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Thabiso KaliEnergy engineer · CEM

For engineers and facility managers

Solar PV yield and self-consumption estimator

How much a rooftop PV system generates depends on where it is; what it is worth depends on how much of that energy the site uses itself. This estimator does both, for South African cities.

Inputs

Pre-filled with an illustrative case. Results update as you type. Jump to results

Site and system

Fills in a typical specific yield. Edit the yield below for your own site.

kWp
kWh/kWp
%

Annual output loss of the modules.

How the energy is used
%

Share of generation used on site.

kWh/yr

Optional: caps self-consumption. 0 if unknown.

R/kWh
R/kWh

0 where exports earn nothing.

%
Cost and evaluation
Cost and evaluation
R/Wp
R/kWp/yr
%
years
%

Results

425,000 kWh in year 1, 85% used on site, saving R795k. Payback 3.3 years.

Year-1 generation
425,000kWh
Capacity factor 19.4%
Used on site
85%
40% of site consumption
Year-1 savings
R795k
Avoided grid cost plus export credit
Payback
3.3years
NPV R8.0m at 12%

NoteTypical estimate for Johannesburg: a well-designed, unshaded, north-facing array. Run a site simulation (PVGIS or similar) before buying.

Check63,750 kWh a year is exported for no credit. A smaller system, load shifting into daylight hours or storage would put it to use.

Where the year-1 energy goes

kWh. Self-consumed energy is worth the grid tariff; exports only the export credit.

Used on site: 361,250 kWh (85%)85%Exported: 63,750 kWh (15%)15%
  • Used on site361,250 kWh · 85%
  • Exported63,750 kWh · 15%

Cumulative cash flow

Rand, nominal, after O&M; year 0 is the R2.8m installed cost. Hover a bar for its value.

R0R10mR20mR30mR40mR50mYear 0: −R2,750,0000Year 1: −R2,000,250Year 2: −R1,193,4622Year 3: −R325,325Year 4: R608,7964Year 5: R1,613,884Year 6: R2,695,2966Year 7: R3,858,795Year 8: R5,110,5748Year 9: R6,457,294Year 10: R7,906,11510Year 11: R9,464,734Year 12: R11,141,43012Year 13: R12,945,101Year 14: R14,885,31414Year 15: R16,972,354Year 16: R19,217,28016Year 17: R21,631,977Year 18: R24,229,22818Year 19: R27,022,771Year 20: R30,027,37720Year 21: R33,258,923Year 22: R36,734,48022Year 23: R40,472,397Year 24: R44,492,40224Year 25: R48,815,698−R2.8mR48.8mYear
Show the numbers
YearGenerationUsed on siteTariff R/kWhSavingsO&MNetCumulative
0–––––−R2,750,000−R2,750,000
1425,000361,2502.20R794,750−R45,000R749,750−R2,000,250
2422,875359,4442.38R854,038−R47,250R806,788−R1,193,462
3420,761357,6472.57R917,750−R49,613R868,137−R325,325
4418,657355,8582.77R986,214−R52,093R934,121R608,796
5416,564354,0792.99R1,059,785−R54,698R1,005,087R1,613,884
6414,481352,3093.23R1,138,845−R57,433R1,081,413R2,695,296
7412,408350,5473.49R1,223,803−R60,304R1,163,499R3,858,795
8410,346348,7943.77R1,315,099−R63,320R1,251,779R5,110,574
9408,295347,0504.07R1,413,205−R66,485R1,346,720R6,457,294
10406,253345,3154.40R1,518,630−R69,810R1,448,821R7,906,115
11404,222343,5894.75R1,631,920−R73,300R1,558,620R9,464,734
12402,201341,8715.13R1,753,661−R76,965R1,676,696R11,141,430
13400,190340,1615.54R1,884,485−R80,814R1,803,671R12,945,101
14398,189338,4605.98R2,025,067−R84,854R1,940,213R14,885,314
15396,198336,7686.46R2,176,137−R89,097R2,087,040R16,972,354
16394,217335,0846.98R2,338,477−R93,552R2,244,925R19,217,280
17392,246333,4097.54R2,512,927−R98,229R2,414,698R21,631,977
18390,284331,7428.14R2,700,392−R103,141R2,597,251R24,229,228
19388,333330,0838.79R2,901,841−R108,298R2,793,543R27,022,771
20386,391328,4339.49R3,118,318−R113,713R3,004,605R30,027,377
21384,459326,79110.25R3,350,945−R119,398R3,231,546R33,258,923
22382,537325,15711.07R3,600,925−R125,368R3,475,557R36,734,480
23380,624323,53111.96R3,869,554−R131,637R3,737,917R40,472,397
24378,721321,91312.92R4,158,223−R138,219R4,020,004R44,492,402
25376,828320,30413.95R4,468,426−R145,129R4,323,297R48,815,698

Over 25 years the system generates about 10,011 MWh. Internal rate of return 34.8% (pre-tax, unlevered).

Add to a business case

What is this?

Save this result as one measure in a project kept in this browser, then combine it with results from other tools for payback, NPV, IRR and a cost curve.

Saving, year 1
R794,750/yrAssumed
Energy saved
361,250 kWh/yrAssumed
Capex
R2,750,000Assumed
Life
25 yearsAssumed
Running cost
R45,000/yrAssumed

Every input above is still the illustrative example, so the result is tagged assumed. Enter your own numbers to make it yours.

Capex, running cost and life come from this tool's own inputs (under Cost and evaluation), so nothing else is needed.

Method

Generation in year y
E_y = kWp × yield × (1 − d)^(y − 1)
Capacity factor
CF = yield ÷ 8,760 h
Self-consumed and exported
self_y = min(E_y × s, site consumption), export_y = E_y − self_y
Value in year y
V_y = (self_y × T + export_y × X) × (1 + e)^(y − 1) − O&M × kWp × (1 + e_om)^(y − 1)
Payback, NPV and IRR
cash flow = [−kWp × 1,000 × R/Wp, V_1, …, V_n]; NPV at r, year 0 undiscounted

Typical specific yields used

Typical estimates, not measurements: a well-designed, unshaded, north-facing fixed-tilt array near the optimum tilt, after typical system losses, rounded to the nearest 50 kWh/kWp. A real site can differ by 10% or more.

CitykWh/kWp/yrCapacity factor
Johannesburg1,70019.4%
Pretoria (Tshwane)1,70019.4%
Cape Town1,65018.8%
George1,50017.1%
Durban (eThekwini)1,45016.6%
Pietermaritzburg1,45016.6%
Gqeberha (Port Elizabeth)1,60018.3%
East London1,50017.1%
Bloemfontein1,80020.5%
Kimberley1,85021.1%
Upington1,95022.3%
Polokwane1,75020.0%
Mbombela (Nelspruit)1,55017.7%
Rustenburg1,75020.0%
Mahikeng1,80020.5%

Assumptions

  • Specific yield already includes typical system losses (inverter, wiring, soiling, temperature). It is the AC output per kWp of DC modules.
  • The self-consumption share is constant across the years; it does not model weekend shutdowns, seasonal load or the half-hourly match.
  • Avoided grid cost is energy only (R/kWh). Rooftop solar rarely cuts the monthly maximum demand reliably, so demand charges are not counted as saved.
  • Pre-tax and unlevered: no section 12B allowance, finance costs, inverter replacement or residual value. See the own vs PPA vs lease tool for the tax view.
  • Default cost (R11/Wp installed) and O&M (R180/kWp a year) are illustrative for a commercial rooftop system; get quotes.

Sources

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