I. Why Grid-tied Retrofit?
Why customers need retrofit: Selling low during the day, buying high at night
Surplus feed-in is not profitable: Solar generation peaks during the day when self-consumption is low. Surplus electricity is sold at low prices, and some spot markets even experience negative prices.
High-priced purchases at night: The cheap electricity sent away during the day must be bought back from the grid at high prices after dark, creating a 'sell low, buy high' loop.
Tightening Grid Policies: More and more regions require 'Self-Consumption, No Export'. Anti-backflow has shifted from a bonus to a mandatory constraint.
Peak-Valley Arbitrage Principle: Charge at Trough, Discharge at Peak
• Trough/Surplus Solar → Charging: Use low-price grid power and otherwise wasted solar surplus to fill the battery.
• Peak Periods → Discharging: Replace expensive grid power. When the peak-valley price difference is ≥ 0.7 RMB/kWh, energy storage provides arbitrage space.
What do customers get after retrofit?
✓ Peak-Valley Arbitrage: Charge low, discharge high; direct reduction in electricity bills.
✓ Improved Solar Utilization: No more low-price selling or wasted surplus energy.
✓ Policy Compliance: Proper anti-backflow avoids restricted or terminated generation.
Comparing Two Solutions: AC Coupling vs. DC Coupling
|
Dimension |
Option 1: AC Coupling |
Option 2: DC Coupling |
|
Efficiency |
Lower (3%-5% loss) |
Higher (3%-5% gain) |
|
Unit Price |
Lower (better LCOE) |
Low |
|
Initial Cost |
Higher |
Lower (Flexible expansion) |
|
Anti-backflow |
Complex link, latency |
Direct CT, ms-level response |
|
Capacity |
261kWh Large Capacity |
108kWh (Limited) |
|
Complexity |
Multiple devices, long setup |
Simple, few failure points |
Key Takeaway: High consumption & strong absorption → Choose AC (Large Capacity). Strict anti-backflow & cost-efficiency → Choose DC (Direct Coupling). The specific boundary is based on the '261 / 108 kWh' criterion.
II. Detailed Analysis of Five Application Scenarios
Scenario 1: Existing PV Feed-in + ESS Addition (Surplus Still Exported)
Context:
• Surplus selling is unprofitable (low or negative prices).
• High nighttime purchase prices.
• Presence of peak electricity pricing (spread ≥ 0.7 RMB/kWh).
Options & Analysis:
|
AC (125kW / 261kWh) |
DC (50kW / 108kWh) |
|
Pros: Capacity is large, lower unit price per kWh, suitable for high-consumption customers. Cons: Lower storage efficiency (3%-5% loss); higher initial investment. |
Pros: Higher efficiency (DC charging); lower initial investment, flexible expansion. Cons: Higher unit price; limited capacity (108kWh) might be insufficient. |
Selection Advice:
• Peak consumption ≥ 261 kWh → Recommend AC Solution.
• 108 kWh ≤ Peak consumption < 261 kWh → Recommend DC Solution.
• Peak consumption < 108 kWh → Storage not recommended.
Scenario 2: Existing PV Feed-in + ESS Addition + New PV (Surplus Still Exported)
Context:
• Existing PV accounts for a small ratio of total usage.
• Need to add PV panels to increase self-consumption ratio.
• Expanded surplus feed-in is still unprofitable; better to store it.
Options & Analysis:
|
AC (261kWh Cabinet + New Inverter) |
DC (50kW Hybrid + 108kWh) |
|
Pros: Large capacity; handles large new PV batches; lower unit price. Cons: Lower storage efficiency; higher initial investment. |
Pros: Higher efficiency; lower initial investment; simpler system (no new grid inverter). Cons: Limited capacity (108kWh); cannot handle massive PV additions. |
Selection Advice:
Same as Scenario 1 based on 261/108 kWh peak consumption threshold.
Scenario 3: Existing PV Feed-in + ESS Addition + New PV (New PV Forbidden from Exporting)
Context:
• Utility requires new PV to be 'Self-Consumption Only, Zero Export'.
• Hybrid policy: Old PV can export, new PV cannot.
• Reliable anti-backflow mechanism is mandatory for the new section.
Options & Analysis:
|
AC (261kWh + New Inverter + EMS) |
DC (50kW Hybrid + 108kWh) |
|
Pros: Handles large capacity and new PV batches. Cons: Anti-backflow relies on long communication chain (CT→EMS→Inverter), prone to latency/instant backflow; complex. |
Pros: Superior anti-backflow (CT direct to Hybrid, ms-level response); no new PV inverter needed. Cons: Limited capacity (108kWh); higher unit price. |
Selection Advice:
• Peak ≥ 261 kWh & Lax anti-backflow requirement → AC.
• Peak ≥ 261 kWh & Strict anti-backflow requirement → DC.
• 108 kWh ≤ Peak < 261 kWh → DC.
Scenario 4: Existing PV Feed-in + ESS Addition (Policy Change to Zero Export)
Context:
• Sudden policy shift: From feed-in allowed to 'Zero Export' required.
• Solar utilization drops as generation doesn't align with load spikes.
• Urgent retrofit deadline imposed by utility.
Options & Analysis:
|
AC (261kWh + Retrofit Anti-backflow) |
DC (50kW Hybrid + 108kWh) |
|
Pros: Large capacity; stores as much as possible in emergencies. Cons: Long anti-backflow link; risk of failed interception; long debugging cycle. |
Pros: Best anti-backflow effect; simplest system; fast installation and debugging. Cons: Limited capacity; higher unit price. |
Scenario 5: Existing PV Zero Export + ESS Addition (Activating Surplus)
Context:
• Built as Zero-Export, but solar often exceeds load during the day.
• Low PV utilization: Solar power is wasted during factory downtime/low load.
• High nighttime purchase prices while daytime energy is wasted.
Options & Analysis:
|
AC (261kWh + Anti-backflow) |
DC (50kW Hybrid + 108kWh) |
|
Pros: Large capacity to capture all daytime surplus. Cons: Complex system; communication latency risk; high initial cost. |
Pros: Best for strict 'Never Export' scenarios; lower investment; simpler system. Cons: Limited capacity; higher unit price. |
III. Decision and Implementation
Primary Selection Criterion: Peak Consumption
Principle: The essence of peak-valley arbitrage is charging at trough and discharging at peak. The stored energy MUST be fully discharged during peak periods to maximize arbitrage ROI. If you can't discharge it, the capacity is wasted.
|
Avg. Daily Peak Usage |
Decision |
Reason |
|
≥ 261 kWh |
AC (261kWh) |
Max arbitrage space; better unit cost. |
|
108 ~ 261 kWh |
DC (108kWh) |
Matches usage; lower investment. |
|
< 108 kWh |
Not Recommended |
Arbitrage cannot cover investment. |
How to get the data?
During site survey, obtain electricity bills or energy management data. Calculate the average daily consumption during peak periods (e.g., 18:00-21:00) and compare against the 261 / 108 kWh thresholds.
Quick Decision Table for Five Scenarios
|
Scenario |
Anti-backflow |
Peak ≥ 261 |
108-261 |
< 108 |
Notes |
|
Add ESS |
No |
AC |
DC |
None |
- |
|
ESS + New PV |
No |
AC |
DC |
None |
- |
|
ESS + New PV (Limited) |
Yes |
AC/DC* |
DC |
None |
DC better ms response |
|
Policy Change |
Yes (Strict) |
DC* |
DC |
None |
Policy compliance priority |
|
Retrofit Zero-Exp |
Yes |
AC/DC* |
DC |
None |
DC better response |
From Survey to Delivery: 5 Steps to Implementation
Site Survey: Assess transformer capacity, PV capacity, load curves, and grid policies.
ROI Calculation: Verify peak-valley spreads, select AC/DC, and estimate payback period.
Solution Design: Design anti-backflow logic, prepare electrical diagrams and equipment quotes.
Installation & Commissioning: Check CT wiring, set Hybrid/EMS parameters, and verify zero-export.
O&M Delivery: Monitor data, optimize strategies, and provide training and manuals.
Survey Memo: Three Essentials
1. Bills/Energy Data: Used to decide between 261kWh and 108kWh.
2. Grid Anti-backflow Requirements: Decides between AC and DC solutions.
3. Connection Point & Transformer Margin: Confirms if the project is feasible.