5 prosumer mistakes — and what each one actually costs

The costliest solar mistakes get made before installation or in the first six months after — when the system is already on the roof, contracts signed, and you can't easily change anything. Below are five real mistakes, with concrete numbers for what each one costs. Three of them happen at planning; two appear after you're already an active prosumer.

1. Oversizing because "I'll sell the surplus to the grid"

The mistake: you install 10 kWp on a house with 4,000 kWh/year consumption "because the grid will buy my surplus at a good price". Many buyers misread net-metering this way — they assume surplus above consumption is bought back at the same tariff the supplier sells at. It isn't.

The reality, in numbers: 10 kWp produces about 12,500 kWh/year in decent conditions. You consume 4,000 kWh/year. Net surplus: 8,500 kWh.

• Of those, 4,000 kWh get 1:1 compensation against your annual consumption (covers everything you draw from the grid) — real gain ~4,800 RON/year at a 1.20 RON/kWh tariff.
• The remaining 4,500 kWh get monetised at year-end at the supplier's buy-back price — 0.30–0.50 RON/kWh, not 1.20.
• At an average 0.40 RON/kWh on the surplus, you get 1,800 RON. Had you bought a 6 kWp system instead (~7,500 kWh/year), you'd have saved ~16,000 RON upfront and lost only ~1,200 RON/year in smaller production.

The lesson: size for your consumption + foreseeable additions (EV, heat pump), not for the "sell-the-surplus" idea. Use the payback calculator to see when each size pays back — oversizing usually extends payback, not shortens it.

2. The "just in case" battery

The mistake: you add a 10 kWh battery to the system without doing the break-even math. The seller talks "backup, autonomy, independence" — partly true but irrelevant as math.

The reality, in numbers: a 10 kWh LFP battery costs about 35,000 RON installed (Pylontech US5000, Dyness Powerbox G2, or similar — see Dyness vs Pylontech). Real annual gain depends on what the battery replaces:

• House with active 1:1 net-metering: the battery avoids injecting kWh into the grid at 1:1, but you'd recover them anyway through compensation. Real gain ≈ 0. The only benefit is paying the fixed component on the lower residual consumption (≈300–500 RON/year).
• House without 1:1 compensation (e.g., a 0.40 RON/kWh fixed buy-back): the battery recovers 1,000 RON/year if it cycles 250 kWh/month from day surplus to night consumption. Battery payback: ~35 years. Longer than battery life.
• TOU day/night tariff with small PV: rare scenario; if your day tariff is 1.80 RON and night 1.00 RON, a 5 kWh battery shifting 1,500 kWh/year nets ~1,200 RON/year. Payback ~15 years.

The lesson: if 1:1 net-metering is active on your contract, a battery doesn't pay back financially — you buy one only if you want backup during outages or you're planning partial off-grid. For the concrete calculation on your system, use the payback calculator with the battery checkbox on.

3. You install before the ATR is in hand

The mistake: you sign with the installer, pay the deposit, panels go on the roof. Then the Technical Connection Approval (ATR) request is filed with the distribution operator. The DSO finds that the service-line cable section is undersized or the technical project differs from the actual installation.

The reality, in numbers:

• Redoing the technical project (another ANRE-certified engineer): 800–2,000 RON
• Re-filing the ATR request with a new document set: another 30–60 days of waiting
• In rare cases, service-line modifications required by the DSO (breaker upgrade, distribution panel additions): 500–3,000 RON
• Total financial loss: 1,500–5,000 RON; time lost: 2–3 months.

Worse: if the DSO finds something that blocks the permit (e.g., the system exceeds the contracted power and the house can't upgrade without grid-side investment), the installation stays on the roof but can't be commissioned as a prosumer. You get direct self-consumption only, no injection.

The lesson: ATR always before installation. A serious installer won't start without DSO approval in hand. See prosumer contract for the correct order of steps.

4. Charging the EV or heat pump at the wrong time

The mistake: you have panels and an EV. You schedule charging at 22:00 (old TOU habit). Your system produces 30 kWh/day and injects it to the grid, then you buy 25 kWh at night at the full tariff.

The reality, in numbers: at 1.20 RON/kWh day/night uniform tariff and 0.40 RON/kWh buy-back, an EV consuming 4,500 kWh/year means:

• Night charging (wrong): 4,500 × 1.20 = 5,400 RON/year paid to supplier. Day surplus sells at 0.40 RON ≈ 1,800 RON/year back. Net: −3,600 RON/year.
• Day charging via direct self-consumption (right): 4,500 kWh covered from production in parallel; remaining surplus smaller but still at 0.40 RON. Net: −0 to −1,000 RON/year.

The annual difference is ~2,500–3,500 RON just from when you charge the EV.

Heat pumps have the same dynamic, less extreme: schedule the big cycles (domestic hot water, daytime space heating) when production is active. For details, see heat pump + EV with solar.

The lesson: direct self-consumption is the best "compensation" — 1.20 RON/kWh saved on consumption vs 0.40 RON/kWh earned on selling. Move EV charging to production hours; use a smart plug for the boiler that turns on when surplus exceeds 1 kW.

5. You confuse "kWh injected" with "kWh compensated"

The mistake: you look at the bill, see "energy injected: 480 kWh", calculate 480 × 1.20 = 576 RON, and expect a negative bill. A positive 35 RON bill shows up. You call the supplier convinced you're being cheated.

The reality, in numbers:

• kWh injected: your surplus pushed to the grid in a month. Shown as-is on the bill.
• kWh compensated: those from injection that auto-offset against your grid consumption (1:1) — limited by the amount you consumed from the grid.

Real example: you inject 480 kWh, consume 180 kWh from the grid. The 1:1 compensation covers the 180 consumed (active energy = 0 RON). The remaining surplus (480 − 180 = 300 kWh) rolls over into the compensation balance for the following months, up to 24 months. It doesn't trigger a payout that month.

The fixed bill components (35–50 RON/month) are:

• subscription + transport: 15–25 RON
• excise + VAT on residual consumption: 5–15 RON
• other ANRE-regulated components: 5–10 RON

Net-metering doesn't cover them — they're grid-usage costs, not active energy.

At the end of 24 months, leftover surplus (if any) is paid out at the buy-back price. That's when you get real cash, but not monthly.

The lesson: read the bill carefully — look for the "compensation balance" row (rolled-over kWh) or "active energy: 0 RON" (compensation applied). If injection isn't compensated 1:1, call the supplier with OUG 163 in hand — see what is a prosumer for compensation details.

How to avoid the five mistakes — short checklist

1. Before installation: use the calculator to size for your consumption + foreseeable additions. Use the payback calculator to check if the battery makes financial sense.
2. Get the ATR from the DSO via the installer before panels go on the roof. See prosumer contract for the order of steps.
3. After installation: schedule EV + heat pump on production hours. Smart plug for the boiler that triggers at surplus > 1 kW.
4. On the first prosumer bill: verify "compensation balance" and "active energy". If injection isn't compensated 1:1, that's when to call the supplier.

For what prosumers regret after 2 years of operation (concrete situations with real costs), see prosumer regrets after 2 years. Request a quote from AFM-verified firms and the installer handles points 1–2 correctly. Points 3–4 are entirely on you — but now you have the numbers.