AC) → home. One inverter pass.
• Flow B — Via battery: panels → inverter → battery (charge) → battery (discharge) → home. Two inverter passes plus the cell round-trip.
• Flow C — Surplus to the grid: compensated at roughly 5 c€/kWh, an order of magnitude below what it costs to buy back.

The efficiency of the battery route is the product of the whole chain:

95.25% (inverter, DC→AC) × 97% (LFP round-trip) × 95.25% (inverter, AC→DC and back) = 88.0%

Of every 100 kWh that enter the battery route, 88 reach the load. That 12% is the energy price of having the power when you need it rather than when the sun offers it.

The LCOE formula and its five traps

The formula is simple. Getting it honest is not. These are the five places the figure gets improved, in ascending order of subtlety:

Trap 1 — Component replacements. A 30-year system needs more than one inverter. Costing a single unit over the full horizon is the most common omission.

Trap 2 — Performance Ratio. Using catalogue Wp instead of PVGIS output inflates the denominator by around 19%, and the LCOE falls by the same proportion for free.

Trap 3 — Injected surplus. Counting exported kWh as "produced" in the denominator lowers the LCOE artificially. You never consume them; they're sold at 4-8 c€ and belong in the payback analysis, not here.

Trap 4 — Discount rate. A euro saved in 25 years is worth less than a euro today. This article uses the simple engineering calculation, with no discounting. If you want the NPV version, multiply the resulting LCOE by roughly 1.2-1.5 at a 3-5% discount rate.

Trap 5 — Subtracting savings inside the LCOE. The subtlest one, and the source of the most spectacular brochure numbers. It means removing things from the numerator that aren't production costs: the tax deduction, the saving on contracted power, the surplus income. The result is an artificially low "LCOE" that then gets compared against the grid price — counting the same benefit twice.

Our rule: the LCOE contains costs only. Savings are real and material, but they belong in the payback analysis, each attributed to whatever makes it possible. Every figure below follows that rule.

If a salesperson shows you a four-year battery payback, or a "cost per kWh" of one or two cents, ask which of these five they used.

Case study: 5.2 kWp + SB Home30 + Victron MultiPlus

Real numbers from a real installation, over a 35-year system horizon.

The battery works out at €173/kWh, and that includes grade-A LFP cells, a SolarBox 100 A continuous BMS, enclosure, electronics and assembly — a battery ready to install, not loose cells.

Separately, and deliberately outside the LCOE (Trap 5), the benefits that belong in the payback analysis: a 40% income-tax deduction capped at €3,000, less €400 for the two mandatory energy performance certificates, so €2,600 net; and about €90/year saved by cutting contracted power from 5.75 to 3.45 kW, which only the battery makes possible.

Where the energy actually goes

The reference household consumes about 6,900 kWh/year — the profile of an electrified home with a heat pump and evening load. The hourly split below is calibrated against real data from SolarBox installations monitored by our fleet manager:

Of the ~6,900 kWh consumed, 6,213 come from the sun and about 690 residual kWh are bought from the grid, mostly on consecutive dull winter days. Self-sufficiency: around 90%.

Note the 55% going through the battery: in a house with a heat pump or an EV, evening and night consumption is far larger than daytime. Without a battery those kWh would be sold as surplus at ~5 c€ and bought back in the evening at 21-32 c€.

LCOE by route: the number that matters

Splitting the costs by route is the whole point. A kWh used directly and a kWh that has been through the battery do not cost the same, and averaging them hides the useful information.

How the battery route gets to €263.78: the shared costs (panels + inverter + installation, €198.98/year) are split by energy share, 58% to the battery route — €115.49 — plus the battery itself at €148.29, which is 100% attributable. The direct route takes the remaining 42%, €83.50, and nothing else.

So the battery adds about 4.1 c€/kWh to the energy that passes through it. That is a real extra cost and we are not hiding it. But it buys a kWh at 7.32 c€ that would otherwise be bought at 31.7 c€ in the evening peak.

One clarification on that 7.32, without breaking Trap 5: it is the gross cost of producing the kWh. The battery also brings a saving no other route can — the €90/year off the contracted power term. Subtract only that, and the effective net cost of battery energy is around 4.8 c€/kWh.

When does the battery pay back?

This is where the answer changes most with the method. Three legitimate methods, three different numbers, for the SB Home30 at €5,190:

Method A — the harshest. Subtracts the gross battery LCOE (7.32 c€) from the grid price, so only the margin left after the battery has already "recovered" its own capital inside the LCOE counts as saving. It makes the battery pay for its capital twice.

Method B — the most optimistic. Battery cost divided by the grid price of the kWh it replaces, no adjustments. This is the one low-cost sellers use: it flatters the result because it ignores the surplus income you no longer earn, now that you store those kWh instead of exporting them.

Method C — the honest one, and the one we recommend. What you stop paying the grid, minus the surplus income you no longer receive, plus the contracted-power saving that only the battery makes possible. Worked out for the realistic case, at an average avoided price of 24.08 c€/kWh:

Gross saving:      24.08 c€ × 3,605 kWh   =  868.08 €/year
− Surplus income forgone (4,097 kWh × 5 c€) = −204.85 €/year
+ Contracted power saving                   =  +90.00 €/year
                                              ─────────────
= NET saving                                =  753.23 €/year

Payback: €5,190 / €753 per year = 6.9 years

If you also attribute the battery's proportional share of the tax deduction — about €1,486 — the amount to recover drops to €3,704 and the payback to 4.9 years.

The important message in that table is not any single figure. It is the sensitivity to when you use the energy: between 5.0 and 9.0 years depending on nothing but timing.

A conclusion that changed in October 2026. Earlier versions of this article said plainly that if all your consumption falls in the off-peak period, a battery makes no sense: at 12 c€ off-peak the payback ran past 16 years. Som Energia's October increase raised the off-peak price by 20.9%, to 19.1 c€. Against a production cost of 7.32 c€, the margin is now there, and the payback is 9.0 years. It remains the worst of the three scenarios — but it is no longer a bad deal, and we would rather say so than leave the old advice standing.

Why your LCOE will be different

Three honest reasons the number in this article won't exactly match yours:

1. Your location. PVGIS production in Catalonia is ~1,432 kWh/kWp; in Germany it's ~950; in Andalusia ~1,600. The same hardware produces different amounts.

2. Your consumption pattern. A family at home during the day (retirees, remote workers) uses much more direct solar; a working family that's out 9-18h depends more on the battery. The split between Flow A and Flow B changes the LCOE.

3. Your installer's pricing. Quality varies. Quotes for the same kit can differ ±30%. Always demand: PVGIS report, real efficiency figures (not catalogue), and clarity on what's included (often electrical work, scaffolding, paperwork are extras).

Sodium-ion: what's coming

Sodium-ion cells (SIB) are reaching commercial maturity in 2026-2027. They promise: lower material cost (sodium is abundant, no lithium needed), better behaviour at low temperatures, and similar cycle life to LFP. The catch in 2026 is lower energy density (~30% bulkier than LFP for the same kWh) and still-limited grade-A supply.

At SolarBox we're closely monitoring sodium-ion. We've covered it in depth in our Catalan article on sodium-ion cells democratising storage. The short version: when grade-A SIB is widely available at €100/kWh, the LCOE numbers above will drop another 15-20%, and the case for storage becomes essentially undeniable.

Conclusion

The real cost of one kWh of self-produced energy is not a single number, and that is the main thing to take away. Used directly it costs 3.20 c€/kWh; once it has been through the battery, 7.32 c€/kWh. The battery adds about 4 c€ to the kWh that passes through it — and even so that kWh is 4.3× cheaper than buying it at the evening peak, and still 2.6× cheaper than buying it at the cheapest hour of the day.

Weighted across both routes, the energy this household produces costs 5.59 c€/kWh against a grid average of about 23 c€ — roughly 4.2× cheaper. Those are gross figures, with no tax deduction and no contracted-power saving subtracted: both are real, and both are counted in the payback section where they belong.

What separates a profitable installation from a so-so one isn't the headline price — it's how well the system is dimensioned for your actual consumption pattern, and how transparent your installer is about real (not catalogue) production figures.