fix: allow negative Export Compensation price

CHANGELOG.md

@@ -22,6 +22,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Fixed
 
+- **Export Compensation can now be set to a negative value** — needed when a grid operator charges for export instead of paying for it; the field silently allowed only non-negative values. ([#666](https://github.com/johanzander/bess-manager/issues/666))
 - **A brief Nordpool hiccup no longer produces a daily "recovered from an earlier issue" notice** — the health check re-fetched today's prices every five minutes instead of using the ones already held. ([#662](https://github.com/johanzander/bess-manager/issues/662))
 - **Growatt VPP now lets the inverter and BMS sleep through a long idle at minimum SoC** — an empty battery was still held under continuous remote control, which nothing was protecting. ([#592](https://github.com/johanzander/bess-manager/issues/592))
 - **A tiny solar surplus is no longer planned as an export the inverter will absorb** — below the export the plan can express, the battery charged anyway and ran fuller than planned, spilling the difference later. ([#630](https://github.com/johanzander/bess-manager/issues/630))

frontend/src/components/settings/PricingFormSection.tsx

@@ -134,7 +134,7 @@ export function PricingFormSection({ form, onChange }: Props) {
                 { unit: 'factor (1.0 = no adjustment)', min: 0.5, max: 2.0, step: 0.0001 })}
               {numField('Export Compensation', form.taxReduction,
                 v => onChange({ ...form, taxReduction: v }),
-                { unit: `${currency}/kWh`, min: 0, step: 0.001 })}
+                { unit: `${currency}/kWh`, step: 0.001 })}
             </div>
             {isEntsoe ? (
               <div className="rounded-lg bg-blue-50 dark:bg-blue-900/20 border border-blue-200 dark:border-blue-800/40 px-4 py-3 space-y-3 text-xs text-gray-600 dark:text-gray-400">

frontend/src/components/settings/__tests__/PricingFormSection.test.tsx

@@ -0,0 +1,42 @@
+import { render, screen, fireEvent } from '@testing-library/react';
+import { describe, it, expect, vi } from 'vitest';
+import { PricingFormSection } from '../PricingFormSection';
+import type { PricingForm } from '../PricingFormSection';
+
+const BASE_FORM: PricingForm = {
+  currency: 'SEK',
+  provider: 'nordpool_official',
+  nordpoolConfigEntryId: '',
+  nordpoolEntity: '',
+  octopusImportTodayEntity: '',
+  octopusImportTomorrowEntity: '',
+  octopusExportTodayEntity: '',
+  octopusExportTomorrowEntity: '',
+  entsoeEntity: '',
+  area: '',
+  markupRate: 0,
+  vatMultiplier: 1.25,
+  additionalCosts: 0,
+  taxReduction: 0,
+  spotMultiplier: 1.0,
+  exportSpotMultiplier: 1.0,
+};
+
+describe('PricingFormSection', () => {
+  it('allows a negative Export Compensation value (nätnytta can be a cost, not just a credit)', () => {
+    const onChange = vi.fn();
+    render(<PricingFormSection form={BASE_FORM} onChange={onChange} />);
+
+    const input = screen.getByLabelText(/Export Compensation/i);
+    expect(input).not.toHaveAttribute('min', '0');
+
+    fireEvent.change(input, { target: { value: '-0.05' } });
+    expect(onChange).toHaveBeenCalledWith(expect.objectContaining({ taxReduction: -0.05 }));
+  });
+
+  it('reflects a negative Export Compensation in the sell price preview', () => {
+    render(<PricingFormSection form={{ ...BASE_FORM, taxReduction: -0.05 }} onChange={vi.fn()} />);
+
+    expect(screen.getByText('0.95 SEK/kWh')).toBeInTheDocument();
+  });
+});