The overwhelming majority of incorrectly sized PV installations are born the same way: the supplier offers "what fits on the roof" and the client buys the larger of two quotes. That skips three questions that alone determine whether the investment pays back — the daily consumption curve, the blackout scenario, and the battery chemistry. This article is a technical sizing exercise, not a marketing ROI sheet.
Step 1 — how much sun your district actually gets
Slovakia isn't homogeneous. 1 kWp installed in Bratislava yields a different annual energy than in Prešov or under the Tatras:
- **Bratislava, Trnava, Nitra (SW Slovakia):** **1,020–1,080 kWh/kWp/year** (PVGIS-SARAH3, 35° tilt, south)
- **Žilina, Trenčín, Banská Bystrica (centre):** **940–1,000 kWh/kWp/year**
- **Prešov, Košice (E Slovakia):** **960–1,020 kWh/kWp/year** (paradoxically better than the centre due to less cloud cover)
- **Tatras, Spiš, Orava (northern highlands):** **850–920 kWh/kWp/year** (longer winter, more snow, but also higher clarity in summer)
**Always look up the PVGIS calculation** for the specific site (geo location + tilt + azimuth + shading). Spreadsheet estimates of "1,000 kWh/kWp" are an average that holds for a prosperous average, not for your house.
Step 2 — the daily consumption curve is decisive
Annual consumption is a trivial number from the bill. What actually determines sizing is the hourly consumption profile. Four scenarios:
Scenario A — "home with nobody during the day" (classic working family)
Consumption: 70% evening (18:00–23:00), 20% morning (06:00–08:00), 10% during the day. The sun shines when nobody is home. **Without a battery, 75% of generation goes to the grid at 4 c/kWh**, while evening consumption is bought at 22 c/kWh.
**Sizing**: 4–6 kWp PV + 8–10 kWh battery. Without a battery, payback stretches to 12–16 years. With a battery 9–12 years.
Scenario B — "home-office + family during the day" (post-covid standard)
Consumption: 35% during the day (laptops, air-con, cooking), 50% evening, 15% morning. **Without a battery, 50–60% of generation goes to self-consumption.** ROI better than Scenario A.
**Sizing**: 5–7 kWp PV + 5–8 kWh battery (no more — the battery wouldn't manage to charge with daytime consumption). Payback 8–11 years.
Scenario C — "heat pump + EV" (growing segment)
Consumption: 50% during the day (heat pump heating when PV warm, EV charging during the day if it's home), 40% evening, 10% morning. **Self-consumption 70–80% even without a battery.**
**Sizing**: 8–12 kWp PV + 10–15 kWh battery. Payback 6–9 years, because absolute consumption is high and every saved kWh goes straight to the wallet.
Scenario D — "off-grid / socially risky location"
The client lives in a location with frequent outages (countryside, flood zones, areas with risk of long WWTP outages). The goal isn't primarily ROI, but **operational continuity during a 1–3 day blackout**.
**Sizing**: 8–12 kWp PV + 30–60 kWh battery + petrol generator 5–7 kW as tertiary backup + careful load management (turn off HP, washing, ironing during the blackout). Investment 35,000–55,000 EUR. ROI isn't the primary metric; the metric is "how many days without grid and without indoor temperature below 18 °C."
Step 3 — battery chemistry decides lifespan
Two main chemistries for residential batteries in 2026:
LiFePO4 (lithium iron phosphate)
- **Nominal voltage**: 3.2 V per cell
- **Cycle life**: 6,000–8,000 cycles at 100% DoD (depth of discharge) declared; **real 4,500–6,500 cycles** on Tier 1 brands (BYD, Pylontech, Huawei, Tesla Powerwall 3 with LFP variant)
- **Calendar life**: 12–18 years at the right temperature regime (10–35 °C)
- **Safety**: thermal runaway at 270 °C+, practically doesn't explode
- **Energy density**: 90–120 Wh/kg
- **Price**: 280–420 EUR/kWh delivered + installed (2026)
**Use**: 95% of residential installations in 2026. **Default choice.**
NMC (nickel manganese cobalt)
- **Nominal voltage**: 3.7 V per cell
- **Cycle life**: 3,000–5,000 cycles at 100% DoD
- **Calendar life**: 8–12 years
- **Safety**: thermal runaway at 150 °C, higher fire risk (Samsung Galaxy Note 7, former Tesla Model S)
- **Energy density**: 150–220 Wh/kg
- **Price**: 320–480 EUR/kWh
**Use**: today mainly in EVs (because of higher density). In residential fixed-installation PV batteries **we no longer recommend it** because of shorter lifespan and higher risk.
Lead-acid — no
Still occasionally seen in cheap off-grid installations. Cycle life 500–1,200 cycles, calendar life 4–8 years, heavy, inefficient. **Don't use for new projects.** Exception: gel batteries for small cabins with consumption under 1 kWh/day, where the 80 EUR/kWh price still beats LFP 350 EUR/kWh.
Step 4 — sizing decision tree
Example 1: Mid-size family, family house without HP, 5,500 kWh/year, Bratislava
- PV: 6 kWp (annual generation ~6,300 kWh, 115% of consumption — optimum)
- Battery: 8 kWh LFP (Pylontech US3000C or BYD Battery-Box Premium HVS 7.7)
- Self-consumption: 65–75% (with battery)
- CAPEX 2026: 13,500–15,800 EUR for the complete bundle including hybrid inverter (Fronius Symo GEN24 or Huawei SUN2000), inspection, design
- ZD IV subsidy: 1,700 EUR (PV) + 1,200 EUR (battery) = 2,900 EUR
- Net CAPEX: 10,600–12,900 EUR
- Annual saving: 720–880 EUR
- **Payback: 12–16 years**
Example 2: Family with HP + EV, Prešov, 11,500 kWh/year
- PV: 10 kWp (annual generation ~10,200 kWh, 89% of consumption — battery helps raise self-use)
- Battery: 12 kWh LFP (BYD HVS 11.5 or a pair of Pylontech US5000)
- 11 kW EV charger with smart-charging (KEBA P30, Wallbox Pulsar Plus, OCPP-compatible)
- Self-consumption: 80–88%
- CAPEX 2026: 22,500–26,000 EUR
- ZD IV subsidy: 2,700 EUR (PV) + 1,800 EUR (battery) + sometimes 1,000 EUR (smart charger) = 5,500 EUR
- Net CAPEX: 17,000–20,500 EUR
- Annual saving: 2,300–2,800 EUR
- **Payback: 7–10 years**
Example 3: Off-grid cabin, Tatras, 3,200 kWh/year
- PV: 8 kWp (annual generation ~6,800 kWh — well over consumption, because winter generation is very low)
- Battery: 30 kWh LFP (BYD HVS for a professional system, or 2× Pylontech Force H2 stack)
- Generator: Honda EU22i or similar 2.2 kW inverter generator for January weeks with 3-day cloud cover
- Hybrid inverter: Victron MultiPlus-II 48/5000 or SMA Sunny Island
- CAPEX 2026: 38,000–48,000 EUR
- No subsidy (off-grid isn't covered by ZD IV)
- ROI isn't the primary metric — the alternative (DSO connection, 800 m distance) would cost 25,000–40,000 EUR + annual connection fee
Step 5 — when a battery makes no sense
Below 2,500 kWh/year annual consumption
A small household (one or two people). With PV without a battery, self-consumption is 30–40%; a battery lifts it to 65–75%. Absolute difference: 600–900 kWh saved per year extra, i.e. 130–200 EUR saved. Battery CAPEX 4,000–6,000 EUR. **Payback > 25 years** — longer than battery lifespan.
Roof area < 25 m² and inadequate tilt
If the PV system is < 3 kWp, the battery only ever soaks up energy from 2–3 hours a day. Self-consumption ends up so low that the battery still only discharges to 30–40%. The cycle life is used but ROI isn't.
Commercial tariff with low variable cost
Some commercial tariffs in SR have an energy component of 130–150 EUR/MWh (vs. 220+ EUR/MWh residential tariff). At that low consumption tariff a battery loses 30–40% of ROI against residential. In commercial projects, batteries are used for peak-shaving (reducing peak draw to manage the capacity tariff), not for self-consumption.
Step 6 — when a battery makes sense beyond ROI
Backup for critical loads
The client runs home-office with a SaaS business. An hour of outage = 200 EUR of lost revenue (client calls, demos, support). 5 outages per year = 1,000 EUR. A 10 kWh battery as backup for router + workstation + monitors + LED lights handles 8–12 hours. ROI isn't in self-consumption, ROI is in continuity.
Time-of-use arbitrage
With a tariff with a substantial peak/off-peak gap (e.g. nightly D2 vs. daily D1 — gap of 6–9 c/kWh) the battery charges at night and discharges during the day. Real saving: 0.5–1.2 kWh × 365 days × 0.07 EUR ≈ 130–300 EUR/year. Without PV. With PV: the battery optimises the combination — charges from solar during the day, plays grid arbitrage at night.
Grid stability fee avoidance
In some Slovak regions (especially Bratislava region, Trnava region) a **capacity tariff** is being prepared for residences with higher installed connection capacity (> 14 kW). A battery makes it possible to keep max grid draw below the threshold, saving 8–15 EUR/month in capacity fees.
Three engineering details that sometimes get forgotten
1. Hybrid inverter vs. AC-coupled battery
**Hybrid inverter** (Fronius Symo GEN24, Huawei SUN2000, Sungrow SH10RT) has an integrated PV inverter + battery inverter + management in one box. Efficiency 96–98%. Price 1,800–3,500 EUR depending on size. **Default choice for new installations.**
**AC-coupled** battery (Tesla Powerwall 3, Sonnen) has its own inverter and ties into the AC bus of an existing PV installation. Efficiency 88–92% (double AC-DC conversion). Higher price (5,500–8,500 EUR for 10 kWh + inverter). **Use only for retrofitting existing PV without a hybrid inverter** (the cost of replacing the inverter offsets the AC-coupled surcharge).
2. EMS — Energy Management System
Without an EMS the battery runs in "greedy" mode: charges when there's sun, discharges on consumption. That's sub-optimal. **Smart EMS** (Solar-Log, Open Energy Monitor, or built into Fronius/Huawei) optimises:
- **Weather forecast integration**: if tomorrow looks bad, tonight the battery keeps a 20% reserve. If tomorrow is sunny, the battery can discharge to 5% today.
- **Time-of-use scheduling**: charge during night off-peak, discharge during daytime peak.
- **EV smart-charging**: the EV charger talks to the EMS, charges only on solar surplus or night off-peak.
EMS surcharge: 400–1,200 EUR (software licence). ROI: 6–18 months.
3. Installation in the right room
The battery wants **5–35 °C** for optimal lifespan. Above 35 °C calendar life drops exponentially (Arrhenius). Below 5 °C the charge current must be limited (LFP must not charge below 0 °C).
**Right rooms**: technical room in the house, boiler room (if it's below 30 °C), basement with ventilation. **Wrong**: unheated garage (winter -10 °C in January), attic (summer +45 °C), outdoors.
Cost of installing the battery in the right room vs. the wrong one: 0 EUR. Shortening of lifespan from the wrong room: 2–5 years out of 12–15.
Our default recommendations
- **Small household (< 4,000 kWh/year)**: 4–5 kWp PV without a battery. The battery doesn't return.
- **Mid-size (4,500–7,000 kWh/year), no HP/EV**: 6 kWp PV + 8 kWh LFP. Payback 11–14 years.
- **Large with HP/EV (8,000–14,000 kWh/year)**: 10–12 kWp PV + 12 kWh LFP + smart EV charger. Payback 7–10 years.
- **Off-grid or high backup demand**: 8 kWp+ PV + 30 kWh+ LFP + 5 kW petrol generator as backup. ROI isn't calculated the classical way.
- **Commercial (> 30 kWp)**: PV 30–100 kWp + 30+ kWh battery with peak-shaving. ROI 5–8 years on the right sizing.
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*We design and install PV + battery + EV chargers for family houses and commercial sites. The first sizing workshop (60 minutes online) walks through your real hourly consumption data from invoices across the 4 scenarios — and usually shows that the cheapest quote isn't the one with the best ROI.*