Figure A. Comparison of diesel generator plus UPS, (Option A), and diesel generator with UPS and EVR (Option B). This figure demonstrates payback period based upon the required hours of diesel backup due to sags. At 200 hours of annual diesel runtime to address voltage brownouts, an EVR pays for itself in under 2 years. From that point on, every hour of usage further reduces the total cost of owning an EVR. If the diesel runtime is reduced by 300 hours annually, the payback period is reduced to 1.2 years. At 400 hours, the payback period is just under 1 year.