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Economics: payback, and what a battery really costs per kWh

Solar PV physics · Intermediate · about 12 minutes · part of Solar PV Systems

A solar system is bought once and paid back by the kWh it replaces: payback = cost ÷ (kWh replaced × tariff). Panels last 25 years losing about 0.5 % a year; batteries are bought by the cycle. That is where lead-acid and lithium part ways: a lead-acid bank costs a third up front and three to five times as much per kWh stored over its life, because it gives half its nameplate for 450 cycles against 85 % for 3000.

Economics: payback, and what a battery really costs per kWh: the circuit on the simulator's bench

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How it works

Two banks with about the same usable energy: a 200 Ah lead-acid (2480 Wh nameplate, 1240 Wh usable, about ₱16,120) and a 100 Ah LiFePO4 (1280 Wh, 1088 Wh usable, about ₱23,040).

Over their lives: lead-acid 450 cycles × 1240 Wh = 558 kWh stored for ₱28.9 per kWh; LiFePO4 3000 × 1088 Wh = 3264 kWh for ₱7.1 per kWh. Prices are the studio’s (editable there).

Where lead-acid still wins: standby duty (a UPS that cycles twice a year), a budget that cannot stretch, places with no lithium supply. Where it loses: anything cycled every night.

Try it on the bench

  1. Let it run: the lead-acid hits 50 % (its working floor) at about the same moment the LiFePO4 is at 80 %.
  2. Open the Solar studio, step 5: change the tariff and the prices and watch the payback.
  3. Change the studio’s chemistry to lead-acid: the bank doubles and the lifetime cost per kWh is shown beside lithium.

Parts on the bench

In the course

Solar PV Systems, module “13 · Economics”.

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